# Patent

## Title
Locking mechanism using a predefined lock for materialized views in a database system

## Abstract

A database system includes a locking mechanism for a materialized view defined on base relations. In response to updates of a base relation, a predefined lock (e.g., Y lock or IY lock) is placed on the materialized view. With the locking mechanism, transaction concurrency is enhanced by enabling concurrent updates of a materialized view by plural transactions in certain cases.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
This is related to U.S. Application entitled “Locking Mechanism Employing a Name Lock for Materialized Views”; and U.S. Application entitled “Locking Mechanism for a Materialized View that Does Not Employ Locks on the Materialized View,” filed concurrently herewith by Gang Luo, Michael W. Watzke, Curt J. Ellmann, and Jeffrey F. Naughton.
BACKGROUND
A database is a collection of stored data that is logically related and that is accessible by one or more users. A popular type of database is the relational database management system (RDBMS), which includes relational tables made up of rows and columns. Each row represents an occurrence of an entity defined by a table, with an entity being a person, place, or thing about which the table contains information. To extract data from, or to update, a relational table, queries according to a standard database query language (e.g., Structured Query Language or SQL) are used. A table (also referred to as a relation) is made up of multiple rows (also referred to as tuples). Each row (or tuple) includes multiple columns (or attributes).
Various other data structures are also typically associated with relations in a relational database system. For example, a view is a derived relation formed by performing a function on one or more base relations. Rather than storing the view, the function is typically recomputed each time the view is referenced. This type of view is referred to as an “ordinary view.”
Unlike an ordinary view, a materialized view is a pre-computed, stored query result that can be used for some queries instead of reconstructing the results directly from the base relations. As with the ordinary view, a function is performed on the base relations to derive the materialized view. However, because the materialized view is stored, fast access to the data is possible without recomputing the view.
After the materialized view is created, subsequent queries are able to use the materialized view, where appropriate, to increase query processing speed. Materialized views can be used to assemble data that come from many different relations. One type of view is the join view, which stores join results of multiple base relations.
A materialized view is updated when the underlying base relations are modified. As the base relations are changed through insertion of new tuples, deletion of tuples, or updates to existing tuples, the corresponding rows in the materialized view are changed to avoid becoming stale. This is known as materialized view maintenance.
Relational database systems can be used for data warehouse applications. A data warehouse collects information from several source databases. The collected information is integrated into a single database to be queried by the data warehouse clients.
Traditionally, data warehouses have been archival stores used for analysis of historical data. More recently, however, there has been a growing trend to use a data warehouse operationally (referred to as a “operational data warehouse” or “real-time data warehouse”), which involves making relatively real-time decisions about data stored in the data warehouse.
Traditional techniques of maintaining views are usually inadequate (in terms of processing speed) for operational data warehouses due to the real-time update requirements. Furthermore, materialized view maintenance in an operational data warehouse requires transactional consistency. If transactional consistency is enforced by traditional concurrency control mechanisms (including locking mechanisms), the ability of the database system to perform concurrent transactions may be reduced. This hurts performance in a database system, especially in a parallel database system having multiple processing modules.
When a base relation is updated (e.g., new row inserted, existing row deleted, or row modified), the update needs to be propagated to a materialized view as part of the materialized view maintenance. In some systems, to increase operational speeds, reduced levels of consistency are used that allow “dirty reads,” which are reads of stale data in relations. However, when such reduced levels of consistency are used in an environment in which materialized views are present, inconsistent query results are often obtained as a result of inaccurate data being captured in materialized views.
SUMMARY
In general, a method and apparatus are provided to improve concurrency control in a database system in which materialized views are maintained. For example, a method of maintaining consistency in a database system includes storing a materialized view that is based on base relations. In response to a request to update a first one of the base relations, an exclusive lock is placed on the first relation and a first predefined lock (e.g., a Y lock or IY lock) is placed on the materialized view. The first predefined lock conflicts with either a shared lock or exclusive lock placed on the materialized view but does not conflict with another predefined lock (e.g., another Y lock or IY lock) placed on the materialized view.
Other or alternative features will become more apparent from the following description, from the drawings, and from the claims.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates an example join view based on multiple base relations.
FIG. 2 is a block diagram of an example arrangement of a database system.
FIG. 3 is a flow diagram of logic for a Y-lock locking mechanism according to one embodiment for join view maintenance.
FIGS. 4A–4C illustrate example join views based on multiple base relations with projection or aggregation applied.
FIGS. 5A–5B illustrate examples of a logical undo mechanism.
FIG. 6 is a flow diagram of logic for a no-lock locking mechanism according to another embodiment for join view maintenance.
FIG. 7 illustrates a name-lock data structure used for a name-lock locking mechanism according to another embodiment.
FIGS. 8A–8B are a flow diagram of logic for the name-lock mechanism for join view maintenance.
FIG. 9 illustrates join view maintenance in the presence of a key-foreign key constraint.
FIG. 10 illustrates auxiliary relations used for join view maintenance.
FIGS. 11A–11B and 12A–12B illustrate join view maintenance without auxiliary relations.
FIGS. 13A–13B illustrate join view maintenance with auxiliary relations.
FIG. 14 illustrates auxiliary indices used for join view maintenance.
FIGS. 15A–15B illustrate join view maintenance with auxiliary indices.
FIG. 16 is a block diagram of a database system with a centralized lock manager mechanism.
FIG. 17 is a block diagram of a database system with a distributed lock manager mechanism.
FIG. 18 illustrates join view maintenance using auxiliary indices and the no-lock locking mechanism.



DETAILED DESCRIPTION
In the following description, numerous details are set forth to provide an understanding of the present invention. However, it will be understood by those skilled in the art that the present invention may be practiced without these details and that numerous variations or modifications from the described embodiments are possible.
I. Introduction
In accordance with some embodiments of the invention, a locking mechanism for materialized views (such as join views) is provided in a relational database system. A common form of materialized view is the join view, which is a view that stores and maintains the result of a join of multiple base relations (also referred to as “base tables”). However, locking mechanisms described here can be extended to other types of views.
The following provides an example query for creating a join view (JV) on two base relations (A, B):

    
    
        CREATE JOIN VIEW JV AS
        SELECT *
        FROM A, B
        WHERE A.c=B.d
        PARTITIONED ON A.e;
    
    


The join view JV includes tuples (also referred to as “rows”) of base relations A and B where the attributes (also referred to as “columns”) A.c and B.d are equal. The join view JV is partitioned on the attribute A.e. In other examples, a join view can be stored for a join of more than two base relations. In the following description, the terms “table” and “relation” are used interchangeably. Also, a table or relation has rows (or tuples) and columns (or attributes). The terms “rows” and “tuple” are used interchangeably, and the terms “column” and “attribute” are used interchangeably.
The join relations A and B and the join view JV created as a result of the query is shown in FIG. 1. Relation A has attributes a, b, c, and relation B has attributes d, e, f, g. The “SELECT *” clause selects all attributes of relations A and B for insertion into the join view JV (which contains attributes a, b, c, d, e, f, g). Note that less than all attributes of relations A and B can be projected into the join view JV, in which case less than all of the attributes a, b, c, d, e, f, g, are stored in the join view JV.
There are various different types of locks that can be placed on data stored in relational tables to restrict access to or the ability to modify the data. Table-level locks are placed on an entire table or relation. Table-level locks include a table-level shared (S) lock and a table-level exclusive (X) lock. Generally, once placed on a table, a table-level S lock blocks a subsequent transaction that attempts to write to any part of the table. A table-level X lock placed on a table is more restrictive, as it blocks any subsequent transaction that attempts to read from or write to any part of the table.
While a table-level locking mechanism locks an entire table, a value locking mechanism locks only a portion of the table. The value locking mechanism specifies a value (or values) of an attribute(s) in a table for which locks are to be placed. Such an attribute, or attributes, is referred to as a value locking attribute, or value locking attributes. A value locking mechanism usually locks only one row or a few rows.
Value locks include a shared (S) value lock and an exclusive (X) value lock. To place an X value lock on an attribute value of a base relation R, a table-level intention exclusive (IX) lock is first placed on R. Similarly, to place an S value lock on an attribute value of the base relation R, a table-level intention shared (IS) lock is first placed on R.
A table-level IX lock is placed on a relation to prevent any subsequent table-level X or S locks on the same relation. The IX lock is a mechanism for the database system to efficiently determine whether a subsequent table-level X or S lock can be placed, without having to find out if there is a conflicting value lock on the relation. For example, suppose the value locking attribute of a relation R is attribute R.a. There can potentially be multiple value locks placed on multiple values of R.a. Thus, a first X value lock can be placed on row(s) of the relation R with R.a=5, a second X value lock can be placed on row(s) of relation R with R.a=2, and so forth. If a subsequent transaction attempts to read the entire relation R, the subsequent transaction will need to acquire a table-level S lock. One way to determine if the table-level lock S lock can be granted is by finding each R.a value for which there is an X value lock. However, this is inefficient. Instead, according to some embodiments, a table-level IX lock is placed on the relation R if there is at least one X value lock on the base relation R. Thus, to determine if the table-level S lock can be placed on the relation R, the database system can quickly detect that there is already an IX lock placed on the relation R, which blocks acquisition of the table-level S lock. Note that only one IX lock is needed for multiple X value locks of a relation. The IX lock also blocks any subsequent table-level X lock.
Similarly, a table-level IS lock placed on a relation R for an S value lock blocks any subsequent table-level X lock.
If a transaction T attempts to update base relation R, the transaction has two choices: (1) T can place a table-level X lock on base relation R; or (2) T can place an IX lock on R and an X value lock on some value locking attribute of R. Similarly, if transaction T attempts to read base relation R, transaction T also has two choices: (1) T can place a table-level S lock on R; or (2) T can place an IS lock on R and an S value lock on some value locking attribute of R.
If materialized views are also stored in the database system, a locking mechanism is also provided for the materialized view. In one embodiment, a “Y-lock” locking mechanism is provided for the materialized view. In another embodiment, a “no-lock” locking mechanism is provided for the materialized view. In yet another embodiment, a “name-lock” locking mechanism is provided for the materialized view.
II. Example Database System Arrangement
FIG. 2 shows an example arrangement of a database system 10 that stores base relations (e.g., A, B) and join views (e.g., JV). The database system 10 is a parallel database system having a plurality of data server modules 12. Each data server module 12 is responsible for managing the access to or modification of data stored in a respective storage module 14. Examples of the responsibilities of each data server module (also referred to as “an access module”) include locking databases, tables, or portions of tables; creating, modifying, or deleting definitions of tables; inserting, deleting, or modifying rows within tables; and retrieving information from definitions and tables. The data server modules, after executing an action, also return responses to a requesting client. In one example implementation, the data server modules 12 are based on access module processors (AMPs) in TERADATA® database systems from NCR Corporation.
According to one embodiment, each data server module 12 includes a lock manager 15 to provide a locking mechanism according to some embodiments of the invention. Thus, the lock manager 15 is responsible for placing locks (e.g. table-level locks or value locks) on base relations and join views. As shown, the locking mechanism is distributed across plural data server modules 12. Alternatively, a centralized lock manager is employed.
In one embodiment, the requesting client that sends commands to the data server modules 12 include one or more parsing engines 16. The parsing engine(s) 16 receive requests from users or applications, which are in the form of queries according to a standard database query language (such as a Structured Query Language or SQL, as provided by the American National Standards Institute or ANSI). In other embodiments, other types of database query languages can be used.
Each parsing engine 16 includes a parser 18 and an optimizer 20. The parser 18 checks a received request for proper syntax and semantically evaluates the request. The optimizer 20 develops an execution plan for the received request. In some embodiments, the optimizer 20 uses a cost-based mechanism to select a least expensive (in terms of system resources utilized) execution plan for the query.
The execution plan includes a series of “steps” that are communicated to one or more of the data server modules 12 over a communications network 22. If the execution plan can be executed by one data server module 12, then the parsing engine 16 sends the one or more steps of the execution plan to the one data server module 12. However, if plural data server modules 12 are involved in the execution plan, the parsing engine 16 sends the step(s) to the plural data server modules 12. The sequence in which steps of an execution plan are executed is controlled by a scheduler 21 in the parsing engine 16.
In the example shown in FIG. 2, base relations A and B are stored in plural storage modules 14 associated with corresponding data server modules 12. Each base relation A or B is partitioned into plural partitions based on one or more selected attributes (referred to as the primary index) of the base relation. Each partition stored on a respective storage module 14 includes a subset of all the rows of the base relation. A join view (JV), such as a join view based on a join of tables A and B, is also partitioned across the plurality of storage modules 14. In the example join view CREATE statement above, the join view is partitioned on attribute A.e of base relation A. Thus, a first partition of the join view JV is stored on a first storage module 14 based on some value(s) of A.e; a second partition of JV is stored on a second storage module 14 base on some other value(s) of A.e; and so forth.
Although the storage modules 14 are shown as separate modules, they can be part of the same storage subsystem. Alternatively, the storage modules 14 can be separate storage devices. In another embodiment, instead of plural data server modules 12, the database system 10 can include only one data server module 12. The locking mechanism according to the various embodiments can be applied to a single-data server module database system rather than the multiple-data server module database system shown in FIG. 2.
III. Y-Lock Locking Mechanism
In accordance with one embodiment of the invention, the locking mechanism provided for the join view is a “Y-lock” locking mechanism. A Y lock has similar functionality as an X lock, with the major difference being that the Y lock does not conflict with itself. In other words, the Y lock is a modified type of exclusive lock placed on a join view that allows subsequent Y locks to be placed on the same join view even though a first Y lock is still active on the join view. In contrast, a table-level X lock placed on a join view blocks a subsequent X lock on the same join view. By enabling multiple Y locks to be concurrently placed on the same join view, throughput of transactions in the database system 10 can be greatly increased, as the possibility of lock conflicts on the join views among different transactions is reduced. A Y lock is also referred to as a “modified-exclusive” lock.
Three types of table-level locks are allowed on a join view (JV): Y locks, S locks, and X locks. The conditions under which such table-level locks are allowed are shown by the logic of FIG. 3. The logic shown in FIG. 3 is performed by a data server module 12 in the database system 10 of FIG. 2. If more than one data server module 12 is involved in a particular transaction, then each data server module 12 performs the acts of FIG. 3 concurrently.
Upon receiving steps associated with a transaction, the data server module 12 first determines (at 102) if the transaction specifies both a read and write of the join view JV. As used here, writing a view is also referred to as updating or modifying the view. If the transaction specifies both a read and write of JV, then the data server module 12 determines (at 104) if an S lock, X lock, or Y lock is currently active on the join view JV. If none of an S lock, X lock, or Y lock is active on the join view JV, the data server module 12 places (at 106) an X lock on the join view JV. Note that the X lock required for the received transaction conflicts with any of an X lock, S lock, or Y lock, so that the presence of any such table-level lock on JV blocks the required X lock for the received transaction.
If the data server module 12 determines (at 104) that any of the S lock, X lock, or Y lock is active on JV, the data server module 12 performs (at 108) conflicting lock processing. For example, the data server module 12 can wait until the current conflicting lock has been released. Different embodiments perform different tasks in response to detecting that a received transaction cannot proceed because of conflicting locks placed on the join view JV.
If the transaction is determined not to involve both a read and write of the join view JV (at 102), the data server module 12 checks (at 110) if the transaction involves a read (but not a write) of the join view. If so, the data server module 12 checks (at 112) if there is either a Y lock or an X lock on JV. If neither a Y lock nor X lock is currently active on JV, the data server module 12 places (at 114) an S lock on the join view JV. Note that the required S lock conflicts with either a Y lock or X lock, so that the presence of either the Y lock or X lock blocks acquisition of the S lock for the received transaction. If either a Y lock or X lock is presently active on JV, the data server module 12 processes (at 108) the conflicting lock condition.
If the transaction does not involve a read of the join view JV, the data server module checks (at 116) if the transaction involves a write (but not a read) of the join view JV. If so, the data server module 12 checks (at 118) if there is either an S lock or an X lock on JV. If not, then the data server module 12 places (at 120) a Y lock on the join view JV. However, if either an S lock or an X lock is presently active on JV, the data server module 12 processes (at 108) the conflicting lock condition.
The discussion above refers to placing a table-level Y lock on a join view. The Y-lock locking mechanism for join views is extendable to also allow value locks on join views. Consider a join view JV defined on base relations R1, R2, . . . , and Rn. For a fixed i (1≦i≦n), suppose that Ri.ai is an attribute of base relation R1 that also appears in JV. This is the case for the example join view JV of FIG. 1. Then X and S value locking on Ri.ai for JV is allowed. For example, consider a transaction T that only updates base relation Ri. If the update to base relation Ri specifies the value(s) of Ri.ai, then transaction T can also place an IY lock on JV and one or several X value locks (not Y value locks) on Ri.ai for JV. If transaction T reads JV by specifying the Ri.ai value(s), then transaction T can put an IS lock on JV and one or several S value locks on Ri.ai for JV. If transaction T both reads and updates JV by specifying the Ri.ai value(s), then transaction T can put an IX lock on JV and one or several S value locks and X value locks on Ri.ai for JV.
The IY lock is similar to the traditional IX lock except that it is compatible with a Y lock or another IY lock. As with the IX lock, the table-level IY lock is placed on the join view JV in conjunction with an X value lock of JV to indicate to subsequent transactions that table-level X or S locks on JV are blocked (however, a table-level Y or IY lock on JV is still possible in the presence of the IY lock with X value lock). Also, a subsequent IS lock with an S value lock on JV is allowed in the presence of an IY lock with X value lock on JV. Note, however, that an X value lock on JV is incompatible with an S value lock placed on the same attribute value of JV.
For a transaction that performs both a (table-level) read and (value) write of the join view JV, both an S lock and X value lock are needed on the join view JV. In this case, a table-level SIY lock (which is equivalent to an S lock and an IY lock) is placed on JV. The SIY lock is similar to the traditional SIX lock. One can think that IX=IS+IY. An SIX lock is equivalent to an S lock and an IX lock (for an X value lock). The SIY lock is only compatible with the IS lock.
Note that SIX=S+IX=S+(IS+IY)=(S+IS)+IY=S+IY=SIY. Thus, the SIX lock is the same as the SIY lock.
If transaction T both updates JV (without specifying the Ri.ai value(s)), which is a table-write, and reads JV (specifying the Ri.ai value(s)), which is a value-read, then transaction T requires both a Y lock and S value lock(s) on JV. In this case, a table-level YIS lock is played on JV (which is equivalent to a Y lock and an IS lock). The YIS lock (Y+IS) is similar to the SIX lock and is only compatible with the IY lock.
The compatibilities of the different locks are listed in Table 1.
















TABLE 1







Y
S
X
IS
IY
IX
SIY
YIS
























Y
yes
no
no
no
yes
no
no
no


S
no
yes
no
yes
no
no
no
no


X
no
no
no
no
no
no
no
no


IS
no
yes
no
yes
yes
yes
yes
no


IY
yes
no
no
yes
yes
yes
no
yes


IX
no
no
no
yes
yes
yes
no
no


SIY
no
no
no
yes
no
no
no
no


YIS
no
no
no
no
yes
no
no
no









According to Table 1, a Y lock on JV is compatible with another Y lock or an IY lock on JV. However, the Y lock is incompatible with a table-level S lock, X lock, IS lock, IX lock, SIY lock, or YIS lock. Note that a table-level X lock is incompatible with any lock. An IY lock on JV is compatible with a table-level Y lock, IS lock, IY lock, IX lock, or YIS lock. However, an IY lock is incompatible with a table-level S lock, X lock, and SIY lock. An IX lock is compatible with an IS, IY, or IX lock, but not with any other locks. An SIY lock (S+IY lock) is compatible with an IS lock, but not with any other locks. A YIS lock (Y+IS lock) is compatible with an IY lock, but not with any other locks.
To show that the Y-lock locking mechanism keeps the isolation property (serializability) of transactions, the following assertions are proven for a join view JV defined on base relations R1, R2, . . . , and Rn:

    
    
        Assertion 1: Transaction T's writes to join view JV are neither read (first part of Assertion 1) nor written (second part of Assertion 1) by other transactions until transaction T completes (aborts or commits).
        Assertion 2: Transaction T does not overwrite dirty data of other transactions (data that is being modified by other transactions) in join view JV.
        Assertion 3: Transaction T does not read dirty data of other transactions in join view JV.
        Assertion 4: Other transactions do not write any data in join view JV that is read by transaction T before transaction T completes.
    
    


The four assertions are first proven for the simple case where JV σ(R1. . . Ri. . . Rn), where σ denotes a selection operator. The assertions are also proven (further below) for the general case where JV=π(σ(R1. . . Ri. . . Rn)), where π is a projection operator. Selection refers to selecting rows of base relations to place into JV based on the rows satisfying the join condition. Projection refers to projecting only those attributes that are in the select list of the join query into the join view JV. Less than all attributes of each base relation can be projected into the join view JV.
It is assumed that join view JV allows duplicate tuples. If no duplicate tuples are allowed in JV, assume that each tuple in JV has a dupcnt (or some other named parameter) attribute recording the number of copies of that tuple. The following assumptions are made (the other more complex cases can be proven in a similar way): (1) any transaction T updates at most one base relation of JV; and (2) if transaction T tries to update base relation Ri(1≦i≦n), it places a Y lock on JV and an S lock on each Rj(1≦j≦n, j≠i). For example, if a join view JV is defined on base relations A, B, and C, an update of A causes an X lock or IX lock to be placed on A, and a Y lock to be placed on JV. In addition, an S lock is placed on each of the other base relations B and C.
If transaction T writes join view JV, T places a table-level Y lock on JV until T completes. If transaction T both reads and writes join view JV, T places a table-level X lock on JV until T completes. Thus transaction T's writes to join view JV are not read by any other transaction T′ until transaction T completes, since T′ requires a table-level S lock on JV (which would conflict with the Y lock or X lock on JV for transaction T). This proves the first part of Assertion 1
Additionally, if transaction T writes join view JV, there are two possible cases:

    
    
        Case 1: Transaction T both reads and updates join view JV. In this case, transaction T puts a table-level X lock on JV until it completes. This X lock will block other transactions from writing JV (by blocking other Y or X lock requests).
        Case 2: Transaction T updates the base relation Ri(1≦i≦n). Transaction T puts an S lock on each Rj (1≦j≦n, j≠i) until T completes. If another transaction T′ tries to write join view JV before transaction T completes, transaction T′ can only update the same base relation Ri. This is because if transaction T′ updates another base relation Rj(1≦j≦n, j≠i) of join view JV, the requirement of an IX or X lock on Rj for transaction T′ will be blocked by the existing S lock on Rj that is placed by transaction T.
    
    


Suppose that transactions T and T′ update ΔRi and ΔRi′ of base relation Ri, respectively. ΔRi refers to the changed portion of Ri made by transaction T, and ΔRi′ refers to the changed portion of Ri made by transaction T′. There are three possible scenarios:

    
    
        Scenario 1: If transaction T puts a table-level X lock on base relation Ri, transaction T′ will get blocked when it tries to get either a table-level X lock or a table-level IX lock on Ri.
        Scenario 2: If transaction T puts a table-level IX lock on base relation Ri, transaction T′ will get blocked if it tries to get a table-level X lock on Ri.
        Scenario 3: Suppose that transaction T puts a table-level IX lock and one or several X value locks on base relation Ri Also, transaction T′ tries to put a table-level IX lock and one or several X value locks on base relation Ri. There are two cases:
        (a) ΔRi∩ΔRi′≠Ø(ΔRi intersects with, or overlaps, ΔRi′). The requirement of X value locks on Ri for transaction T′ will be blocked by the existing X value locks on Ri that is put by transaction T, since T and T′ are changing the same portion of Ri(the portion that overlaps).
        (b) ΔRi∩ΔRi′=Ø(ΔRi does not intersect with ΔRi′). Then σ (R1. . . ΔRi. . . Rn)∩σ(R1. . . ΔRi. . . Rn)=Ø. In other words, the intersection of the updates to JV by transactions T and T′ is empty.
    
    


Thus transaction T's writes to join view JV are not written by any other transaction T′ until transaction T completes (this proves the second part of Assertion 1).
Based on a similar reasoning to the proof of Assertion 1, transaction T does not overwrite dirty data of other transactions in join view JV (this proves Assertion 2).
Suppose that transaction T reads join view JV by requiring a table-level S lock on JV. If some other transaction T′ is writing any data in join view JV, T′ will place a table-level Y lock (or X lock) on JV until T′ completes. The required table-level S lock on JV for transaction T will be blocked by the table-level Y or X lock on JV for transaction T′. Thus transaction T does not read dirty data from transaction T′ in join view JV (this proves Assertion 3). The case that transaction T tries to both read and update the join view JV can be proved similarly, since the required X lock will be blocked by the Y or X lock for transaction T′.
If transaction T reads join view JV, T will place a table-level S lock on JV until T completes. If transaction T both reads and writes join view JV, T will place a table-level X lock on JV until T completes. Thus no other transaction T′ can write any data in JV until transaction T completes, since T′ requires a table-level Y lock (or X lock) on JV. This proves Assertion 4.
The proof for the general case where JV=π(σ(R1. . . Ri. . . Rn)) is discussed here. When projection is applied, less than all the attributes of the base relations Ri (1≦i≦n) will be in a join view JV based on the base relations R1, R2, . . . , Rn. This is illustrated in FIG. 4A. In the example, the join view JV contains only attributes A.a and A.b. The other attribute A.c of relation A and attributes B.d, B.e, B.f, B.g of relation B are not in JV. In this general case, the proofs of the four assertions are the same except for the case where ΔRi∩ΔRi′=Ø in Assertion 1. In this case, a virtual join view JV′=σ(R1. . . Ri . . . Rn) is defined conceptually (that is, JV′ is an imaginary table that does not exist in the database system). The virtual join view is a join view without the projection applied. Therefore, JV′ contains all attributes of the base relations. Each tuple in the actual join view JV comes from (and is a subset of) one tuple in JV′. Conceptually, if the tuples in JV are expanded to include all the attributes of JV′, then the tuples in JV are different from each other. After computing the change Δ=σ(R1. . . ΔRi. . . Rn), for each tuple TA1 in Δ, there are three possible cases:

    
    
        Case 1: Tuple TA1 is inserted into Δ. This means that π(TA1) needs to be inserted into JV. We insert π(TA1) into JV. Conceptually, the expanded tuple of π(TA1) is TA1.
        Case 2: Tuple TA1, is updated into TA2 in Δ. This means that tuple π(TA1) needs to be updated into π(TA2) in JV. A tuple TA3 in JV that is equal to π(TA1) can be identified. Note that there may be several tuples in JV that are equal to π(TA1). However, conceptually, it can be thought that the expanded tuple of the identified tuple TA3 is equal to TA1. Then tuple TA3 in JV is updated from π(TA1) to π(TA2). Conceptually, it can be thought that the expanded tuple of the updated TA3 is TA2.
        Case 3: Tuple TA1, is deleted from Δ. This is similar to case 2.
    
    


For example, suppose each tuple without projection originally has three attributes (a, b, c). After projection, the tuple only contains two attributes (a, b). Suppose there are tuples T1=(1, 2, 3), T2=(1, 2, 4), and T3=(1, 5, 6). After projection, the tuples become T1′=(1, 2), T2′=(1, 2), T3=(1, 5). Thus, the expanded tuple of T1′ is T1, the expanded tuple of T2′ is T2, and the expanded tuple of T3′ is T3, Suppose the tuple T2=(1, 2, 4) is updated to (1, 8, 5). Then, after projection, one of the tuples (1, 2) is changed to (1, 8). However, T1′ and T2′ look the same. Suppose T1′ is changed from (1, 2) to (1, 8). Note T2′ originally comes from T2, so T2′ should be changed instead of T1′. However, a distinction between T1′ and T2′ in JV cannot be made as they look the same. The tuples before the projection become T1=(1, 2, 3), T2=(1, 8, 5), and T3=(1, 5, 6). The tuples after projection become T1′=(1, 8), T2′=(1, 2), T3′=(1, 5).
However, conceptually, it can be thought that the expanded tuple of T1′ is T2, the expanded tuple of T2′ is T1, and the expanded tuple of T3′ is T3. That is, conceptually it can be thought that the expanded tuple of the identified tuple, T1′, is equal to tuple T2, even if T1′ originally comes from tuple T1. Note tuples in the relation have no order. If the order of tuples in the relation is switched, the same relation can be obtained. Thus, if the order of T1′ and T2′ is switched, the tuples after projection become T1′=(1, 2), T2′=(1, 8), T3′=(1, 5). Then it can be thought that the expanded tuple of T′ is T1, the expanded tuple of T2′ is T2, and the expanded tuple of T3′ is T3. The key point is that tuples in the JV with the same value have no difference and thus can be treated in any way.
Thus, conceptually,

π(σ(R1. . . ΔRi. . . Rn))∩π(σ(R1. . . ΔRi. . . Rn))=Ø.

That is, the intersection of the updates to JV by transactions T and T′ is empty. Consequently, transaction T's writes to join view JV are not written by any other transaction T′ until transaction T completes (this proves part 2 of Assertion 1 for the general case).
The Y-lock locking mechanism for join views also applies to aggregate join views. An example aggregate join view AJV is created as follows:

    
    
        CREATE JOIN VIEW AJV AS
        SELECT A.a, SUM (A.b)
        FROM A, B
        WHERE A.c=B.d
        GROUP BY A.a;
    
    


As shown in FIG. 4B, the aggregate join view AJV contains an attribute A.a and a sum of the attribute A.b. SUM is one type of aggregate function, with other aggregate functions including COUNT, AVG, and so forth.
Consider an aggregate join view AJV=γ(π(σ(R1. . . Ri. . . Rn))), where γ is the aggregation operator. A virtual (imaginary) join view JV=π(σ(R1. . . Ri. . . Rn)) is defined so that AJV=γ(JV). In other words, the aggregate join view AJV is equal to the virtual join view after aggregation.
An example of this is illustrated in FIG. 4C. Assume that the schema of AJV is (A.a, SUM(B.d))—in other words, AJV has two attributes: A.a and the sum of B.d. Assume that initially AJV has one tuple (1, 3). Subsequently, as a result of a new transaction(s), join result tuples (1, 1) and (1, 5), which are results due to a join of base relations A and B, are to be inserted into AJV. The join result tuples are not inserted into new rows of AJV, but rather, the existing row in AJV is updated by summing B.d values of the join result tuples to the existing tuple of AJV. Thus, AJV is updated by updating SUM(B.d), which is 3+1+5=9. Thus, after the update, AJV has the tuple (1, 9).
For purposes of proving the assertions above, the virtual or imaginary JV remains consistent with AJV. The schema of the virtual join view JV is (a, d). Aggregation is not performed on the virtual join view JV so that the join result tuples are added into new rows of the virtual join view JV. However, note that aggregation of the virtual join view JV will cause it to result in AJV.
Thus, whenever AJV is updated by Δ, conceptually, the corresponding tuples that produce Δ are updated in the virtual join view JV. By the above reasoning, if the virtual join view JV is considered instead of the aggregate join view AJV, then any parallel execution of the transactions are equivalent to some serial execution of these transactions. AJV is always set equal to γ(JV). Thus if the virtual join view JV is replaced by AJV, any parallel execution of the transactions are still equivalent to some serial execution of the transactions. This provides the four assertions above.
As noted above, a benefit of the Y-lock locking mechanism is that multiple transactions can place Y locks concurrently on the same join view JV to enhance database performance. However, in the context of projection join views and aggregate join views, a physical undo of an update of a join view due to a first transaction (T1) that aborts may not be possible. An example is illustrated in FIG. 5A. Assume that projection JV initially contains three tuples (1, 1), (1, 2), and (1, 2), shown as 150 in FIG. 5A. In this example, the projection JV allows duplicate tuples. The scheme of JV is (A.a, B.d). Due to projection, less than all of the attributes of base relations A and B are in the projection JV.
The first transaction T1 updates the tuple (1, 1) in JV to (1, 2). The modified projection JV is shown as 152, where (1, 1) has been changed to (1, 2). In the example, another transaction T2 is also active. T2 updates tuple (1, 2) in JV to (1, 3). The modified projection JV is shown as 154.
After the T2 update, the first transaction T1 aborts. In that case, the tuple (1, 2) that was changed from (1, 1) needs to be un-updated back to (1, 1). However, that tuple has already been changed to (1, 3) by transaction T2, so a physical undo of that tuple is not feasible.
In accordance with some embodiments, a logical undo is performed. In the logical undo, the database system looks for another tuple in the join view JV that has the attribute values (1, 2). That other tuple is changed to the value (1, 1) for a logical undo of transaction T1, shown as 156 in FIG. 5A.
A logical undo is also used for aggregate join views. As shown in FIG. 5B, assume AJV has the scheme (A.a, SUM(B.d)) and initially has the tuple (1, 5), shown as 160. Transaction T1 causes a join result tuple (1, 3), with scheme (A.a, B.d), to be inserted into AJV. This causes the AJV entry to be updated to (1, 8), shown as 162. Before T1 completes, T2 causes a join result tuple (1, 4) to be inserted into AJV, which causes the tuple in AJV to be updated to (1, 12), shown as 164.
If T1 aborts, a physical undo is not possible, since the entry (1, 8) no longer exists in AJV. Therefore, a logical undo is performed to change the tuple (1, 12) in AJV to (1, 9), shown as 166. This removes the contribution of the (1, 3) tuple for transaction T1 from the AJV.
By reducing the occurrences in which transactions block each other due to locks placed on a materialized view, such as a join view, database system performance is enhanced. This is especially beneficial in a parallel database system having plural data server modules, such as that shown in FIG. 2, where the ability to perform steps of multiple transactions on separate data server modules increases database throughout. For example, a portion of one transaction that updates the join view can be performed in a first data server module concurrently with a portion of another transaction that updates the join view on a second data server module.
IV. No-Lock Locking Mechanism
In accordance with another embodiment, instead of a Y-lock locking mechanism, a “no-lock” locking mechanism is employed for the join view JV. One purpose of placing a Y lock on a join view JV during an update of the join view in a first transaction is to prevent other transactions from reading JV, as write conflicts on the join view JV have already been handled by placing proper locks on the base relations of JV. However, in another embodiment, according to the “no-lock” locking mechanism, a Y lock (or any other type of lock) on the join view JV can be omitted while still protecting the join view JV as well as maintaining transaction serializability. A benefit of not placing any lock on the join view reduces the overhead associated with maintaining join views in response to update of base relations. Many transactions in the database system 10 are small updates that involve single-tuple or few-tuple updates to base relations of a join view. If locks (such as Y locks) are required for all such small updates, then the required locks may cause a bottleneck, since the lock manager may become tied up with many lock requests.
In some embodiments, a join view cannot be directly updated or modified. In other words, a join view JV is updated or modified by updating the base relations of the join view JV. FIG. 6 illustrates the logic according to one embodiment of the no-lock locking mechanism.
In the discussion below, it is assumed that a join view JV is defined on base relations R1, R2, . . . , and Rn. As shown in FIG. 6, a data server module 12 determines (at 402) if a transaction requires both a read and write of the join view JV. A write of JV occurs through write(s) of one or more base relation(s) Rt1, Rt2, . . . , Rth({t1, t2, . . . , th})⊂(1, 2, . . . , n}) on which JV is defined. If the data server module 12 determines (at 402) that the transaction involves both a read and write of JV, the data server module 12 next checks (at 404) if any conflicting locks are placed on the base relations R1, R2, . . . , and Rn. If so, then the data server module 12 performs (at 406) conflicting lock processing, which in one example involves waiting for the conflicting lock(s) to be released.
If no conflicting locks on the base relations are detected, then the data server module 12 places a table-level X lock (at 408) on each base relation Rtu (1≦u≦h) that is being updated. Note that the required table-level X lock on each relation Rtu would conflict with an X lock, S lock, IX lock, or IS lock. The data server module 12 also places a table-level S lock on each other base relation Rj (jε{1, 2, . . . , n}−{t1, t2, . . . , th}). Note that the table-level S lock on each Rj required for the received transaction would conflict with an X lock or IX lock that has been placed on Rj. Thus, assuming no conflicting locks are present, an X lock is placed on each base relation that is updated, while an S lock is placed on the base relation(s) not being updated.
The table-level X lock on each base relation that is being updated can be replaced with a less restrictive SIX lock (S+IX) if the following condition is satisfied: the update of Rtu(1≦u≦h) specifies value locking attribute values of Rtu.
If the data server module 12 determines (at 402) that the transaction does not involve both a read and write of the join view JV, then the data server module 12 determines (at 410) if the transaction involves a read (but not a write) of JV. If so, then the data server module 12 checks (at 412) for any conflicting locks placed on the base relations R1, R2, . . . , Rn. If a conflict is present, lock conflict processing is performed (at 406). If no conflicting lock exist, then the data server module 12 places (at 414) a table-level S lock on each base relation Ri(1≦i≦n) of JV. Note that the required table-level S lock on each base relation would conflict with an X lock or IX lock placed on any of the base relations.
The requirement (by the no-lock locking mechanism) of an S lock on each base relation of JV for a transaction that reads the JV differs from that of the Y-lock locking mechanism. In the Y-lock context, to read JV, only a Y lock is needed on JV, with no S locks needed on the base relations. Therefore, for reads of JV, the Y-lock locking mechanism requires less locks than the no-lock locking mechanism.
If, however, the data server module 12 determines (at 416) that the transaction involves a write (but not a read) of the join view JV through updating one or several base relations Rt1, Rt2, . . . , Rth({t1, t2, . . . , th}⊂(1, 2, . . . , n}) of JV, then the data server module 12 checks (at 418) for conflicting locks on any of the base relations. If no conflicting locks are present, then the data server module 12 places (at 420) an X lock or IX lock on each base relation Rtu (1≦u≦h) being updated. An X lock is placed if a table-write of Rtu is needed. An IX lock is placed if a value-write of Rtu is performed. In addition, the data server module 12 places an S lock or IS lock on each other base relation Rj(jε{1, 2, . . . , n}−{t1, t2, . . . , th}) based on whether a table-read or value-read of Rj is performed.
A value-write or value-read is a write or read requested by a query with a WHERE clause condition that specifies a specific value or values for the value locking attribute of the base relation. An example query that specifies a value-write is as follows:

    
    
        UPDATE A
        SET A.f=A.f+1
        WHERE A.e=2;

where A.e is the value locking attribute of base relation A.

    
    


On the other hand, a table-write or table-read is a write or read requested by a query with a WHERE clause condition that does not specify a specific value or values for the value locking attribute of the base relation. An example query that specifies a table-read is as follows:

    
    
        SELECT *
        FROM A
        WHERE A.f=2;

where A.f is not the value locking attribute of base relation A.

    
    


If h≧2, which means that more than one base relation Rtu is being updated, then an IX lock placed on each Rtu may need to be changed to an SIX lock (S+IX). Placing the extra S lock on each Rtu is needed because when one base relation (e.g., Rt1) is being updated, then all other base relations on which JV is based are read for join view maintenance. Similarly, when Rt2 is being updated, then all the other base relations (including Rt1) are read for join view maintenance.
The no-lock locking mechanism requires less locks than the Y-lock locking mechanism for transactions that cause the join view JV to be updated. In the Y-lock context, for updates of JV, a Y lock is placed on JV along with an X lock (or IX lock) on each updated base relation and an S lock on each non-updated base relation. On the other hand, in the no-lock context, for updates of JV, no lock is placed on JV, with an X lock (or IX lock or SIX lock) placed on each updated base relation and an S lock placed on each non-updated base relation.
The no-lock locking mechanism for join views can be extended to allow value locks on join views. Consider a join view JV defined on base relations R1, R2, . . . , and Rn. For a fixed i (1≦i≦n), suppose that Ri.ai is the value locking attribute of base relation Ri that also appears in JV. Then value locking on Ri.ai for JV is allowed. However, note that no lock is placed on JV according to the no-lock locking mechanism.
If transaction T reads JV by specifying Ri.ai value(s), then transaction T places an IS lock on Ri, one or several S value locks on Ri.ai for Ri(not JV), and an S lock on each other base relation Rj(j≠1≦j≦n) of JV. If transaction T both updates base relation Ri by specifying the Ri.ai value(s) and reads JV, an SIX lock is placed on Ri, one or several X value locks are placed on Ri.ai or Ri, and an S lock is placed on each other base relation Rj (j≠i, 1≦j≦n) of JV.
To show that the no-lock locking mechanism maintains the isolation properly (serializability) of transactions, the same four assertions as for the Y-lock locking mechanism are proven. It is assumed that join view JV allows duplicate tuples. If no duplicate tuples are allowed in JV, assume that each tuple in JV has a dupcnt attribute (or some other named parameter) recording the number of copies of the tuple. The following assumptions are made (the other more complex cases can be proved in a similar way): (1) any transaction T updates at most one base relation of JV; and (2) if transaction T tries to update base relation Ri(1≦i≦n), it places an S lock on each Rj(1≦j≦n, j≠i).
If transaction T writes join view JV, T places a table-level IX or X lock on the base relation being updated until T completes. Thus transaction T's writes to join view JV are not read by any other transaction T′ until transaction T completes, since T′ requires a table-level S lock (or X lock if T′ tries to both read and update JV) on each base relation of JV. This proves the first part of Assertion 1.
In addition, if transaction T writes join view JV, there are two possible cases:

    
    
        Case 1: Transaction T both reads and updates join view JV. Transaction T places a table-level X lock on the base relation being updated and an S lock on each other base relation of JV until T completes. These X and S locks will block other transactions from writing JV (by blocking other IX or X lock requests on the base relations of JV).
        
            Case 2: Transaction T updates the base relation Ri(1≦i≦n). This is similar to the proof in the Y-lock context.
        
        
    
    


Thus transaction T's writes to join view JV are not written by any other transaction T′ until transaction T completes (this proves the second part of Assertion 1).
Based on a similar reasoning to the proof of Assertion 1, transaction T does not overwrite dirty data of other transactions in join view JV (this proves Assertion 2).
Suppose that transaction T reads join view JV by requiring a table-level S lock on each base relation of the join view JV. If some other transaction T′ writes any data in join view JV, T′ will place a table-level IX or X lock on the base relation being updated until T′ completes. Thus transaction T does not read dirty data from transaction T′ in join view JV (this proves Assertion 3). The case that transaction T tries to both read and update the join view JV is similarly proved.
If transaction T reads join view JV, T places a table-level S lock on each base relation of the join view JV until T completes. Thus no other transaction T′ can write any data in JV until transaction T completes, since T′ requires a table-level IX or X lock on the base relation being updated (this proves Assertion 4). The case that transaction T tries to both read and update the join view JV is similarly proved.
Similar to the Y-lock locking mechanism, the no-lock locking mechanism can also be used for aggregate join views.
Since multiple transactions may also be able to update a join view JV concurrently using the no-lock locking mechanism, a logical undo mechanism similar to that discussed for the Y-lock mechanism is used in case a transaction aborts.
V. Name-Lock Locking Mechanism
According to another embodiment, instead of the Y-lock or no-lock locking mechanism discussed above, an alternative locking mechanism is a name-lock locking mechanism. As shown in FIG. 7, each join view is associated with a corresponding name-lock data structure. Thus, join view JV1 is associated with a first name-lock data structure, while join view JV2 is associated with a second name-lock data structure.
According to one embodiment, each name-lock data structure is of the following format: (NAME, COUNT). NAME refers to a base relation name, while COUNT refers to the number of transactions updating join view JV through updating base relation NAME. For join view JV defined on base relations R1, R2, . . . , and Rn, the value of NAME can be one of R1, R2, . . . , and Rn. Furthermore, NAME can have a special value (referred to as “Q”) that is not any base relation name, but indicates that a transaction is attempting to update multiple base relations of JV or that a transaction requires both a read and update of the join view JV. Note that the designation of “Q” is provided as an example only, as other designations can be used in other embodiments.
Use of the name-lock data structure is described in connection with the name-lock locking logic shown in FIGS. 8A and 8B. The value of COUNT is initialized (at 502) to zero and the value of NAME is initialized to any base relation Ri(1≦i≦n) or Q. In other words, the value of NAME is a “don't care” because the value of COUNT is equal to zero. Upon receipt of a transaction (e.g., receiving commands or steps associated with the transaction), the data server module determines (at 504) if the transaction requires a read of (but not a write to) the join view JV. If so, the data server module determines (at 506) if an X lock is already on JV. If an X lock is already on JV, then conflicting locks are processed (at 508) by the data server module. If an X lock is not already on JV, as determine at 506 by checking that the value of COUNT is equal to zero, then the data server module places (at 510) a table-level S lock on the join view JV.
If the transaction does not involve a read of the join view JV, as determined at 504, the data server module determines (at 512) if the transaction involves a write to (but not a read of) the join view JV through the update of a single base relation Ri of JV. If JV is being updated through the update of a single base relation Ri, the data server module checks (at 514) if there is an S or X lock present on JV. The write of base relation Ri in the received transaction requires an X lock on Ri. Thus, if an S lock already exists, then the X lock request is blocked. Similarly, if an X lock for another transaction is already placed on Ri, then the required X lock for the received transaction will be blocked except for the case where the X lock for the other transaction is placed on the same base relation Ri. The data server module checks the name-lock data structure (NAME, COUNT) to determined if an X lock is present. COUNT equals zero means that there is no other X lock granted on JV. COUNT greater than zero indicates that there is another X lock granted on JV. However, the X lock for the received transaction can be granted if the prior X lock is on the same base relation Ri(that is, NAME=Ri). If the X lock is possible, which means that there is no S lock on Ri and that either (i) COUNT=0, or (2) COUNT>0 but NAME=Ri, then the data server module places (at 516) the X lock on JV. The name-lock data structure is then updated (at 518) by the data server module. The update is as follows: (a) if COUNT=0, then NAME is changed to Ri and COUNT is incremented by 1; or (b) if COUNT is greater than zero and NAME is equal to Ri, then COUNT is incremented by 1.
The acts performed at 512, 514, 516, and 518 are for a write to the join view JV through an update of a single base relation Ri However, if the write to JV is a result of updating multiple base relations Ri of JV, as determined (at 520), then the data server module checks (at 522) if no S lock is present on JV and no X lock is present on JV (that is, COUNT=0). In other words, an X lock for the received transaction (which involves a write to JV through the update of multiple base relations of JV) is allowed only if there is no prior S lock or X lock on JV. If this condition is true, then the data server module places (at 524) an X lock on JV.
In addition, the name-lock data structure is updated (at 526) by changing the value of NAME to Q and incrementing COUNT by 1. As a result of this X lock (with NAME=Q), all other transactions that try to read or update JV are blocked. This is contrasted to the situation where a write to JV results from an update of a single base relation Ri of JV, in that the X lock (with NAME=Ri) does not block a subsequent transaction that tries to update JV if the subsequent transaction also involves a write to JV that results from an update of the same base relation Ri.
Another type of transaction, which is not any one of the transactions above, involves both a read and write of the join view JV. If the data server module detects (at 528) this type of transaction, then the acts at 522, 524, and 526 are performed to determine if an X lock, with NAME set to Q, can be placed on the join view JV.
When a transaction finishes execution (aborted or committed), the data server module checks (at 552) if this transaction has placed an X lock on the join view JV or not. If so, then the data server module updates (at 556) the name-lock data structure by decrementing COUNT by 1. Then the data server module releases the X lock (at 558) on the join view JV held by this transaction. Otherwise, the data server module checks (at 554) if this transaction has placed an S lock on the join view JV or not. If so, the data server module releases the S lock (at 560) on the join view held by this transaction.
With the name-lock locking mechanism, there are five possible scenarios in the database system at any time. In a first scenario, there is no transaction trying to read or update the join view JV. In a second scenario, several transactions are reading the join view JV concurrently. These transactions place table-level S locks on the join view JV, blocking other transactions from updating the base relations of JV.
In the third scenario, there are several transactions updating the same base relation Ri(1≦i≦n). These transactions all place table-level X locks on the join view JV with the parameter NAME=Ri Provided the transactions updating Ri do not block each other without join views, they will not block each other in the presence of join views. However, such transactions will block other transactions from either reading the join view JV or updating other base relations Rj(j≠i, 1≦j≦n).
In a fourth scenario, one transaction is updating several base relations of JV simultaneously. This transaction places a table-level X lock on the join view JV, with the parameter NAME set to Q. This will block other transactions from either reading the join view JV or updating base relations Ri(1≦i≦n) of the join view. In a fifth scenario, one transaction is both reading the join view JV and updating one or several base relations of JV simultaneously. This scenario is similar to the fourth scenario.
As with the Y-lock and no-lock locking mechanisms, the name-lock locking mechanism can be extended to allow value locking on join views. Given a join view JV defined on base relations Ri, R2, . . . , Rn, and given Ri.ai is the value locking attribute of base relation Ri that also appears in JV, then X or S value locking on Ri.ai for JV is allowed. For example, consider a transaction that only updates Ri If the update of Ri can be performed by value locking, then transaction T can also place an IX lock on JV and one or several X value locks on Ri.ai for JV (instead of a table-level X lock on JV with NAME=Ri).
To show that the name-lock locking mechanism maintains the isolation property (serializability) of transactions, for a join view JV defined on base relations R1, R2, . . . , and Rn, the same four assertions (Assertions 1–4) as for the Y-lock locking mechanism above are shown.
The four assertions are first proven for the simple case where JV=σ(R1. . . Ri. . . Rn) It is assumed that join view JV allows duplicate tuples. If no duplicate tuples are allowed in JV, assume that each tuple in JV has a dupcnt attribute (or some other named parameter) recording the number of copies of that tuple.
If transaction T writes join view JV, T places a table-level X lock on JV until it completes. Thus transaction T's writes to join view JV are not read by any other transaction T′ until transaction T completes, since T′ requires a table-level S lock on JV. This proves the first part of Assertion 1.
If transaction T writes join view JV, there are three possible cases:

    
    
        Case 1: Transaction T updates several base relations of join view JV simultaneously. Transaction T places a table-level X lock on JV whose NAME=Q until T completes. The “NAME=Q” X lock will block other transactions from writing JV.
        Case 2: Transaction T both reads and updates join view JV. This is similar to case 1.
        Case 3: Transaction T updates one base relation Ri(1≦i≦n). Transaction T places a table-level X lock on JV whose NAME=Ri until T completes. If another transaction T′ tries to write join view JV before transaction T completes, transaction T′ can only update the same base relation Ri. This is because if transaction T′ updates several base relations of join view JV, the requirement of a “NAME=Q” X lock on JV for transaction T′ will be blocked by the existing “NAME=Ri” X lock on JV that is put by transaction T. Similarly, if transaction T′ updates another base relation Rj(1≦j≦n, j≠i) of join view JV, the requirement of a “NAME Rj” X lock on JV for transaction T′ will be blocked by the existing “NAME Ri” X lock on JV that is placed by transaction T.
    
    


Suppose that transactions T and T′ update ΔRi and ΔRi′ of base relation Ri, respectively. There are three possible scenarios:

    
    
        Scenario 1: If transaction T places a table-level X lock on base relation Ri, transaction T′ will get blocked when it tries to get either a table-level X lock or a table-level IX lock on Ri.
        Scenario 2: If transaction T places a table-level IX lock on base relation R1, transaction T′ will get blocked if it tries to get a table-level X lock on Ri.
        Scenario 3: Suppose that transaction T places a table-level IX lock and one or several X value locks on base relation Ri Also, transaction T′ tries to place a table-level IX lock and one or several X value locks on base relation Ri. There are two cases:
        (a) ΔRi∩ΔRi′≠Ø. The requirement of X value locks on Ri for transaction T′ will be blocked by the existing X value locks on Ri that is placed by transaction T.
        (b) ΔRi∩ΔRi′=Ø. Then σ(R1. . . ΔRi. . . Rn)∩σ(R1. . . ΔRi′. . . Rn)=Ø. That is, the intersection of the updates to JV by transactions T and T′ is empty.
    
    


Thus transaction T's writes to join view JV are not written by any other transaction T′ until transaction T completes (proof of second part of Assertion 1).
Based on a similar reasoning to the proof of Assertion 1, transaction T does not overwrite dirty data of other transactions in join view JV (proof of Assertion 2).
Suppose that transaction T reads join view JV by requiring a table-level S lock on JV. If some other transaction T′ writes any data in join view JV, T′ places a table-level X lock on JV until T′ completes. Thus transaction T does not read dirty data from transaction T′ in join view JV, since the required S lock on JV conflicts with the X lock on JV by transaction T′ (proof of Assertion 3).
If transaction T reads join view JV, T places a table-level S lock on JV until T completes. Thus no other transaction T′ can write any data in JV until transaction T completes, since T′ requires a table-level X lock on JV. This proves Assertion 4.
In the general case where JV=π (σ(R1. . . Ri. . . Rn)), the proofs of the four assertions are the same except for the case where ΔRi∩ΔRi′=Ø in Assertion 1. For this general case, the proof is the same as the above for the Y-lock locking mechanism.
The name-lock locking mechanism for join view also applies to aggregate join view. The proof is the same as for the Y-lock locking mechanism, described above.
A first special case for which the name-lock locking mechanism can be modified to enhance parallel execution of transactions is described. Consider a join view JV=AB. Suppose that the join condition of AB is A.c=B.d, where A.c and B.d are the value locking attributes of base relations A and B, respectively. Consider the following two transactions T1 and T2. Transaction T1 inserts tuple TA1 into base relation A and tuple TB1 into base relation B. Transaction T2 inserts tuple TA2 into base relation A and tuple TB2 into base relation B. Assume that TA1.c, TB1.d, TA2.c, and TB2.d are different from each other. Then transactions T1 and T2 can be executed in parallel by changing the name-lock locking mechanism as follows:

    
    
        (1) For transaction T1, an IX lock and an IS lock are placed on both A and B, an X value lock is placed on A for TA1.c, an S value lock is placed on B for TA1.c, an X value lock is placed on B for TB1.d, and an S value lock is placed on A for TB1.d. No table-level X or S lock is placed on either A or B.
        (2) For transaction T2, an IX lock and an IS lock are placed on both A and B, an X value lock is placed on A for TA2.c, an S value lock is placed on B for TA2.c, an X value lock is placed on B for TB2.d, and an S value lock is placed on A for TB2.d. No table-level X or S lock is placed on either A or B.
        (3) For transaction T1, a table-level X lock is placed on JV whose NAME=A (instead of NAME Q).
        (4) For transaction T2, a table-level X lock is placed on JV whose NAME=A (instead of NAME=Q).
    
    


Thus, even though each of transactions T1 and T2 is updating multiple base relations of the join view JV, the requirement of an X lock with NAME=Q can be avoided if the join condition AB is A.c=B.d and A.c and B.d are the value locking attributes of A and B, respectively. Instead of NAME=Q, the X lock placed on JV is performed with NAME=Ri(the name of the base relation being updated).
In this way, when transactions T1 and T2 do not conflict with each other without the join view JV, they also do not conflict with each other in the presence of the join view JV. The general principle for this first special case is as follows: consider a join view JV defined on base relations R1, R2, . . . and Rn. Suppose that the join condition among base relations Rt1, Rt2, . . . , and Rth({t1, t2, . . . , th⊂}{1, 2, . . . , n}) is Rt1.a1=Rt2.a2 (h≧2), where Rt1.ai (1≦i≦h) is the value locking attribute of base relation Rt1. A fixed Rtu, (1≦u≦h) is picked in advance. Consider a transaction T updating base relations Rs1, Rs2, . . . , and Rsv (v≧1) simultaneously. If {s1, s2, . . . , sv}⊂{t1, t2, . . . , th} and the update to any base relation Rs1 (1≦i≦v) can be done by value locking on Rs1, then only an X lock needs to be placed on the join view JV whose NAME=Rtu (instead of NAME=Q if v≧2).
A second special case for which the name-lock locking mechanism can be modified to enhance performance involves the key-foreign key constraint. An example to illustrate the second special case is as follows: consider a join view JV=AB, as shown in FIG. 9. Suppose that the join condition of AB is A.c=B.d, where A.c is a key of A and B.d is a foreign key of B that references A.c. Also suppose that A.c and B.d are the value locking attributes of base relations A and B, respectively. If transaction T inserts a tuple TA into base relation A, transaction T does not need to put any lock on B or JV for join view maintenance, as there can be no matching tuple in B that can be joined with TA.
A foreign key identifies table relationships. For example, table A can be a customer table, with A.c being the customer identifier and the other attributes of A describing various aspects of each customer. There is one row per customer in table A. Table B is a customer order table that contains orders of each customer, with B.d being the customer identifier. Inserting a new row into A means that a new customer (associated with a new customer identifier A.c) is being added to A. Therefore, since A.c is new, there can be no row in the customer orders table (table B) that contains order information for the new customer.
Transaction T only needs to put an IX lock on A and an X value lock on A for TA.c for the newly inserted TA. On the other hand, if transaction T inserts a tuple TB into base relation B, transaction T needs to put an IS lock on A and an S value lock on A for TB.d to do join view maintenance. Transaction T also needs to put an IX lock on B, an X value lock on B for TB.e, and a table-level X lock on JV whose NAME=B.
The general principle is as follows: consider a join view JV defined on base relations R1, R2, . . . , and Rn. Suppose that the join condition between base relations Ri(1≦i≦n) and Rj(j≠i, 1≦j≦n) is Ri.ai=Rj.aj, where Ri.ai is a key of Ri and Rj.aj is a foreign key of Rj that references Ri.ai. Also suppose that Ri.ai is the value locking attribute of base relations Ri, and Rj.bj is the value locking attribute of base relations Rj. Then if transaction T inserts a tuple Ti into base relation Ri, transaction T requires the following locks: an IX lock on Ri and an X value lock on Ri for Ti.ai. If transaction T inserts a tuple Tj into base relation Rj, transaction T requires the following locks: an IS lock on Ri, an S value lock on Ri for Tj.aj, an IX lock on Rj, an X value lock on Rj for Tj.bj, and a table-level X lock on JV whose NAME=Rj.
A logical undo mechanism is similarly used for the name-lock locking mechanism.
VI. Auxiliary Relations
Several locking mechanisms have been described above to improve transaction concurrency in the database system by reducing the occurrences in which locks placed on a join view, and/or base relations on which the join view is based, will cause transactions to block each other from execution. According to one embodiment, auxiliary relations are also optionally used to maintain join views. The use of auxiliary relations makes join view maintenance more efficient, especially in the parallel database system shown in FIG. 2. As depicted in FIG. 10, in one example, auxiliary relations ARA and ARB are derived from base relations A and B. Additionally, a join view JV is generated from relations A and B, as shown. Assume the join view JV contains tuples selected by the join condition A.c=B.x. Also, the join view JV is partitioned on A.e. Auxiliary relation ARA is a copy of relation A that is partitioned on the join attribute A.c. Likewise, auxiliary relation ARB is a copy of relation B that is partitioned on the join attribute B.x. Where relation A (B) is already partitioned on attribute A.c (B.x), no auxiliary relation ARA(ARB) is generated, as it is not needed.
Additionally, in one embodiment, a clustered index is maintained on each auxiliary relation. Index IA is maintained on attribute A.c for auxiliary relation ARA. Likewise, index IB is maintained on attribute B.x for auxiliary relation ARB. In FIG. 10, the index is depicted as a triangle adjacent to the auxiliary relation upon which the index is maintained.
The auxiliary relations ARA and ARB are constructed based on a reorganization of the tuples TA and TB of base relations A and B, respectively, in which the join attributes (A.c and B.x) influence the construction. Auxiliary relations ARA and ARB thus include all the tuples of relations A and B, respectively, with the tuples simply being rearranged.
In one embodiment, the data structures of FIG. 10 are maintained in the parallel database management system 10 of FIG. 2, which has L data server modules (L being greater than one). In such a parallel database system, tuples of relations A and B are distributed across multiple data server modules.
Auxiliary relations ARA and ARB, as well as the join view JV, are also distributed across the data server modules of the parallel database system. The partitioning strategy for each of auxiliary relations ARA and ARB determines the node upon which the tuples TARA and TARB of the auxiliary relations are stored.
In an example join view definition, tuple TA of relation A is to be joined with one or more tuples TB of relation B in which attribute c of TA equals attribute x of one or more tuples TB. In one embodiment, the join operation is performed using, not relations A and B, but the auxiliary relations ARA and ARB. Then, the join result tuples are stored in the join view JV.
As noted above, a materialized join view must be maintained as base relations of the join view are modified. This means that, as each existing tuple is updated or deleted or as each new tuple is inserted into either relation A or B, the materialized view is updated so as not to become stale. The efficiency of maintenance of the materialized view depends on how the data is organized. Auxiliary relations are used to perform materialized join view maintenance, in accordance with some embodiments, to enhance efficiency. FIGS. 11A, 11B, 12A, and 12B show how materialized view maintenance may be performed without the use of auxiliary relations.
Take, for example, a join view JV, constructed from relations A and B. If base relations A and B are partitioned on the join attributes A.c and B.x, respectively, performing the join operation is relatively efficient, since tuples TA whose “c” attributes are equal to the “x” attribute of tuples TB are stored on the same node. In the ensuing discussion, the term “node” is used broadly to refer to a data server module 12.
In a database system with multiple nodes, assume node i includes tuples TA in which attribute “c” is between 1 and 5. Also, node i includes tuples TB in which attribute “x” is between 1 and 5. Other tuples TA and TB of relations A and B are stored at nodes other than node i in the parallel DBMS. The join of (A.c=B.x) of tuples TA and TB is relatively efficient in this arrangement, since the tuples TA and TB are stored on the same node i.
Where the join view JV is also partitioned according to either attribute “c” of relation A or attribute “x” of relation B, the join result tuples also remain at node i. However, where the join view JV is not partitioned on these attributes, however, the join result tuples may need to be sent to the other node(s).
More generally, assume the join view JV is partitioned on an attribute of relation A. Incoming tuple TA is joined with the appropriate tuple or tuples of relation B at node i. The join result tuples (if any) are sent to some node k based on the attribute value of TA, as shown in FIG. 11A. The join result tuples are inserted into the join view JV there. Node k may be the same as node i.
If, instead, the join view JV is not partitioned on an attribute of relation A, the distribution of join result tuples is depicted in FIG. 11B. Since the join view JV is not partitioned on an attribute of base relation A, the join result tuples are distributed to multiple nodes to be inserted into the join view JV there.
The situation is worse when the base relations A and B are not partitioned on the join attributes. Instead of going to a node (such as node i) in which “like” tuples will be present, multiple nodes of the parallel database system may have to be accessed to find tuples TA or TB that meet the join criteria.
In FIGS. 12A and 12B, maintenance of the join view JV is illustrated where the base relations A and B are partitioned on attributes that are not the join attributes, i.e., not A.c and B.x, respectively. In such a situation, not only are the join result tuples possibly distributed to multiple nodes, but the incoming tuple TA itself is distributed to all the nodes of the parallel database system, to ensure that all possible tuples TB that meet the join criteria are considered. In FIG. 12A, for example, the join view JV is partitioned on an attribute of A. Tuple TA is distributed to every node to search for tuples TB which meet the join criteria (A.c=B.x). The join result tuples (if any) are sent to some node k to be inserted into the join view JV based on the attribute value of TA. Again, node k may be the same as node i.
In FIG. 12B, the join view JV is not partitioned on an attribute of relation A. As in FIG. 12A, the tuple TA is redistributed to every node to search for the tuples TB that meet the join condition. The join result tuples, if any, are distributed to multiple nodes to be inserted into the join view JV there. The dashed lines in FIG. 12B indicate that the network communication is conceptual as the message is sent to the same node.
Thus, as can be seen from the example of FIGS. 12A, and 12B, without the use of auxiliary relations, materialized view maintenance may be inefficient and costly (in terms of system resources) where the base relations A and B are partitioned on attributes that are not join attributes, since substantial network communication costs may be incurred. Further, a join operation has to be performed at every node.
According to one embodiment, auxiliary relations are used to overcome the shortcomings of the join view maintenance techniques described above. In general, an auxiliary relation is maintained for each relation involved in the join operation. For the general case, it is assumed that neither base relation is partitioned on the join attribute. If, however, some base relation is partitioned on the join attribute, the auxiliary relation for that base relation is unnecessary, in one embodiment.
Where a join view JV is defined on base relations A and B, two auxiliary relations, ARA for relation A, and ARB for relation B, are maintained. Relation ARA is a copy of relation A that is partitioned on the join attribute A.c. Relation ARB is a copy of relation B that is partitioned on the join attribute B.x. Additionally, as depicted in FIG. 10, index IA on attribute c of relation A is maintained for auxiliary relation ARA. Likewise, index IB on attribute x of relation B is maintained for auxiliary relation ARB.
By maintaining auxiliary relations ARA and ARB for relations A and B, respectively, assurance can be made that, at any node i of the parallel database system, tuples TA coexist with tuples TB in which the join attributes are of the same value. In other words, where the tuples TA and TB of relations A and B are not organized such that tuples meeting the condition A.c=B.x coexist at the same node, such a condition is provided using tuples TA and TB of auxiliary relations ARA and ARB.
One procedure for maintaining a join view using auxiliary relations is depicted in FIGS. 13A and 13B, according to one embodiment. When a tuple TA is inserted into relation A at node i, the tuple is also redistributed to a specific node j, based on the join attribute value A.c of the tuple. Node j may be the same as node i. Tuple TA is inserted into the auxiliary relation ARA at node j. At node j, TA is joined with the appropriate tuples TB in the auxiliary relation ARB, where the auxiliary relation ARB utilizes the index IB to quickly obtain the appropriate tuples TB.
In FIG. 13A, the join view JV is partitioned on an attribute of A. Thus, it is possible that the join result tuple is stored on the same node as TA. The join result tuples (if any) are sent to some node k to be inserted into the join view JV based on the attribute value of TA. Node k may be the same as node j.
In FIG. 13B, the join view JV is not partitioned on an attribute of A. Accordingly, the join result tuples (if any) are distributed to multiple nodes to be inserted into the join view JV there. For example, each join result tuple may be sent to a different node in the parallel database system.
Operations in which tuple TA is deleted from base relation A or updated in base relation A are similarly performed. Also, when a tuple TB is inserted into, deleted from, or updated in base relation B, similar operations are performed.
By using one or more auxiliary relations, the join view may be maintained more efficiently. In one embodiment, network communication is reduced. For each inserted (updated, deleted) tuple of base relation A, the join work to be done occurs at one node rather than at every node of the parallel database system. Further, for each inserted (updated, deleted) tuple of base relation A, the auxiliary relation ARB at one node (rather than at all nodes) is locked when performing the join operation. In one embodiment, this improves accessibility to the base relation B while join view maintenance is being performed.
In one embodiment, the storage overhead for each auxiliary relation may be kept relatively small in many cases. For example, if a join view has some selection condition on the base relation A in the where clause, such as:

    
    
        CREATE JOIN VIEW JV AS
        SELECT *
        FROM A, B
        WHERE A.c=B.x AND A.e=3;

only those tuples of A that satisfy the selection condition (A.e=3) need be maintained in the auxiliary relation ARA.

    
    


As another example, a join view does not contain all attributes of the base relation A, such as:

    
    
        CREATE JOIN VIEW JV AS
        SELECT A.e, B.z
        FROM A, B
        WHERE A.c=B.x;
    
    


The auxiliary relation ARA may maintain fewer than all the attributes of relation A. In the above example, the auxiliary relation ARA may maintain only the join attribute (A.c) and the attributes appearing in the select clause (A.e). Accordingly, ARA would include attributes c and e of base relation A.
Another example involves the join condition in the join view that is based on key and referential integrity restraints, such as:

    
    
        CREATE JOIN VIEW JV AS
        SELECT *
        FROM A, B
        WHERE A.c=B.x;
    
    


Where A.c is a key of relation A and B.x is a foreign key of relation B that references to A.c. If a tuple TA is inserted into relation A, there is no matching tuple in relation B that can be joined with TA. However, if a tuple TB is inserted into relation B, there must be a tuple of relation A that can be joined with it. The case for deletion is similar. Thus, in one embodiment, if only insertion and deletion in the base relations is considered, only the auxiliary relation ARA is maintained. There is no need for auxiliary relation ARB.
A further discussion of maintaining a view with auxiliary relations is described in U.S. Ser. No. 09/900,280, filed on Jul. 6, 2001, by Gang Luo, Curt J. Ellmann, and Jeffrey F. Naughton, which is hereby incorporated by reference.
VII. Auxiliary Indices
Instead of auxiliary relations, which require relatively substantial storage overhead in some cases, auxiliary indices can be used instead. For each base relation A and B on which a join view JV is maintained, an auxiliary index AIA(for relation A) and an auxiliary index AIB(for relation B) are also maintained, as shown in FIG. 14.
Auxiliary index AIA is an index on the join attribute A.c. AIA is partitioned on A.c. Each entry of the auxiliary index is in the form (value of A.c, list of global row identifiers), where the list of global row identifiers contains all the global row identifiers of the tuples of relation A whose attribute A.c is of that value. Each global row identifier is of the form (node identifier, local row identifier at the node). A local row identifier uniquely identifies a row of a relation at a node. A global row identifier uniquely identifies a row of a relation among all the nodes.
The auxiliary index AIA is distributed clustered if the base relation is clustered on the join attribute A.c at each node. On the other hand, the auxiliary index AIA is non-distributed clustered if the base relation is non-clustered on the join attribute at each node. An auxiliary index AB is similarly maintained for relation B, and is of the form (value of B.x, list of global row identifiers).
As shown in FIG. 15A, when a tuple TA is inserted into relation A at node i, the tuple TA is also redistributed to a specific node j (node j may be the same as node i) based on the join attribute value of TA. A new entry containing the global row identifier of tuple TA is inserted into the auxiliary index AIA at node j. The auxiliary index AIB at node j is then searched to find the list of global row identifiers for these tuples TB of relation B that satisfy TB.x=TA.c. Suppose those tuples TB reside at K (K≦L) nodes. TA with the global row identifiers of the node is sent to each of the K nodes. In the example of FIG. 15A, the K nodes include nodes 1, 3, and L. The tuple TA is joined with tuple TB of relation B identified by those global row identifiers in the K nodes. If JV is partitioned on an attribute of A, then the join result tuples (if any) are sent to some node k (which may be the same as node j) to be inserted into JV based on the attribute value of TA.
However, if JV is not partitioned on an attribute of A, then the join result tuples are distributed to plural nodes to be inserted into JV, as shown in FIG. 15B. The procedure for handling deletion of rows of A or updates of rows of A are similar to the procedure performed in the case of insertion.
Generally, auxiliary indices have less storage overhead than auxiliary relations. However, join view maintenance with auxiliary indices incur more inter-node communication than for join view maintenance with auxiliary relations, but less inter-node communication than for join view maintenance without either auxiliary indices or auxiliary relations (see FIG. 12B). For each inserted (deleted, updated) tuple of base relation A, the join work needs to be done at (i) only one node with the auxiliary relation mechanism, (ii) several nodes for the auxiliary index mechanism, and (iii) all the nodes without either the auxiliary index or auxiliary relation mechanism.
The auxiliary index mechanism can also be extended to maintain a join view JV defined on more than two base relations. For optimal performance, a hybrid mechanism that includes both the auxiliary relation mechanism and the auxiliary index mechanism can be used.
VII. Locking Mechanisms for Auxiliary Relations and Auxiliary Indexes
When base relations are updated and materialized views are incrementally updated, the proper locks are placed on the base relations; there is no need to place any X or S lock on any auxiliary relation according to one embodiment. However, short-term latches on pages and locks on access paths placed by the database system are needed. In an alternative embodiment, locks can be placed on the auxiliary relation when reading or updating the auxiliary relation.
For example, in one embodiment, an X or S lock on an auxiliary relation ARR for base relation R is not needed because the auxiliary relation ARR is only updated when base relation R is updated. Therefore, the X or IX lock placed on R will prevent a subsequent conflicting update on R that causes ARR to be updated. Similarly, an auxiliary relation ARR is only read when some join view defined on base relation R is being maintained (with an S or IS lock placed on R). Thus, while a first transaction is ready for join view maintenance, the S lock or IS lock on R blocks any subsequent conflicting update of ARR through the update of R (which requires an X or IX lock on R).
Similarly, in the auxiliary index context, according to one embodiment, only proper locks are needed on base relations when base relations are updated and materialized views are being incrementally maintained. Locks on the auxiliary index are not needed. In an alternative embodiment, locks can be placed on the auxiliary index when reading or updating the auxiliary index.
Generally, for an auxiliary relation ARR of base relation R, there are three possible scenarios in the parallel database system:

    
    
        (1) No transaction is trying to read or updated auxiliary relation ARR.
        (2) One or several transactions are updating base relation R, and thus the auxiliary relation ARR is also being updated. Table-level IX or X locks are placed on R so no other transaction can read ARR (because such other read requires an S lock on R).
        (3) One or several transaction are reading auxiliary relation ARR to perform join view maintenance. Table-level S locks are placed on R so no other transaction can update auxiliary relation ARR.
    
    


Similar scenarios exist for auxiliary index AIR kept on base relation R.
A logical undo mechanism is also applied to the lock mechanism for auxiliary relations and auxiliary indices.
VIII. Example Application of No-Lock Locking Mechanism for Join View with Join View Maintenance Performed with Auxiliary Indices
FIG. 16 shows a traditional (S, X, IS, IX) locking mechanism in a parallel database system with L data server modules. Each data server module is also referred to as node i (1≦i≦L). Also, for each relation, there is a centralized node on which the lock manager resides. As a result, to place a value lock, a data server module has to send a message to the lock manager at the centralized node to place a corresponding IX or IS lock for an entire relation R that is distributed or partitioned across the multiple data server modules. Thus, with the centralized (S, X, IS, IX) locking mechanism of FIG. 16, placing a value lock on the relation R is relatively expensive, as the data server module has to send a message across a communications network to the lock manager at the centralized node. Thus, value locking cannot be performed locally with the centralized locking mechanism. On the other hand, placing the table-level X or S lock is relatively efficient with the centralized locking mechanism, as a message can be sent to the centralized node to place a lock on the entire relation R.
FIG. 17 shows a distributed (S, X, IS, IX) locking mechanism, in which value locks for R are partitioned among all L nodes. Thus, at each node i (1≦i≦L), there is a symbol Ri for R. If a table-level X or S lock is to be placed on R, then a message is sent to every node i (1≦i≦L) to X or S lock the symbol Ri. To place a value X or S lock on R, there is no need to go to a centralized node to get the table-level IX or IS lock on R. Instead, assuming that the value lock resides at node i, an IX or IS lock is first placed on the symbol Ri, followed by acquiring the value X or S lock at node i. Thus, for placing value locks, there is no centralized node that can become a bottleneck in the distributed locking mechanism. However, compared to the centralized locking mechanism, acquiring a table-level X or S lock is more expensive in the distributed locking mechanism, as an X or S lock has to be placed on the symbol Ri at each node i (1≦i≦L).
Join view maintenance can be implemented using the distributed locking mechanism in a parallel database system. Consider a join view JV=AB based on the join condition A.c=B.d. Suppose neither A nor B is partitioned on the join attribute, and suppose A.e (B.f) is the value locking attribute of A (B). In this case, two auxiliary indices AIA and AIB are created, with AB shown in FIG. 18. Auxiliary index AIA, which is partitioned on A.c, is an index on the join attribute A.c. Each entry of the auxiliary index AIA is of the form (value of A.c, list of two-tuples (global row identifier, value of A.e)). Note that this implementation of each auxiliary index differs slightly from that described above. The list of two-tuples contains all the global row identifiers (with associated A.e values) of the tuples of relation A whose attribute A.c is of that value. The same technique is applied to base relation B. Table-level S, X, IS, and IX locks are allowed on A, B, AIA, and AIB. S or X value locks are allowed on A for A.e, on B for B.f, on AIA for A.c, and on AIB for B.d. Note that, in this embodiment, locks are placed on auxiliary indices.
When a tuple TA is inserted into base relation A, the following operations are performed to maintain the join view JV using the no-lock locking mechanisms for JV. First, an IX lock is placed on A and an X value lock is placed on a TA.e value for A. The tuple TA is then inserted into the base relation A. Next, an IX lock is placed on AIA and an X value lock is placed on TA.c for AIA. The data structure (TA.c, (global row identifier of TA, TA.e)) is then inserted into the auxiliary index AIA.
Next, an IS lock is placed on the auxiliary index AIB and an S value lock is placed on TA.c for the auxiliary index AIB. The data server module next finds the entry in the auxiliary index AIB according to TA.c. Next, the data server module finds all the two-tuples (global row identifier of TB, value of TB.f) for those tuples TB of base relation B that match with TA. For each two-tuple (global row identifier of TB, value of TB.f), an IS lock is placed on the base relation B and an S value lock is placed on TB.f for the base relation B.
The join of tuples TA and TB is then performed, with the join result tuple inserted into the join view JV.
A benefit of the join view maintenance mechanism described above is that no table-level S or X locks are placed on A, B, or JV for join view maintenance. Only IX or IS locks and X or S value locks on A, B, AIA, and AIB are needed while they are distributed among all the nodes. Thus, no centralized node will become a bottleneck and many transactions can be executed concurrently in a parallel database system where each transaction updates one or several base relations of a join view.
IX. System Environment
Instructions of the various software routines or modules discussed herein (such as the data server modules, lock managers, and so forth) are stored on one or more storage devices in the corresponding systems and loaded for execution on corresponding control units or processors. The control units or processors include microprocessors, microcontrollers, processor modules or subsystems (including one or more microprocessors or microcontrollers), or other control or computing devices. As used here, a “controller” refers to hardware, software, or a combination thereof. A “controller” can refer to a single component or to plural components (whether software or hardware).
Data and instructions (of the various software routines or modules) are stored in respective storage devices, which are implemented as one or more machine-readable storage media. The storage media include different forms of memory including semiconductor memory devices such as dynamic or static random access memories (DRAMs or SRAMs), erasable and programmable read-only memories (EPROMs), electrically erasable and programmable read-only memories (EEPROMs) and flash memories; magnetic disks such as fixed, floppy and removable disks; other magnetic media including tape; and optical media such as compact disks (CDs) or digital video disks (DVDs).
The instructions of the software routines or modules are loaded or transported to each system in one of many different ways. For example, code segments including instructions stored on floppy disks, CD or DVD media, a hard disk, or transported through a network interface card, modem, or other interface device are loaded into the system and executed as corresponding software modules or layers. In the loading or transport process, data signals that are embodied in carrier waves (transmitted over telephone lines, network lines, wireless links, cables, and the like) communicate the code segments, including instructions, to the system. Such carrier waves are in the form of electrical, optical, acoustical, electromagnetic, or other types of signals.
While the invention has been disclosed with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover such modifications and variations as fall within the true spirit and scope of the invention.

## Claims

1. A method of maintaining consistency in a database system, comprising:
storing a join view that is based on base relations; and
in response to a request to update a first one of the base relations, placing an exclusive lock on the first base relation and a first predefined lock on the view, the first predefined lock conflicting with shared lock placed on the view and with an exclusive lock placed on the view but not conflicting with another predefined lock placed on the view, wherein placing the first predefined lock comprises placing a Y lock on the view.

2. The method of claim 1, further comprising blocking a subsequent, second transaction requiring either a shared lock or an exclusive lock on the view.

3. The method of claim 2, further comprising allowing a subsequent, third transaction requiring another predefined lock placed on the view.

4. The method of claim 1, further comprising updating the first base relation with a value lock on a first value of a locking attribute.

5. The method of claim 4, further comprising:
performing a second update on the first base relation with a value lock on a second value of the locking attribute; and
placing a second predefined lock on the view for the second update.

6. The method of claim 1, wherein placing the first predefined lock comprises placing an IY lock on the view.

7. The method of claim 6, wherein placing the first predefined lock further comprises placing an exclusive value lock on the view on a locking attribute of the first relation.

8. The method of claim 1, further comprising:
in response to a request to read the view, placing a shared lock on the view.

9. The method of claim 8, further comprising:
in response to a transaction that both writes to and reads from the view, placing an exclusive lock on the view.

10. The method of claim 1, wherein placing the first predefined lock on the view comprises placing a table-level lock on the view.

11. The method of claim 1, wherein placing the first predefined lock on the view comprises placing an exclusive value lock on the view.

12. An article comprising at least one storage medium containing instructions that when executed cause a database system to:
store a join view containing data that is based on a join of the plural base relations;
in response to a first transaction that both reads from and writes to the view, place an exclusive lock on the view;
in response to a second transaction that reads, but does not write to, the view, place a shared lock on the view; and
in response to a third transaction that writes to, but does not read from, the view, place a modified-exclusive lock on the view,
the modified-exclusive lock conflicting with a shared on the view and with an exclusive lock on the view but not conflicting with another modified-exclusive lock on the view.

13. The article of claim 12, wherein the instructions when executed cause the database system to store the view by storing an aggregate join view containing data that is based on the join of the plural base relations and further containing data that is an aggregate of at least one attribute of at least one of the base relations.

14. The article of claim 12, wherein the instructions when executed cause the database system to:
store auxiliary relations corresponding to the base relations, each auxiliary relation containing at least a portion of a corresponding base relation but partitioned differently from the base relation; and
in response to updating the first one of the base relations, place a lock on the first base relation without placing a lock on a corresponding auxiliary relation.

15. The article of claim 12, wherein the instructions when executed cause the database system to:
store auxiliary indices corresponding to the base relations, each auxiliary index containing a join attribute of a corresponding base relation, the auxiliary index further identifying tuples in the corresponding base relation containing a given join attribute value; and
in response to updating the first one of the base relations, place a lock on the first base relation without placing a lock on a corresponding auxiliary index.

16. The article of claim 12, wherein the instructions when executed cause the database system to, in response to a fourth transaction that writes to, but does not read from, the view, place another modified-exclusive lock on the view while the modified-exclusive lock for the third transaction is still active.

17. The article of claim 12, wherein the instructions when executed cause the database system to check if either a shared lock or exclusive lock has been placed on the view to determine if the modified-exclusive lock can be granted for the third transaction.

18. The article of claim 12, wherein the instructions when executed cause the database system to check if either a modified-exclusive lock or exclusive lock has been placed on the view to determine if the shared lock can be granted for the first transaction.

19. The article of claim 12, wherein the instructions when executed cause the database system to check if any of a modified-exclusive lock, exclusive lock, or shared lock has been placed on the view to determine if the exclusive lock can be granted for the second transaction.

20. The article of claim 12, wherein the instructions when executed cause the database system to perform each of the first, second, and third transactions on plural data server modules in the database system,
the data server modules to manage access to data stored in corresponding storage modules, each of the base relations and view being partitioned across the plural storage modules.

21. The article of claim 20, wherein the instructions when executed cause the database system to perform the third transaction concurrently with a fourth transaction that writes to, but does not read from, the view, the fourth transaction also to place a modified-exclusive lock on the view.

22. The article of claim 21, wherein the instructions when executed cause the database system to concurrently perform a portion of the third transaction on one data server module and a portion of the fourth transaction on another data server module.

23. A database system comprising:
a storage to store plural base relations and a join view based on the plural base relations; and a database management controller adapted to place a modified-exclusive lock on the view in response to a first transaction that writes to the view,
the modified-exclusive lock conflicting with an exclusive lock on the view and with a shared lock on the view but not conflicting with another modified-exclusive lock on the view.

24. The database system of claim 23, wherein the database management controller is adapted to place another modified-exclusive lock on the view in response to a second transaction that writes to the view,
the modified-exclusive locks for the first and second transactions being active concurrently.

25. The database system of claim 23, wherein the database management controller is adapted to check if either a shared lock or an exclusive lock has been placed on the view to determine if the modified-exclusive lock for the first transaction can be granted.

26. The database system of claim 25, wherein the database management controller is adapted to block the modified-exclusive lock for the first transaction if either a shared lock or an exclusive lock has been placed on the view.

27. The database system of claim 23, wherein the join view comprises an aggregate join view.

28. The database system of claim 23, wherein the database management controller blocks another transaction requesting a read or write of data in the join view that is being written by the first transaction.

29. The database system of claim 23, wherein the database management controller is adapted to prevent the first transaction from overwriting modified data of another transaction.

30. The database system of claim 23, wherein the modified-exclusive lock comprises a value lock.

31. The database system of claim 23, wherein the storage comprises plural storage modules across which each of the base relations and view is partitioned, and wherein the database management controller comprises plural data server modules to manage access of respective storage modules,
the data server module to concurrently perform the first transaction and another transaction that writes to the view.

32. The method of claim 1, wherein the predefined lock is different from the shared lock and exclusive lock.

33. The article of claim 12, wherein the modified-exclusive lock is different from the shared lock and exclusive lock.

34. The database system of claim 23, wherein the modified-exclusive lock is different from the shared lock and exclusive lock.