Patent Application
20050065926
1. Field of the Invention
This invention relates in general to database management systems performed by computers, and in particular, to query transformation for multiple UNION ALL view join queries using join predicates for pruning and distribution.
2. Description of Related Art
Computer systems incorporating Relational DataBase Management System (RDBMS) software using Structured Query Language (SQL) interface are well known in the art. The SQL interface has evolved into a standard language for RDBMS software and has been adopted as such by both the American National Standards Institute (ANSI) and the International Standards Organization (ISO).
For most RDBMS software, combinations of tables and views are used to access data stored in tables in the database. A view definition includes a query that, if processed, provides a temporary result table based on the results of the query at that point in time.
To enhance maintainability, usability, and performance in database applications with very large data volumes, a UNION ALL view feature has been widely used to partition large tables into smaller tables. A typical partitioning mechanism is based on time. For example, data can be organized into monthly tables, and a UNION ALL view of these monthly tables is used to provide a logical view of all data. Some sophisticated applications have many views with UNION ALL, and join these views in queries.
Existing query optimization techniques will use local predicates in a query to prune useless branches of a UNION ALL view. Moreover, the distribution will generate joins for the remaining branches. These techniques may not be effective for queries with joins of multiple UNION ALL views, for the following reasons.
Although each branch of a UNION ALL view has a local predicate, such as YEAR=2001, the referencing query may not have a local predicate on the view, or it may only have local predicates on other columns. This makes pruning based on local predicates useless, and none of the branches can be pruned.
The distribution transformation will generate joins of branches of the joined views. The number of joins is the product of the number of the branches of the joined views.
Consider the distribution of A*B*C, wherein A, B, C are UNION ALL views of four base tables each, A=(A1+A2+A3+A4), B=(B1+B2+B3+B4), C=(C1+C2+C3+C4), * indicates a join and + indicates a UNION ALL. Performing A*B*C without sub-select pruning due to lack of local predicates will result in:
(A1+A2+A3+A4)*(B1+B2+B3+B4)*C1+
(A1+A2+A3+A4)*(B1+B2+B3+B4)*C2+
(A1+A2+A3+A4)*(B1+B2+B3+B4)*C3+
(A1+A2+A3+A4)*(B1+B2+B3+B4)*C4
after just distributing A*B into view C. The complete list, after performing distribution into view B and then view A, would contain 64 branches of the UNION ALL view.
When the number of branches of the UNION ALL views gets a little larger, this creates two major problems during the compilation (bind) time. First, it exceeds the query processing limits, the processing cannot continue, or if the limit is not exceeded, it consumes excessive storage at the bind time. Second, bind time is excessively long.
Among the joins generated, many of them are non-overlapping and produce empty results. For example, a branch with YEAR=2001 of view A joining a branch with YEAR=2002 of view B results in an empty result. All the resources at both the bind time and the run time on these empty-result joins are wasted.
Thus, there is a need in the art for a solution for this class of multiple UNION ALL view join queries that solves the above shortcomings. The present invention satisfies these needs.
To overcome the limitations in the prior art described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the present invention discloses a method, apparatus, and article of manufacture for optimizing a query in a computer system, wherein the query is performed by the computer system to retrieve data from a database stored on the computer system. The optimization includes: (a) combining join predicates from a query with local predicates from each branch of one or more UNION ALL views referenced by the query; (b) analyzing the combined predicates; and (c) not generating the join when the analysis step indicates that the combined predicates lead to an empty result for the join.
Referring now to the drawings in which like reference numbers represent corresponding parts throughout:
In the following description of the preferred embodiment, reference is made to the accompanying drawings, which form a part hereof, and in which is shown by way of illustration a specific embodiment in which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural and functional changes may be made without departing from the scope of the present invention.
Operators of the client systems 102 use a standard operator interface 108 to transmit commands to and from the server system 100 that represent commands for performing various search and retrieval functions, termed queries, against the databases. In the present invention, these queries conform to the Structured Query Language (SQL) standard, and invoke functions performed by Relational DataBase Management System (RDBMS) software. In the preferred embodiment of the present invention, the RDBMS software comprises the DB2 product offered by IBM for the MVS, LINUX, UNIX, WINDOWS or OS/2 operating systems. Those skilled in the art will recognize, however, that the present invention has application to any RDBMS software.
As illustrated in
At the heart of the RDBMS architecture is the Database Services module 114. The Database Services module 114 contains several submodules, including a Relational Database System (RDS) 116, Data Manager 118, Buffer Manager 120, and SQL Interpreter 122. These submodules support the functions of the SQL language, i.e., definition, access control, retrieval, and update of user and system data.
Generally, each of the components, modules, and submodules of the RDBMS comprises instructions and/or data, and are embodied in or retrievable from a computer-readable device, medium, or carrier, e.g., a memory, a data storage device, a remote device coupled to the server computer 100 by a data communications device, etc. Moreover, these instructions and/or data, when read, executed, and/or interpreted by the server computer 100, cause the server computer 100 to perform the steps necessary to implement and/or use the present invention.
Thus, the present invention may be implemented as a method, apparatus, or article of manufacture using standard programming and/or engineering techniques to produce software, firmware, hardware, or any combination thereof. The term “article of manufacture”, or alternatively, “computer program carrier”, as used herein is intended to encompass a computer program accessible from any computer-readable device, carrier, or media.
Of course, those skilled in the art will recognize many modifications may be made to this configuration without departing from the scope of the present invention. Specifically, those skilled in the art will recognize that any combination of the above components, or any number of different components, including computer programs, peripherals, and other devices, may be used to implement the present invention, so long as similar functions are performed thereby.
The present invention discloses an improved optimization technique that is typically performed at step 202 of
The method performed at step 202 of
For a query that contains a join of n views, each of which is a union of m base tables, the query would have been expanded into a union of mn n-table joins with prior art techniques. With the method proposed herein, however, this number mn could be dramatically reduced, even possibly to m in some cases.
Moreover, the technique of the present invention reduces both space and time at the bind time, and also eliminates the need to evaluate the empty joins at the runtime.
Assumptions
The present invention is best described by example. Consider an SQL query (or a query block in a larger query) of the following form:
1. The above query block joins two or more UNION ALL views. It can optionally contain other views, tables or table expressions, in the FROM clause, as part of the overall join.
2. Each UNION ALL view contains a UNION ALL of one base table with a local predicate specifying a data range in the base table. (For simplicity of description, it is assumed that there is one table in each branch, but the technique also applies to cases with two or more base tables in a branch).
3. The join predicates are on columns of UNION ALL views that correspond to the local predicates.
4. The query has one of the following characteristics:
Block 400 is a decision block that represents a loop through all query blocks. For each query block, control transfers to Block 402. Upon the end of the loop, the logic exits.
Block 402 is a decision block that determines whether the query block contains multiple views or table expressions (wherein the term “view” is used in the following discussion to refer to either a view or a table expression) with the UNION ALL patterns defined above (i.e., Cases A, B or C). If not, control transfers back to Block 400; otherwise, control transfers to
Referring now to
Block 502 is a decision block that determines whether the query block is Case B, as described above, based on the select-list and the GROUP BY of the query block. If so, control transfers to Block 504; otherwise, control transfers to Block 506.
Block 504 represents the processing of query blocks that are Case B, wherein a new query block is created that includes a DISTINCT modifier with the same select-list as in the original query block. This new query block is designated as the parent query block for the new UNION ALL. Thereafter, control transfers to Block 510.
Block 506 is a decision block that determines whether the query block is Case C, as described above, based on the select-list and the GROUP BY of the query block. If so, control transfers to Block 508; otherwise, control transfers to Block 510.
Block 508 represents the processing of query blocks that are Case C, wherein a regrouping query block is created with the regrouping select-list and the regrouping GROUP BY clause derived from the original query block, and the select-list is used for distribution. Thereafter, control transfers to Block 510.
Block 510 represents generating a UNION ALL of all the resultant query blocks for overlapping joins (with the understanding that if there is only one result overlapping join, there is no need for a UNION ALL). Thereafter, control transfers to
Referring now to
Blocks 602-614 represent pruning logic that determines whether the combined predicates are contradictory.
Block 602 represents storing all applicable predicates (intervals) on a column into a list from local predicates on columns of the branches, through column correspondence and join predicate transitive closure.
Block 604 is a decision block that determines whether the predicates always evaluate to FALSE (indicating that the predicates are contradictory). If so, then no join is generated and control transfers to Block 600; otherwise, control transfers to Block 606.
Block 606 represents generating a join for the branch if the predicates are not contradictory as follows:
A) The FROM clause includes branches in place of views, and the rest of the FROM clause of the original view referencing query block is copied;
B) The distribution select-list with column mappings (references to views are mapped to references to branches) is copied;
C) The WHERE predicate from the branches, and the WHERE predicate from the original view referencing query block, with the column mapping (as above), are copied and connected with the AND Boolean operator; and
D) The GROUP BY clause with column mapping is copied.
Thereafter, control transfers to Block 608.
Block 608 is a decision block that determines whether this is the first query block. If not, then control transfers to Block 610; otherwise, control transfers to Block 600.
Block 610 represents placing the query block into the UNION ALL tree with previously constructed query block or UNION ALL tree. Thereafter, control transfers to Block 600.
Upon completion of the loop, Block 612 is a decision block that determines whether the query block is a Case B or C. If so, then control transfers to Block 614; otherwise, control transfers to Block 616.
Block 614 represents, once all the branches are generated, linking to the regrouping/DISTINCT query block. Thereafter, control transfers to Block 616.
Block 616 represents linking the query block constructed, either UNION ALL query block for Case A, or regrouping/DISTINCT query block for Case B and C, to the parent query block, in place of the original query block. Thereafter, control transfers to
More Details on the Pruning Logic
The pruning logic, given a list of predicates on a column, including those generated from column equivalence transitive closure, determines whether the predicates will always be FALSE.
Consider the example of predicates C=a, C>b, C<c. They are AND-connected. The method of determining whether the predicates will always be FALSE comprises the following:
1. Place the constants into a list of intervals: [a, a], (b, infinite), (−infinite, c), wherein ( ) indicate exclusive boundaries to the intervals and [] indicate inclusive boundaries to the intervals.
2. Loop through the list to consolidate the intervals. If the consolidated interval is empty, the predicate will always be FALSE, and the corresponding branch combination is pruned.
For example, consider [a, a] and (b, infinite). If a>b, then the consolidated interval is [a, a]; otherwise, if (a<=b), then the result interval is empty.
In another example, consider [a, a] and [−infinite, c). If a<c, then the result interval is [a, a]; otherwise, if a>=c, then the result interval is empty.
This logic only considers AND-connected predicates. To extend the logic to OR-connected predicates (or an IN-list) on a column, a hierarchical structure is needed. For OR-connected predicates, one entry in the AND-connected list is used, which points to a list of OR-connected intervals.
For example, C=a AND C>b AND C<c AND (C=d1 OR C=d2 OR C>=d3) can be referred to as:
When the logic addresses the OR-list, the current interval (or interval list) is AND-connected to each interval in the OR-list. The result is OR-connected. If all intervals are empty, then the result is FALSE always. If one interval remains, pruning cannot be performed.
For example, assume the result interval is [a, a] before the OR-list is encountered in the above example. The system considers [a, a] and [d1, d1], assuming a !=d1, then the result interval is empty. Then, the system considers [a, a] and [d2, d2], it is empty again (with a !=d2). Finally, the system considers [a, a] and [d3, infinite), if a>=d3, then [a, a] is the result interval and no pruning is possible. If a<d3, the result is an empty interval, and pruning is performed. In the case of multiple OR-connected predicates, nested loops are needed to iterate over all the combinations of intervals.
Example:
Note that the following query that joins these two UNION ALL views, UIVA and UIVB, can be rewritten with the proposed method as follows:
Case A:
The proposed method will rewrite the above query as follows:
Note that the original query is expanded into three sub-selects tied together with a UNION ALL. The predicates from the definition of the views, UIVA and UIVB, have been “bubbled up” into the referencing query and distributed appropriately to the branch that has the associated joins of the base tables. Without the application of this proposed method, the intermediate structure, in effect, will require storage space for nine sub-selects tied together with a UNION ALL, instead of three sub-selects.
Case B:
The proposed method will rewrite the above query as follows:
Case C:
Notice an extra table TC in the FROM clause. The proposed method will rewrite the above query as follows:
This concludes the description of the preferred embodiment of the invention. The following describes some alternative embodiments for accomplishing the present invention. For example, any type of computer, such as a mainframe, minicomputer, or personal computer, could be used with the present invention. In addition, any software program performing database queries with grouping and/or aggregation could benefit from the present invention.
In summary, the present invention discloses a method, apparatus, and article of manufacture for optimizing a query in a computer system, wherein the query is performed by the computer system to retrieve data from a database stored on the computer system. The optimization includes: (a) combining join predicates from a query with local predicates from each branch of one or more UNION ALL views referenced by the query; (b) analyzing the combined predicates; and (c) not generating the join when the analysis step indicates that the combined predicates lead to an empty result for the join.
The foregoing description of the preferred embodiment of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching.
