Abstract:
A query processor for parallel processing translates an input query which references data stored in one or more homogenous or heterogenous databases into a plurality of parallel output queries each of which is directed to a single one of the databases or a partition thereof. A runner combines the results of each of the output queries and integrates them into a single coherent answer set.

Description:
BACKGROUND OF THE INVENTION 
     It is known in the art of database management to organize and store data in electronically readable form for subsequent shared access by a multiplicity of computer users. Database engines enable a population of users to submit queries addressing such data, which is organized conceptually in relational, or tabular, form for convenience, and to receive in response an output table known as an answer set. Under adverse circumstances, answer sets take an inordinate amount of time to produce. As the tables comprising a database become larger, and the queries addressing them more complex, the time required to extract answer sets increases. This effect can be seen most dramatically in computer systems having a single processor. If it were generally possible, in the presence of many independent processors, to break requests into tasks that could be executed in parallel, database management systems could respond to even the most difficult queries in a reasonable time. 
     This is so for the same reason that ten men working on a job can complete it in one-tenth of the time providing they have equivalent skills and are able to share the work in an optimal fashion. Cooperating computer processors, like cooperating individuals, can not always function effectively in parallel. It often takes outside intervention to facilitate cooperation and, even then, the end result can only approach the ideal. 
     Consider, for example, a powerful computer system equipped with an unlimited supply of processors managing a database comprised of a single, monolithic, table. If, and this is very often the case, only one processor can use the table at one time, the power of the system is no greater than it would be if only one processor were available. This scenario is roughly analogous to the human situation in which ten workers are forced to share an important tool. At times only the person with the tool can work. The rest are forced to wait. 
     To make effective use of parallel processing computer database systems require outside intervention, primarily to encourage effective resource sharing amongst available processors. In part, this can be accomplished by breaking up large tables into small, disjoint, subsets to facilitate sharing. Suppose, for example, the customer file for a commercial establishment had grown very large, and assume that we wish to list those customers who have placed an order in the past month. Satisfying a query of this sort would normally require the database management system to scan the file from beginning to end extracting those records, or rows, exhibiting the desired characteristics, in this case evidence of a recent purchase. This could be a lengthy process. If the file were known to consist of ten non-overlapping subsets, the system could, in theory, assign ten processors to do the job. Each would scan one of the subsets and each would contribute part of the answer set. A controlling processor would be required to combine the intermediate results into a coherent result. 
     In this hypothetical situation, the actual structure of the information need not be known to the end user, who would prefer to view the customer file as a monolithic table. The ideal system would automatically take physical data partitioning into account when it processes a query, and it would do so without revealing this knowledge to its clientele. Of course, even under ideal conditions someone would have to determine the actual physical structure of the customer file. 
     The prior art has not produced a parallel processing database management system approaching the hypothetical ideal herein described for the following reasons. First, the most popular database management systems have had a long history. They are likely to have been conceived at a time when no premium was placed on parallel processing. Second, most actual data repositories are heterogeneous in nature. That is, the information base for a typical enterprise is, more likely than not, a composite of several dissimilar databases managed by jointly incompatable database management systems. In an environment in which no one system has the ability to coordinate the activities of the others, the parallel processing ideal posited here is difficult, if not impossible to realize. Third, adequate tools for partitioning files and tables to organize data in a fashion suited to parallel processing have been lacking. 
     SUMMARY OF THE INVENTION 
     The aforementioned problems of the prior art are overcome by the instant invention which provides apparatus for splitting a given query into a plurality of related queries which, when submitted to suitable independently functioning database management engines, produce information that can subsequently be transformed into the requested answer set. More specifically, the instant invention processes signals representing an input query addressing one or more tables, each of which may be physically partitioned into discrete subsets, by breaking the input query into a family of related queries addressing the aforementioned subsets, issuing each such related query to a database engine for processing, combining the resulting intermediate signals produced by the database engines into signals representing a combined answer set and producing the signals representing the combined answer set at its output terminal. The instant invention includes query processor means for analyzing and splitting queries, submitting queries to database engines, combining answer sets and producing suitable signals representing answer sets at its output port, meta-data analysis means having an input port for receiving signals representing a database table name and an output port for producing signals representing the structure of the table corresponding to the table name, translator means having one terminal operatively connected to the meta-data analysis means and a plurality of output ports, each adapted to be connected to an independent database engine and answer set aggregation means having a multiplicity of input terminals, each adapted to be connected to an independent database engine, through which signals representing intermediate answer sets are received, and an output port at which signals representing a combined answer set may be produced. The translator means includes lexical analysis means for breaking signals representing the text of a query into signals representing the sequence of tokens or words comprising the query, syntax analysis means for converting the signals generated by the lexical analysis means into signals corresponding to a tree representation of the query, semantic analysis means for refining the tree representation of the query and determining its meaning, normalizer means for re-expressing the tree prepared by the semantic analysis means in such a way as to remove all references to sets known as views which have no direct physical counterpart, planner means for determining how best to recast the input query as a family of related, but independently processable, queries, splitter means for expressing the plan generated by the planner means in the form of signals representing query trees and code generator means for converting queries expressed in the form of trees into queries expressed in the text form understood by the independently operating database engines. 
     It is an object of the invention to decrease the time required to process queries that address large databases by distributing the workload among many database engines that can run in parallel. 
     Another object of the invention is to provide a means for processing queries addressing one or more partitioned tables or files in parallel. 
     Still another object of the invention is to provide a means for processing queries addressing tables managed by an arbitrary collection of heterogeneous database management systems in parallel. 
     Other and further objects of the invention will become apparent from the following drawings and description of a preferred embodiment of the invention in which like reference numerals have been employed to indicate like parts. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an overall schematic view of the apparatus of the preferred embodiment of the invention. 
     FIG. 2 is a detailed schematic view of the apparatus of the preferred embodiment of the invention. 
     FIG. 3 is a flowchart depicting the operation of a component of the invention. 
     FIGS. 4a, 4b and 4c form a flowchart depicting the operation of another component of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 1 of the drawings there is shown a query processor 1 whose communications handler 2 receives signals representing a source query from a client 69. The source query, which can be expressed in the ANSI 1989 dialect of SQL, is shown addressing a database that has been decomposed into 7 partitions, 43, 45, 47, 49, and 51a, 51b, and 51c, four of the partitions 43, 45, 47, and 49, being mutually incompatible, and managed, respectively, by independent database engines, 23, 25, 27 and 29, and the remaining three partitions 51a, 51b, and 51c being incompatible with partitions 43, 45, 47, and 49 and managed by independent database engine 31. The communications handler 2 passes the query it receives to the translator 3, which recasts it in terms of base tables and transforms it into multiple SQL statements, each conforming to the language standards of the independent database engine that will eventually process it. 
     The signals generated by the translator typically represent many queries, each of which addresses information found within a single partition 43, 45, 47, 49, 51a, 51b, 51c, of the database and each of which conforms to the query language standards of the engine managing that partition. A given partition may be managed by one and only one of the engines 23, 25, 27, 29 and 31. Thus, database engine 23 manages partition 43, database engine 25 manages partition 45, database engine 27 manages partition 47, database engine 29 manages partition 49 and database engine 31 manages partitions 51a, 51b, 51c, and 51d. 
     A database engine requires at least one processor in order to function, but a given engine might have more than one processor at its disposal. The translator 3 may, therefore, direct multiple queries to database engine 31, which is shown presiding over multiple sub-partitions, 51a, 51b and 51c. Database engine 31 has many processors, and its component of the database has been partitioned to take advantage of parallel processing. 
     The database engines are not part of the invention. The invention serves to enhance the utility of the engines by making it possible for them to work together in an efficient fashion. 
     A meta-data database 70 contains a definition of the hardware environment, a description of the database as it is understood by the client 69, a description of the partitioned structure of the database and a definition of the relationship between individual partitions and database engines. Information stored in the metadata database is used by the translator 3, which sends signals representing specific requirements to a Master File Parser (MASPAR) module 15. MASPAR 15 responds to such requests with signals representing the requisite meta-data. Many such exchanges between the translator 3 and MASPAR 15 may be required to process a single source query. 
     The application of an output query to a participating database engine 23, 25, 27, 29 or 31, triggers the production of an intermediate answer set. Each participating database engine generates a result signal representing an intermediate answer set and directs the result signal to a runner 67, which in turn produces a final result signal satisfying the source query, and directs that signal to the communications handler 2 which, in turn, sends the result signal to the client 69. 
     Initially, the translator 3 transforms the source query into signals representing a tree structure. Consider the following SQL source query: 
     SELECT KEYCOLS, KEYNAME 
     FROM SYSTEM.SYSKEYS 
     WHERE KEYNAME=`Fred` 
     ORDER BY KEYCOLS 
     The translator reduces this to a J-tree, represented below as an indented list. The nodes that comprise the list contain a major and a minor operation code and are written in the form, operation: sub-operation. 
     
         ______________________________________Node Description    Comment______________________________________SELOP:NOOP          SELECT Node (the root)SCOLSOP:NOOP        Column list NodeQNAMEOP:NOOP        1st Column ReferenceIDENT:KEYCOLS       an SQL identifierQNAMEOP:NOOP        2nd Column ReferenceIDENT:KEYNAME       an SQL identifierFROMOP:NOOP         FROM Clause NodeQNAMEOP:NOOP        1st Table Ref.IDENT:SYSTEM        Table QualifierIDENT:SYSKEYS       Table NameemptynodeWHEREOP:NOOP        WHERE Clause NodeEQLOP:NOOP          &#34;=&#34; ComparisonQNAMEOP:NOOP        Left Operand NodeIDENT:KEYNAME       Name of OperandSTRNG:`FRED;        Right Operand NodeORDEROP:NOOP        ORDER Clause NodeQNAMEOP:NOOP        1st Column Ref.IDENT:KEYCOLS       Column Name______________________________________ 
    
     The translator 3 also obtains from the MASPAR 15 information about the structure of the database file containing the information that is being referenced, i.e., SYSTEM.SYSKEYS, and constructs a J-tree representation of this file which is stored as a single segment file having 11 fields as follows. 
     
         ______________________________________Node Description     Comment______________________________________EN.sub.-- USERID:SYSTEM                Table nameEN.sub.-- SEGMENT:SQLOUT                Segment within tableEN.sub.-- FIELD:TNAME                Field (1) within segmentEN.sub.-- ALIAS:E01  Alias for field (1)EN.sub.-- FIELD:TCREATOR                Field (2)EN.sub.-- ALIAS:E02  Alias (2)EN.sub.-- FIELD:KEYTYPE                Field (3)EN.sub.-- ALIAS:E03  Alias (3)EN.sub.-- FIELD:KEYNAME                Field (4)EN.sub.-- ALIAS:E04  Alias (4)EN.sub.-- FIELD:KEYCOLS                Field (5)EN.sub.-- ALIAS:E05  Alias (5)EN.sub.-- FIELD:INAME                Field (6)EN.sub.-- ALIAS:E06  Alias (6)EN.sub.-- FIELD:REFTNAME                Field (7)EN.sub.-- ALIAS:E07  Alias (7)EN.sub.-- FIELD:REFTCREATOR                Field (8)EN.sub.-- ALIAS:E08  Alias (8)EN.sub.-- FIELD:DELETERULE                Field (9)EN.sub.-- ALIAS:E09  Alias (9)EN.sub.-- FIELD:STATUS                Field (10)EN.sub.-- ALIAS:E10  Alias (10)EN.sub.-- FIELD:TIMESTAMP                Field (11)EN.sub.-- ALIAS:E11  Alias (11)______________________________________ 
    
     At this point, the translator 3 consults its meta-data files and finds that SYSTEM.SYSKEYS is partitioned into SYSTEM.SYSKEYS1 and SYSTEM.SYSKEYS2. Each partition has an associated set membership condition. The system generates an SQL query corresponding to the first such condition as follows. 
     SELECT X FROM Y WHERE 
     (KEYNAME&gt;=`MM`) AND 
     ((KEYNAME&lt;&gt;`MM`) OR (KYCOLS&gt;=3)) 
     The query may be expressed in terms of dummy column and table names, &#34;X&#34; and &#34;Y&#34;, because only the WHERE clause is of consequence. A parser, more fully described below, reduces the query to the following tree structure. 
     
         ______________________________________Node Description      Comment______________________________________SELECT SELOP          The root nodeSCOLSOP               Dummy SELECT listQNAMEOPQNAMEPART IDENT XFROMOP                Dummy FROM clauseQNAMEOPQNAMEPART IDENT YEMPTYNODEWHEREOP               The split conditionANDOP                 ANDGEQOP                 &gt;=QNAMEOP               left operand of &gt;=QNAMEPART IDENT KEYNAMELITSTRING STRNG `MM`  right operand of &gt;=OROP                  ORNE NEQOP              &lt;&gt;, left opr of ORQNAMEOP               left operand of &lt;&gt;QNAMEPART IDENT KEYNAMELITSTRING STRNG `MM`GEQOP                 &gt;=, right opr of ORQNAMEOP               left opr of &gt;=QNMAEPART IDENT KEYCOLSLITINT FIXED 3        right opr of &gt;=______________________________________ 
    
     The forgoing specifies the condition predicate for the first partition, SYSTEM.SYSKEYS1 of the database SYSTEM.SYSKEYS. The &#34;WHEREOP&#34; subtree of this tree is then merged (&#34;ANDed&#34;) with the tree representing the input query. This results in a new tree containing the following &#34;WHEREOP&#34; subtree. 
     
         ______________________________________Node Description     Comment______________________________________WHEREOP:NOOP         Root of sub-treeANDOP:NOOP           AND (a)ANDOP:NOOP           AND (b)GEQOP:NOOP           &gt;=QNAMEOP:NOOP         left opr of &gt;=IDENT:KEYNAME        operand nameSTRNG:`MM`           right opr of &gt;=OROP:NOOP            OR (a)OROP:NOOP            OR (b), left opr of (a)LESSOP:NOOP          &lt;, left opr of (b)QNAMEOP:NOOP         left opr of &lt;IDENT:KEYNAME        Operand nameSTRNG:`MM`           right opr of &lt;GRTROP:NOOP          &gt;, right opr of (b)QNAMEOP:NOOP         left opr of &gt;IDENT:KEYNAME        Operand nameSTRNG:`MM`           right opr of &gt;GEQOP:NOOP           &gt;=, right opr of (a)QNAMEOP:NOOP         left opr of &gt;=IDENT:KEYCOLS        Operand nameFIXED:3              right opr of &gt;=EQLOP:NOOP           =, right opr of aQNAMEOP:NOOP         left opr of =IDENT:KEYNAME        Operand nameSTRNG:`FRED`         right opr of =______________________________________ 
    
     The foregoing tree will not contribute to the end result due to an incompatibility between the conditions KEYNAME&gt;=`MM` and KEYNAME=FRED. Hence, this particular split query is slated to be removed from consideration, i.e., pruned from the tree, since it can not contribute to the answer to the input query. 
     The translator now examines the set membership condition, i.e., condition predicate, for the second partition, SYSTEM.SYSKEYS2, which is stated in SQL as: 
     
         ______________________________________SELECT X FROM Y WHERE(KEYNAME &lt;= `MM`)AND((KEYNAME &lt;&gt;`MM`) OR (KEYCOLS &lt;3));______________________________________ 
    
     The translator 3 parses the foregoing query to produce another &#34;WHEREOP&#34; subtree and merges (ANDs) it with the sub-tree of the input query to obtain the following tree. 
     
         ______________________________________Node Description     Comment______________________________________SELOP:NOOP           The rootSCOLSOP:NOOP         Column list nodeQNAMEOP:NOOP         1st column referenceIDENT:KEYCOLS        Column nameQNAMEOP:NOOP         2nd column referenceIDENT:KEYNAME        Column nameFROMOP:NOOP          FROM clause nodeQNAMEOP:NOOP         1st table referenceIDENT:SYSTEM         QualifierIDENT:SYSKEYS2       Table name (2nd part)emptynodeWHEREOP:NOOP         WHERE clause nodeANDOP:NOOP           AND (a)ANDOP:NOOP           AND (b)LEQOP:NOOP           &lt;=, left of opr of bQNAMEOP:NOOP         left opr of &lt;=IDENT:KEYNAME        name of operandSTRNG:`MM            right opr of &lt;=OROP:NOOP            ORNEQOP:NOOP           &lt;&gt;, left opr of ORQNAMEOP:NOOP         left opr of &lt;&gt;IDENT:KEYNAME        operand nameSTRNG:`MM`           right opr of &lt;&gt;LESSOP:NOOP          &lt;, right opr of ORQNAMEOP:NOOP         left opr of &lt;IDENT:KEYCOLS        name of operandFIXED:3              right opr of &lt;EQLOP:NOOP           =, right opr of aQNAMEOP:NOOP         left opr of =IDENT:KEYNAME        name of operandSTRNG:`FRED`         right opr of =______________________________________ 
    
     This tree contributes to the end result. The translator 3 then prunes the tree and transforms the resulting WHERE clause to obtain the following WHERE clause sub-tree. 
     
         ______________________________________Node Description     Comment______________________________________WHEREOP:NOOP         WHERE clause nodeANDOP:NOOP           AND (a)ANDOP:NOOP           AND (b)LEQOP:NOOP           &lt;=, left opr of bQNAMEOP:NOOP         left opr of &lt;=  IDENT:KEYNAME name of operandSTRNG:`MM`           right opr of &lt;=OROP:NOOP            OR (c), right opr of bOROP:NOOP            OR (d), left opr of c  LESSOP:NOOP   &lt;, left opr of dQNAMEOP:NOOP         left opr of &lt;  IDENT:KEYNAME operand nameSTRNG:`MM`           right opr of &lt;GRTROP:NOOP          &gt;, right opr of dQNAMEOP:NOOP         left opr of &gt;  IDENT:KEYNAME operand nameSTRNG:`MM`           right opr of &gt;LESSOP:NOOP          right opr of cQNAMEOP:NOOP         left opr of &lt;IDENT:KEYCOLS        operand nameFIXED:3              right opr of &lt;EQLOP:NOOP           =, right opr of aQNAMEOP:NOOP         left opr of =IDENT:KEYNAME        operand nameSTRNG:`FRED`         right opr of =______________________________________ 
    
     From the above tree, the translator 3 produces the following SQL query. 
     
         ______________________________________SELECT KEYCOLS ,KEYNAMEFROM SYSTEM.SYSKEYSWHERE(KEYNAME &lt;=`MM`) AND((KEYNAME &lt;&gt;`MM`) OR (KEYCOLS &lt;3))AND(KEYNAME = `FRED`)______________________________________ 
    
     This SQL output query is applied to SYSTEM.SYSKEYS2, the second partition. The system wastes no time searching SYSTEM.SYSKEYS1. 
     The query processor 1 of the invention will now be described in greater detail with particular reference to FIG. 2. The translator 3 comprises a lexical analyzer 5, a parser 7, a semantic analyzer 11, a normalizer 13, a planner 17, a splitter 19 and a code generator 21. As will be known to those skilled in the art, each of these components may be realized on a computer processor having associated random access memory. A server computer may be configured to perform the functions of these components, to receive a query from a client computer 69, process it in accordance with the invention and return the answer set specified by the query to the client computer 69. 
     The lexical analyzer 5 transforms digital signals representing the text of an SQL query to digital signals representing a sequence of SQL tokens and passes them, on request, to the parser 7. There are many kinds of SQL tokens: character string literals, delimited identifiers, special characters, relational operators, numeric literals, national character strings, identifiers and key words. The lexical analyzer 5 extracts the next word or other significant symbol of the SQL language from the source query string when it receives a signal from the parser and delivers signals representing the aforementioned word or symbol to the parser 7. The parser 7, having received and analyzed the tokens comprising the source query, constructs an abstract syntax tree (AST) depicting the source query and directs signals representing that AST to the semantic analyzer 11 for further processing. 
     The semantic analyzer 11 scans the AST and constructs from the information contained therein another representation of the source query hereinafter referred to as a &#34;J-tree.&#34; The J-tree encapsulates the latent information contained in the source query in a form suitable for manipulation in a computer memory. The semantic analyzer 11 determines whether the source query is consistent with the database schema encoded in the meta-data database 70 before it permits the translation process to continue. It rejects all queries that do not conform to the semantic rules of SQL. 
     Query validation requires the services of MASPAR 15 which, upon receipt of the appropriate signals, assembles signals representing the database objects referenced in the J-tree. MASPAR 15 contains circuitry for comparing the table and column names found in the J-tree with table and column names found in the meta-data database. For each table or column reference signal it receives, MASPAR 15 returns either a &#34;not found&#34; signal or a signal representing the internal structure of the database object corresponding to the table or column name. The semantic analyzer 11 uses the signals generated by MASPAR 15 in this context both to validate the query and to augment selected nodes of the J-tree with additional information. 
     Having accepted the source query, the semantic analyzer 11 passes a signal to the normalizer 13, which ensures that the query represented by the J-tree at that moment has been expressed soley in terms of base tables. SQL queries may reference two kinds of tables: base tables, the contents of which are actually recorded on external digital storage media, and views, which have no direct physical representation. A view, in SQL terminology, is an object defined in terms of any number of other base tables and views that retains the important characteristics of a base table. Since the planner 17, splitter 19 and code generator 21 require information about base table partitions, view references must be systematically replaced by equivalent base table references before the planner 17, splitter 19 and code generator 21 can perform their respective functions. 
     The operation of the normalizer is illustrated in FIG. 3 with the assumption that a J-tree, T0, exists and that the FROM list of T0 contains a set, v, of view references. The procedure first invokes itself recursively to process nested SELECT sub-trees of T0. Having normalized all such nested SELECTS, the procedure considers every remaining view reference x in v, replacing each with one or more table references. This step entails recursively normalizing the view definition, dx, of x and merging T0 and dx. The merge step, shown as a single box in FIG. 3 involves substituting the view column references with corresponding table column references taken from dx, replacing the view reference, x, with the entire FROM sub-tree of dx and ANDing the WHERE sub-tree of dx with that of T0. Should dx contain contain a GROUP BY subtree, the normalizer 13 ANDs the corresponding HAVING sub-tree with the WHERE sub-tree of dx. To avoid introducing ambiguous references, the normalizer 13 replaces all column references in T0 and dx with uniquely qualified column references. Should this process generate an operation that can not be performed, the normalizer 13 rejects the query rather than continuing. 
     The normalization process is illustrated by the following example. Intermediate results, which are shown here in flattened text form for readability, should be understood to describe J-trees. 
     View Definitions: 
     CREATE VIEW SALES 
     (PNAME, PCODE, PDESCR, PCOST, VOL, REGION, MONTH) AS 
     SELECT P.PNAME, S.PCD, P.PDESCR, P.PCOST, S.VOL, S.REG, S.MON FROM SALES --  BASE S, PROD --  BASE P WHERE S.PCD=P.PCD; 
     CREATE VIEW HIGHLIGHTS AS 
     SELECT PNAME, PCODE, VOL, REGION, MONTH FROM SALES WHERE VOLUME&gt;(SELECT AVG(VOL) FROM SALES WHERE MONTH=`DEC`); 
     CREATE VIEW OUR --  SALES (PRODUCT --  NAME, PRODUCT --  CODE, VOLUME) AS 
     SELECT * FROM HIGHLIGHTS 
     WHERE REGION IN (`Hither`, `Yon`); 
     Sample Query (T0): 
     SELECT, FROM OUR --  SALES 
     WHERE PRODUCT --  NAME LIKE `%cycle`; 
     The Normalization of T0 takes place as follows. The normalizer 13 retrieves the &#34;CREATE VIEW OUR --  SALES . . .&#34; statement, reduces the statement to a J-tree, T1, and normalizes the tree by calling itself recursively. Initially, T1, takes the following form: SELECT * FROM HIGHLIGHTS 
     WHERE REGION IN (`Hither`, `Yon`); 
     In the process of normalizing T1 the system must access another view definition, T2. Initially, T2 takes the following form: 
     SELECT PNAME, PCODE, VOL, REGION, MONTH 
     FROM SALES 
     WHERE VOLUME&gt; 
     (SELECT AVG(VOL) FROM SALES 
     WHERE MONTH=`DEC`); 
     T2 contains a sub-query, T3, that must be normalized. But T3 is also cast in terms of a view. The normalizer 13 accesses the SALES view definition and converts it into yet another J-tree, T4. Initially, T4 takes the following form: 
     SELECT P.PNAME, S.PCD, P.PDESCR, P.PCOST, S.VOL, S.REG, S.MON 
     FROM SALES --  BASE S, PROD --  BASE P 
     WHERE S.PCD=P.PCD 
     Since T4 is in normal form, it can be merged with T3 to produce a new version of T3: 
     SELECT AVG(S.VOL) FROM SALES --  BASE S, PROD --  BASE P 
     WHERE (S. PCD=P. PCD ) AND (S. MONTH=`DEC`); 
     T2, once it has been normalized, has the following appearance: 
     SELECT PNAME, PCODE, VOL, REGION, MONTH 
     FROM SALES 
     WHERE VOLUME&gt; 
     (SELECT AVG(VOL) FROM SALES --  BASE S1, PROD --  BASE P1 
     WHERE (S1.PCD=P1.PCD) AND (MONTH=`DEC`)); 
     But T2 still contains a reference to the SALES view, T4. Merging and T2 and T4 produces another T2 revision: SELECT P2.PNAME, S2.PCD, S2.VOL, S2.REG, S2.MON FROM SALES --  BASE S2, PROD --  BASE P2 WHERE 
     (S2.PCD=P2.PCD) AND 
     (S2.VOLUME&gt;(SELECT AVG(VOL) FROM SALES --  BASE S1, PROD --  BASE P1 WHERE (S1.PCD=P1.PCD) AND (MONTH=`DEC`)); 
     Merging this with T1 yields T1 in its final form: SELECT P2.PNAME, S2.PCD, S2.VOL, S2.REG, S2.MON FROM SALES --  BASE S2, PROD --  BASE P2 WHERE 
     (S2. PCD=P2. PCD) AND 
     (S2.VOLUME&gt; 
     (SELECT AVG(S1.VOL) FROM SALES --  BASE S1, PROD --  BASE P1 WHERE (S1.PCD=P1.PCD) AND (MONTH=`DEC`)) AND 
     (S2.REGION IN (`Hither`, `Yon`)); 
     When it has completed its work, the normalizer 13 signals the splitter which partitions the source query into independently executable units. The splitter bases its decisions on meta-data descriptions which, by this time, have been brought into memory and stored in a variant of J-tree used to retain such information. If the source query addresses only monolithic tables or is thought to be optimal as it stands, the system makes no attempt to split it. The splitter 19 breaks divisible queries into tasks that can take place in parallel. A normalized J-tree is considered to be a candidate for decomposition if (1) any base table, T, referenced in its FROM subtree is the union of multiple, disjoint, SQL union-compatible base tables, (2) any base table referenced in its FROM sub-tree has been partitioned into disjoint subsets on the basis of key field ranges or (3) its root node contains a union operator. The system processes SQL UNION statements in parallel even if the individual queries that comprise the union cannot be decomposed. 
     FIGS. 4a, 4b, and 4c depict the overall logic of the splitter, which begins by determining whether J-tree Q (FIG. 4a) represents a UNION operation. If it does, the splitter invokes itself recursively to partition both branches of the tree. Note that if additional UNION operations were embedded in either branch of the tree, the splitter would detect them and, once again, invoke itself recursively to partition each branch. Since UNION operations must, by definition, occur at a higher level than SELECT operations, this strategy effectively removes UNIONS from consideration before the SELECT node is detected. 
     Having dispensed with UNIONs, the splitter 19 scans for subqueries, constructs which may have been employed to specify individual values or columns of values in the predicate. SQL defines two kinds of sub-queries: correlated and uncorrelated. Correlated sub-queries require special handling because they cannot be evaluated independently of the query in which they are embedded. The splitter attempts to replace every uncorrelated subquery with a value or column of values before continuing. This entails (1) detecting an uncorrelated sub-query, Qs, (2) splitting Qs, (3) executing Qs and (4) recasting the Qs sub-tree in terms of literal values. Thus, the subtree representing &#34;A=(SELECT AVG(Age) FROM Personnel&#34; might be replaced by the equivalent of &#34;A=37&#34; and the subtree representing &#34;X IN (SELECT ModelNumber FROM Products WHERE Qty --  On --  Hand&lt;100)&#34; might be replaced by the equivalent of &#34;X IN (100, 221, 085)&#34;. 
     IN lists, because their size can not be known apriori, present a special problem. If the number of elements exceeds a DBMS-dependent threshold, the splitter must store them in a temporary table, Tmp, for example, and replace the predicate in question with the equivalent of `X IN (SELECT * FROM Tmp)&#34;. 
     A false predicate, as referred to in FIG. 4a, is an SQL predicate containing an inexpensive sub-query that is guaranteed to produce a suitable result. For example &#34;1=SELECT 2 FROM EMPTYTABLE&#34;. 
     Once all uncorrelated sub-queries have been replaced, the splitter examines the FROM list of the J-tree. FROM lists, at this stage in the process, can contain an unspecified number of table references (view references have already been replaced by the normalizer 13). References to &#34;concatenated&#34; tables, the components of which are seen as separately addressable tables by participating DBMSs, and &#34;partitioned&#34; tables, which are not, must be treated differently. 
     A query containing a concatenated table reference, T, always gives rise to one task for every component of T while a query containing a partitioned table reference need not be split at all. Thus, FIG. 4 shows Q being split relative to its concatenated table reference before invoking the DBMS-dependent &#34;Explain&#34; function (FIG. 4b). The Explain function, which may not necessarily be available, is a generic name for a facility that examines a proposed query and returns information about how a particular DBMS would process it. The system uses such information to determine (1) whether to split and (2) what partitioned table or tables in the FROM list can best be used as the basis for splitting. Query optimization at this level is highly DBMS-dependent and can only be brought into play when partitioned tables are addressed. 
     The logic illustrated in FIG. 4b is capable of splitting a query Q relative to every partitioned table it references. In practice, splitting is constrained by DBMS-dependent rules. 
     Finally, the splitter 19 looks for correlated sub-queries referencing concatenated tables. Such queries can not be processed as stated by the SQL DBMS engines because the engines are unaware of concatenated table names. Concatenated tables, which are logical entities, are defined as the union of one or more base tables. A DBMS can address the component parts of a concatenated table but not, as is required in the case of correlated sub-queries, the table as a whole. To solve the addressability problem the system must materialize the information required to satisfy the correlated sub-query and store it in a temporary table local to a selected DBMS. It attempts to do so by generating a suitably qualified SQL UNION request. It is crucial that the UNION operation produce answer sets of a manageable size. The system requests only those rows and columns that are required to evaluate the predicate under consideration. It derives the column list by enumerating column references in the correlated sub-query and forms a predicate by copying relevant conditions from its WHERE clause. Normally it is possible to guarantee a priori that the answer set produced in this fashion will be far smaller than a straightforward materialization of the concatenated table in question. But if this is not the case, and the projected size of the intermediate result exceeds a user-defined threshold value, the splitter 19 aborts the source query and returns a diagnostic message to the client. 
     The correlated sub-query strategy for concatenated tables is illustrated by the following example. Consider the following query: 
     SELECT EName 
     FROM emp e1 
     WHERE Salary&gt; 
     (SELECT AVG(Salary) 
     FROM emp e2 
     WHERE e1. Dpt=e2.Dpt); 
     If emp were partitioned on Dpt N, queries of the following form could be generated. This is possible because e1.Dpt is known to be the same as e2.Dpt. Since Dpt is the partitioning key, the sub-query need can be evaluated without crossing subset boundaries. 
     SELECT EName 
     FROM emp e1[i] 
     WHERE Pred(e1[i]) AND 
     Salary&gt; 
     (SELECT AVG(Salary) 
     FROM emp e2[i] 
     WHERE e1.Dpt=e2.Dpt AND pred(e2[i])); 
     But if emp is not partitioned on Dpt (i.e., the sub-query spans subset boundaries) the inner query can not be split. An intermediate table, T, defined as follows, must be introduced. 
     T(Dpt, Number, Amount) 
     Having created T, the query processor 1 then populates it with N result sets. 
     
         ______________________________________INSERT INTO TSELECT Dpt, SUM(VALUE(LENGTH(EName), 1)*0+1),SUM(Salary)FROM emp[1]GROUP BY DptHAVING COUNT(*) &gt; 0;INSERT INTO TSELECT Dpt, SUM(VALUE(LENGTH(EName), 1)*0+1),SUM(Salary)FROM emp[2]GROUP BY DptHAVING COUNT(*) &gt; 0;. . .INSERT INTO TSELECT Dpt, SUM(VALUE(LENGTH(EName), 1)*0+1),SUM(Salary)FROM emp[N]GROUP BY DptHAVING COUNT(*) &gt; 0;______________________________________ 
    
     Once T is fully populated, the query processor issues N correlated queries of the form: 
     SELECT EName FROM emp [i] 
     WHERE emp. Salary&gt; 
     (SELECT SUM(T.Amount) / SUM(T.Number) 
     FROM T 
     WHERE emp. Dpt=T.Dpt); 
     Finally, the result sets are merged and T is dropped. 
     A split query must be issued for every component of a concatenated table reference. When the client calls for concatenated tables, T1, T2, . . . , Tn to be joined, for example, the query generator 1 is forced to generate a split query for every permutation and combination of the components of T1, T2, . . . , Tn. In consequence, the method depicted in FIGS. 4a, b, c is capable of deriving every possible split query from a given source request. The query generator can actually optimize performance by splitting the source request selectively. Selective splitting is possible for tables partitioned on split key ranges. For such tables, the splitter 19 identifies which queries are worth submitting for parallel execution by examining a set of DBMS-specific rules designed to optimize the overall performance of the system. 
     The following rules have been developed specifically for the IBM DB/2 DBMS engine. In the case of DB/2, the splitter 19 recognizes six general classes of SQL query (called Q below) for the purpose of split range key table parallelization: 
     (1) A UNION [ALL] query: Break n-way Union operations into n separate subordinate query specifications and split each of them individually. 
     (2) A simple ungrouped query (Q) with one table reference (T) in the FROM list, no HAVING clause and no correlated sub-queries: Providing T is composite, and the EXPLAIN data provides ample justification to proceed, split Q on the partitioning key of T as shown in the previous example. 
     (3) A simple grouped query whose GROUP BY specification is the partitioning key (or an ungrouped query with a HAVING clause): If T is composite, split Q relative to T. 
     (4) A 2-way join(T1 X T2): If T1 and T2 are partitioned m and n ways, respectively, the splitter 19 is capable of generating as many as m*n tasks. Do not attempt a two-way split unless DB/2 Explain data shows it to be advantageous. &#34;Star&#34; joins, in which all the join columns are partitioning keys (with identical range or subsetted boundaries), may be split into one replica per partition. For other queries, do an EXPLAIN on the source query and split on the partitioning key of the outer table, T1, of the join. If T2 happens to be partitioned on the same columns as T1 generate a 2-way split. 
     (5) An n-way join (T1×T2 . . . X Tj): The strategy for n-way joins is a generalization of the 2-way join strategy. In the concatenated case as many as n1, n2 . . ,nj queries could result. In the split key case, partition on the outermost table of the DB/2-selected join order and then expand to two-way partitioning in the event of a star join. Attempt to expand to three-way partitioning in the case of a second star join. 
     (6) A query, Q, with a correlated sub-query, SQ: do not split SQ unless it addresses a concatenated table. Limit splitting to the parent query. 
     When it completes its work the splitter 19 passes two signals, the address of the root node (r) of the J-tree and the address (d) of a descriptor list indicating how to produce many statements from r. The purpose of the code generator 21 is to generate signals representing the text of the SQL statements encoded by r and d. 
     To produce text, the code generator 21 traverses r in a top-to-bottom, left-to-right fashion, emitting the text of one or more tokens for every node it visits. For J-tree nodes representing column names in the select list, for example, the code generator 21 copies the identifier, suitably delimited, to its result string. Other nodes representing higher level constructs such as sub-selects and expressions require special treatment. Sub-selects, for example, might give rise to a leading &#34;(&#34; followed by the result of flattening the sub-select, followed by a closing&#34;)&#34;. 
     The code generator 21 suppresses HAVING clauses that have been marked as post-processing steps by the splitter 19. Assume a tree, T, representing the following grouped query is submitted to the code generator 21. 
     SELECT DEP --  NBR, SUM(SALES) FROM SALES --  INFO 
     WHERE REGION=`East` 
     GROUP BY DEP --  NBR 
     HAVING AVG (SALES)&gt;100000, 
     From this, the code generator 21 might produce the following text: 
     SELECT DEP --  NBR, SUM(SALES), SUM(VALUE(LENGTH(COST),0+1,0)) 
     FROM SALES --  INFO 
     GROUP BY DEP --  NBR; 
     Eventually, when the runner 67 receives intermediate answer sets, it merges the data, computes the aggregate functions, applies the suppressed HAVING clause and builds an answer set. 
     The runner 67 controls parallel query execution. It submits tasks to selected DBMSs, processes the result sets returned by participating DBMSs and produces answer sets satisfying the source query. 
     Like other components of the query processor 1 the runner 67 is driven by information organized in the form of a J-tree (T). Initially, the runner 67 traverses T, submitting a request for every query specification encoded by T. It issues SQL requests to selected DBMSs and awaits results. When the first row of an answer set arrives, it starts processing data. 
     What it does then depends on T. For an unsplit straightforward end-user request, the runner 67 relays output rows more or less unchanged to the client; for an unordered UNION ALL the runner 67 merges answer sets; for an ordered UNION, it is obliged to remove duplicates as it merges; for a query with a HAVING clause, it may apply SQL aggregate functions to column values; for a distributed join, it combines rows that satisfy a specified join condition; and finally, for an uncorrelated sub-query, the runner 67 writes a result (a value or file reference) into a designated memory location. 
     It is to be appreciated that the foregoing is a description of a preferred embodiment of the invention to which variations and modifications may be made without departing from the spirit and scope of the invention.