Abstract:
A method for executing after-triggers in an active database. A tree is constructed for each after-trigger and an operator tree is constructed for the statement that activates the trigger. The method joins each of the trees for the activated row-after triggers to the operator tree for pipelined execution with the operator tree. The trees for the activated row-after triggers form a group and each of the trees within the group execute in parallel with each other. The method joins trees for activated statement-after triggers to the operator tree for execution subsequent to the execution of the operator tree, the statement after trigger trees receiving rows from a temporary table that accumulates affected rows from the operator tree. Trees for activated statement after triggers form a group and each of the trees within the group execute in parallel with each other.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a continuation of copending application Ser. No. 09/823,337, filed Mar. 29, 2001, which is hereby incorporated by reference herein. This application is related to U.S. application entitled “A METHOD OF EXECUTING CONFLICTING TRIGGERS IN AN ACTIVE DATABASE”, Ser. No. 09/823,340, filed on Mar. 29, 2001; and to U.S. application entitled “A METHOD OF EXECUTING BEFORE-TRIGGERS IN AN ACTIVE DATABASE, Ser. No. 09/822,996, filed on Mar. 29, 2001. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates generally to executing triggers in active relational databases and more specifically to the concurrent execution of after-triggers in a relational data base management system.  
         DESCRIPTION OF THE RELATED ART  
         [0003]    Database management systems (DBMS)  11 , such as the system shown in FIG. 1, have become the dominant means of keeping track of data, especially for servers connected to the Internet. These systems take an organized approach to the storage of data by imposing a data model, typically a relational data model, on the data  17  that is stored in the database  15 . Included in the typical DBMS are a Query Processing Engine  13 , a File Access and Storage Management subsystem  21  for accessing the database  15 , a Concurrency Control subsystem  19  for managing locks needed for concurrency on database items (tables and rows) and a Recovery Control Subsystem  23  for restoring the DBMS  23  to a consistent state after a fatal error. The latter two subsystems  19 ,  23 , are interconnected with the File Access and Storage Management subsystem  21 .  
           [0004]    In the relational data model, data is stored as a relation, which has two aspects, the relation schema and the relation instance. The relation schema specifies the relation&#39;s name, and the name and domain of each column in the relation. The relation instance is a set of records (also called rows or tuples) that conform to the relation schema. A relation instance is therefore a table of records, each of which has a column that meets the domain constraints imposed by the schema.  
           [0005]    Not only does the DBMS impose a constraint on storage of data, a DBMS usually formalizes the means by which information may be requested from the database. In particular, a query language is specified by which questions may be put to the database. The language is usually based on a formal logic structure such as relational algebra or calculus. Queries are usually carried out in the DBMS  11  by a Query Processing Engine  13 , which has a number of components for parsing a query, creating a query plan, and evaluating the query plan. In particular, a component of the Query Processing Engine  13 , a Query Optimizer, creates one or more query plans, each in the form of a tree of relational operators, that are evaluated for execution of the query based on some efficiency metric.  
           [0006]    Relational operators take one or more tables as inputs and generate a new table as the output. For example, a selection operator selects one or more rows of an input table meeting the selection criteria to produce an output table having only those rows. Operators can be composed since an operator may take as input a table generated as the output of another operator. A tree of operators is the representation of a composition of the relational operators appearing as the nodes of the tree.  
           [0007]    A tree of such operators for a particular query plan is shown in FIG. 3. As can be observed from the tree of FIG. 3, relational operators are connected to each other and to base tables T 1  and T 2  by means of queues Q 1 -Q 4 . These queues supply input rows to a particular operator and store output rows from the operator. The queues allow an operator to start processing rows as soon as the operator that supplies the rows begins to produce them and before all rows are produced. Such pipelining improves the efficiency of the system because intermediate results need not be stored in a temporary table and then read again for input.  
           [0008]    The standard language for implementing a DBMS is the Structured Query Language (SQL). This language includes Triggers, which are actions executed by the DMBS under certain conditions.  
           [0009]    A database having a set of triggers is called an active database and each trigger in the database has three parts, an event, a condition and an action. The event part is a change to the database, such as an insertion, deletion, or modification of a table, that activates the trigger. The SQL statement which is the activating event, is termed the activating statement. A condition is a test by the activated trigger to determine whether the trigger action should occur and an action is an SQL statement that is executed if the trigger event and trigger condition are both satisfied. The set of rows affected (i.e., inserted, updated, or deleted) by the activating statement is termed the affected set of rows for the relevant trigger.  
           [0010]    The action part of the trigger can occur either before or after the activating statement. If before, it is called a before-trigger and if after, it is called an after-trigger. In addition, triggers can operate at the row level or the statement level. A statement trigger executes its action once per activating statement and a row trigger executes its action for each row in the affected set. The combination of “before” and “after” with “row” and “statement” creates four different types of triggers. Chain reactions of trigger actions and recursive trigger actions are also possible.  
           [0011]    The execution of triggers in a relational database is governed by the proposed ANSI standard for SQL (SQL:1999) which places certain restrictions on trigger execution. A chief restriction is that the triggers be executed serially in their creation time order or at least that the serial execution of triggers be equivalent in outcome and effect on the database to the execution of triggers in their creation time order. However, the serial execution of triggers, in accordance with the proposed ANSI:99 standard, would seriously affect the performance of the DMBS, especially if many trigger actions are involved. Thus, there is a need for the improved execution of multiple trigger actions which leads to improved performance of trigger actions over a purely sequential execution, but still conforms to the ANSI standard.  
         BRIEF SUMMARY OF THE INVENTION  
         [0012]    The present invention is directed towards the above need. A method of forming an execution plan in accordance with the present invention includes the following steps. First, any triggers that may be activated by an activating statement and any rows in database tables that are affected by the activating statement are determined. An operator tree for the activating statement is then formed and a tree for the trigger that is activated by the activating statement is formed. The activated trigger is either a row-after trigger or a statement-after trigger. If the activated trigger is a row-after trigger, the tree for the row-after trigger is joined to the operator tree for pipelined execution with the operator tree and any rows affected by the activating statement are pipelined to the row-after trigger for input. If the activated trigger is a statement-after trigger, the tree for the statement-after trigger is joined to the operator tree for execution subsequent to the operator tree. The statement-after trigger obtains input during execution from a temporary table that accumulates affected rows from the execution of the activating statement.  
           [0013]    If a plurality of row-after triggers is activated by the activating statement, each of the trees for the row-after triggers is joined to the operator tree for pipelined execution with the operator tree. In one embodiment, the plurality of trees for activated row-after triggers is connected to a parallel union operator to form a group and a flow operator is interconnected between the parallel union operator and the operator tree.  
           [0014]    If a plurality of statement after triggers is activated by the activating statement, each of the statement-after trigger trees is joined to the operator tree for execution subsequent to the execution of the operator tree. In one embodiment, the activated statement-after actions are connected to a parallel union operator to form a group, a flow operator is interconnected between the operator tree and a temporary table that accumulates affected rows from the operator tree and an ordered union operator is interconnected between the parallel union operator and the flow operator.  
           [0015]    Joining both a plurality of activated row-after triggers and a plurality of statement-after triggers to the operator tree is such that the activated row-after triggers execute in a pipelined fashion with the operator tree and the activated statement-after triggers execute subsequently to the execution of the operator tree. Each trigger tree within either the statement-after group or the row-after group executes in parallel with the other trigger trees in the group.  
           [0016]    An advantage is that row after-triggers are executed substantially in parallel with each other and in a pipeline with the execution of the operator tree for the activating statement thereby substantially reducing the execution time of row-after triggers compared to purely sequential execution of the activating statement and the triggers.  
           [0017]    Another advantage is that statement-after triggers are executed substantially in parallel with each other thereby substantially reducing the execution time of statement-after triggers compared to the purely sequential execution of the activating statement and the triggers.  
           [0018]    Another advantage of the invention is that triggers execute in parallel with the activating statement and groups of triggers that are activated by the same activating statement execute in parallel. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    These and other features, aspects and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where:  
         [0020]    [0020]FIG. 1 illustrates a typical database management system;  
         [0021]    [0021]FIG. 2A illustrates a Flow operator;  
         [0022]    [0022]FIG. 2B illustrates an Ordered Union Operator;  
         [0023]    [0023]FIG. 2C illustrates a Parallel Union Operator;  
         [0024]    [0024]FIG. 3 shows an operator tree for a statement;  
         [0025]    [0025]FIG. 4 shows a trigger tree and a representative statement for a trigger;  
         [0026]    [0026]FIG. 5 shows an overview of an aspect of the present invention;  
         [0027]    [0027]FIG. 6A illustrates a more detailed execution plan in accordance with the present invention;  
         [0028]    [0028]FIG. 6B illustrates a timing chart for the plan of FIG. 6A; and  
         [0029]    [0029]FIG. 7 shows a flow chart for creating an execution plan in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0030]    The present invention relies on a number of operators to control the execution of operations in both an activating statement and its associated trigger trees. The first of these operators is illustrated in FIG. 2A which shows a Flow Operator. The function of this operator is to move the output of operator op 1   12  to the input of operator op 2   14 , as the output of operator op 1  is produced. For example, if op 1  is a selection operator on a table which selects rows of the table meeting a certain condition, then as the rows meeting the condition are found, say by scanning the table, the rows are sent to the input of op 2 . This permits the op 2  operator to function in parallel to the op 1  operator, though, of course, not on the same row that op 1  is operating on. FIG. 2A illustrates this “pipelining” operation in a timing chart which shows the activity of op 1  overlapped with the activity of op 2 .  
         [0031]    [0031]FIGS. 2B and 2C illustrate the Union Operators. The Ordered Union operator  16  of FIG. 2B forces op 2  to operate only after op 1  has completed its operations, in effect serializing the op 1 , op 2  operations as shown in the timing chart. The Parallel Union operator  18  allows op 2  to operate concurrently with op 1 , and assumes that op 2  has no data access conflict with op 1 . As is evident from FIGS. 2A and 2C, the flow operator  10  and the parallel union operator  18  reduce the time to carry out the functions of the op 1  and op 2  operators compared to the ordered union operator  16 .  
         [0032]    Referring to FIG. 3, an operator tree  20  is shown for the given SQL statement  22 . The SQL statement  22  projects a desired column F 1  from the table created by joining tables T 1 , T 2  and selecting the rows that meet the conjunction of conditions C 1 , C 2  and C 3 . The operator tree  20  shows one way of implementing the SQL statement  22 . According to the tree, first T 1  and T 2  are joined based on condition C 1  by the join operator  24 . Next, a selection operator  26  selects the rows of the joined table that meet the condition which is the conjunction of C 2  and C 3 . Finally, a projection operator  28  selects the column F 1  from any rows that result from the prior operations. As described above, the function of a Query Optimizer is to form alternative execution plans for a query so that the plans can be evaluated in terms of some performance metric. The tree in FIG. 3  is only one such tree that a Query Optimizer can produce for the given SQL statement.  
         [0033]    [0033]FIG. 4 shows an SQL statement  30  for a row after-trigger, rt 1 . The event, condition and action for the trigger are shown in block  32 . The event for rt 1  is a row insertion into a table T 1 ; the condition is C 1 , which can be an arbitrary relational condition and the ACTION part of the trigger can be practically any sequence of SQL statements. The trigger tree  34  represents both the condition and the action parts of the trigger.  
         [0034]    [0034]FIG. 5 shows an overview of the present invention. In FIG. 5, an operator tree  42  for an activating statement S is combined, i.e., “inlined,” with a trigger tree  44  of a trigger T activated by the statement to create an inlined tree  46 . The inlined tree  46  is then processed by an optimizer to create an optimized execution plan  50  for the operators and trigger trees caused by the activating statement S.  
         [0035]    [0035]FIG. 6A illustrates a more detailed execution plan formulated in accordance with the present invention illustrated in FIG. 5. In FIGS. 6A and 6B it is assumed that there are no data access conflicts among the activated triggers and between the activated triggers and the activating statement and that all of the activated triggers are after-triggers.  
         [0036]    Referring to FIG. 6A, statement S is represented by an operator tree  42 , row triggers rt 1  and rt 2  are represented by trees  52 ,  54 , respectively, and statement triggers st 1  and st 2  are represented by trees  56  and  58 , respectively. It is assumed that statement S is the event that causes activation of the row and statement triggers. In accordance with the present invention, the operator tree  42  produces, as output, the set of affected rows. A flow operator  60  connects the operator tree  42  for statement S to a temporary table, TempTable  62 , so that rows that are output by the operator tree  42  are pipelined to the temporary table, TempTable  62 . Parallel union operators  64  and  66  connect the trees  52 ,  54  for rt 1  and rt 2  and the trees  56 ,  58  for st 1  and st 2  so that trees  52  and  54  execute in parallel and trees  56  and  58  execute in parallel.  
         [0037]    Another flow operator  68  connects the parallel union operator  64  for rt 1  and rt 2  to the flow operator  60  connected to the operator tree  42  for statement S so that action trees  52  and  54  execute pipelined to the execution of the statement tree  42 . Finally, an ordered union operator  70  connects the flow operator  68  to the parallel union operator  66  for st 1  and st 2  so that the trees  56  and  58  execute subsequent to the execution of the statement tree  42 . The statement trees  56  and  58  receive their inputs by scanning the temporary table, TempTable  62 , as represented by the scan functions  72  and  74 .  
         [0038]    The effect of structure of FIG. 6A is that the row triggers execute in parallel with each other and pipelined with the activating statement and statement triggers execute in parallel with each other but subsequent to the activating statement. Specifically, the structure operates as follows. The operator tree  42  of S operates to generate a stream of affected rows. As the operator tree for S produces the stream of rows, each row is pipelined by the flow operator  60  to the TempTable  62  to prepare for the operation of the statement trigger st 1  and st 2 , which must execute only after statement S is completed. TempTable  62  accumulates the set of affected rows that were produced by the operator tree  42  for S. These changes may need to be made available to the statement trigger trees st 1  and st 2 . Additionally, each row produced by statement S operator tree  42  is pipelined to the row trigger trees rt 1  and rt 2 , which execute in parallel on the pipelined rows. Upon completion of the execution of statement S, and the row triggers rt 1  and rt 2 , the statement triggers st 1  and st 2  are allowed to execute because of the ordered union operator  70 . The statement trigger trees execute in parallel with each other by scanning the TempTable  62  for input data as needed. After the temporary table is used, the contents of the temporary table are deleted by a special delete operator The timing of the execution plan  76  of Statement S, rt 1 , rt 2 , st 1  and st 2 , according to the structure of FIG. 6A, is illustrated in FIG. 6B, where S represents the time to execute the statement tree  42 , rt 1 , the time to execute the rt 1  action tree  52 , rt 2  the time to execute the rt 2  action tree  54 , st 1  the time to execute the st 1  action tree  56 , and st 2  the time to execute the st 2  action tree  58 . As can be noted from the figure, rt 1  and rt 2  execute in parallel and overlap with the execution of statement S because of pipelining. Statement triggers st 1  and st 2  execute in parallel but only after the execution of the row triggers. This gives a large decrease in the time to execute the statement S and its associated triggers compared to the case of sequential execution  74  shown in the figure.  
         [0039]    [0039]FIG. 7 shows a flow chart of the process for creating an execution plan such as is shown in FIG. 6A. In the process depicted, first the triggers that may be activated by the activating statement are determined in step  90  and an operator tree of the activating statement is formed in step  92 . Next, a trigger tree for each of the activated triggers is formed in step  94  and, in step  95 , the process then verifies that there are no conflicts among activated triggers and between the activated triggers and the activating statement. An activated trigger is either a row or statement trigger as determined by step  96 . If a row trigger is activated, it is joined to the action tree for pipelined execution with the execution of the statement tree in step  98 . If a statement trigger is activated, it is joined, in step  100 , to the statement tree for execution after the execution of the statement tree using a temporary table as input for the action of the statement trigger. The temporary table accumulates the set of affected rows. The statement trigger scans the temporary table for its input.  
         [0040]    The above covers the case of a single row trigger or statement trigger. If more than one row or statement trigger is activated by the activating statement, the row or statement triggers must be combined into the execution plan. In particular, if a number of row triggers is activated, the activated row triggers are combined together into a parallel row group (Group  1  in FIG. 6A) and this parallel row group is the object that is attached to the statement tree for pipelined execution. Internal to the parallel group, each trigger is interconnected by means of a parallel union operator to permit parallel execution of each row trigger within the group. Thus, the execution plan according to the present invention prescribes that each trigger in the parallel group executes in parallel with the other triggers in the group and the entire group execute in a pipeline with the activating statement tree.  
         [0041]    If a number of statement triggers is activated, the activated statement triggers are combined together into a parallel statement group (Group  2  in FIG. 6A) and this parallel statement group is the object that is attached to the statement tree for execution subsequent to the statement tree. Again, internal to the parallel group, each trigger is interconnected by means of a parallel union operator to permit parallel execution of each statement trigger within the group. Additionally, each statement trigger during its execution typically scans the TempTable  62  for its input. The execution plan thus prescribes that the statement triggers execute in parallel and the entire group executes subsequent to the execution of the activating statement tree.  
         [0042]    Of course, it is possible that both a plurality of row triggers and a plurality of statement triggers are activated by the activating statement. This means that the final execution plan combines the actions trees of both the activated statement triggers and row triggers according to FIG. 6A.  
         [0043]    Although the present invention has been described in considerable detail with reference to certain preferred versions thereof, other versions are possible. Therefore, the spirit and scope of the appended claims should not be limited to the description of the preferred versions contained herein.