112 lines
6.4 KiB
HTML
112 lines
6.4 KiB
HTML
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<meta name="DC.Title" content="Optimizing grouping by using index skip key processing" />
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<meta name="abstract" content="Index Skip Key processing can be used when grouping with the keyed sequence implementation algorithm which uses an existing index. It is a specialized version of ordered grouping that processes very few records in each group rather than all records in each group." />
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<meta name="description" content="Index Skip Key processing can be used when grouping with the keyed sequence implementation algorithm which uses an existing index. It is a specialized version of ordered grouping that processes very few records in each group rather than all records in each group." />
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<title>Optimizing grouping by using index skip key processing</title>
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<body id="rzajqgroupidx"><a name="rzajqgroupidx"><!-- --></a>
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<!-- Java sync-link --><script language="Javascript" src="../rzahg/synch.js" type="text/javascript"></script>
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<h1 class="topictitle1">Optimizing grouping by using index skip key processing</h1>
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<div><p>Index Skip Key processing can be used when grouping with the keyed
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sequence implementation algorithm which uses an existing index. It is a specialized
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version of ordered grouping that processes very few records in each group
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rather than all records in each group.</p>
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<div class="section"><p>The index skip key processing algorithm:</p>
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</div>
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<div class="section"> <ol><li>Uses the index to position to a group and</li>
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<li>finds the first row matching the selection criteria for the group, and
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if specified the first non-null MIN or MAX value in the group</li>
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<li>Returns the group to the user</li>
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<li>"Skip" to the next group and repeat processing</li>
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</ol>
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</div>
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<div class="section"><p>This will improve performance by potentially not processing all
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index key values for a group.</p>
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</div>
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<div class="section"><p>Index skip key processing can be used: </p>
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<ul><li>For single table queries using the keyed sequence grouping implementation
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when: <ul><li>There are no column functions in the query, or</li>
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<li>There is only a single MIN or MAX column function in the query and the
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operand of the MIN or MAX is the next key column in the index after the grouping
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columns. There can be no other grouping functions in the query. For the
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MIN function, the key column must be an ascending key; for the MAX function,
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the key column must be a descending key. If the query is whole table grouping,
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the operand of the MIN or MAX must be the first key column. <div class="p">Example 1,
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using SQL: <pre><strong>CREATE INDEX</strong> IX1 <strong>ON</strong> EMPLOYEE (SALARY DESC)
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<strong>DECLARE</strong> C1 <strong>CURSOR FOR</strong>
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<strong>SELECT MAX</strong>(SALARY) <strong>FROM</strong> EMPLOYEE;</pre>
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</div>
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<p>The query optimizer will chose to use the index IX1. The
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SLIC runtime code will scan the index until it finds the first non-null value
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for SALARY. Assuming that SALARY is not null, the runtime code will position
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to the first index key and return that key value as the MAX of salary. No
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more index keys will be processed.</p>
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<div class="p">Example 2, using SQL: <pre><strong>CREATE INDEX</strong> IX2 <strong>ON</strong> EMPLOYEE (DEPT, JOB,SALARY)
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<strong>DECLARE</strong> C1 <strong>CURSOR FOR</strong>
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<strong>SELECT</strong> DEPT, <strong>MIN</strong>(SALARY)
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<strong>FROM</strong> EMPLOYEE
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<strong>WHERE</strong> JOB='CLERK'
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<strong>GROUP BY</strong> DEPT</pre>
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</div>
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<p>The query optimizer will chose to use Index IX2. The database
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manager will position to the first group for DEPT where JOB equals 'CLERK'
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and will return the SALARY. The code will then skip to the next DEPT group
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where JOB equals 'CLERK'.</p>
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</li>
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</ul>
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</li>
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<li>For join queries: <ul><li>All grouping columns must be from a single table.</li>
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<li>For each dial there can be at most one MIN or MAX column function operand
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that references the dial and no other column functions can exist in the query.</li>
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<li>If the MIN or MAX function operand is from the same dial as the grouping
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columns, then it uses the same rules as single table queries.</li>
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<li>If the MIN or MAX function operand is from a different dial then the join
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column for that dial must join to one of the grouping columns and the index
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for that dial must contain the join columns followed by the MIN or MAX operand. <p>Example
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1, using SQL: </p>
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<pre><strong>CREATE INDEX</strong> IX1 <strong>ON</strong> DEPARTMENT(DEPTNAME)
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<strong>CREATE INDEX</strong> IX2 <strong>ON</strong> EMPLOYEE(WORKDEPT, SALARY)
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<strong>DECLARE</strong> C1 <strong>CURSOR FOR</strong>
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<strong>SELECT</strong> DEPTNAME, <strong>MIN</strong>(SALARY)
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<strong>FROM</strong> DEPARTMENT, EMPLOYEE
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<strong>WHERE</strong> DEPARTMENT.DEPTNO=EMPLOYEE.WORKDEPT
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<strong>GROUP BY</strong> DEPARTMENT.DEPTNO;
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</pre>
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</li>
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</ul>
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</li>
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</ul>
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</div>
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</div>
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<div>
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<div class="familylinks">
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<div class="parentlink"><strong>Parent topic:</strong> <a href="groupopt.htm" title="DB2 Universal Database for iSeries has certain techniques to use when the optimizer encounters grouping. The query optimizer chooses its methods for optimizing your query.">Grouping optimization</a></div>
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</div>
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</div>
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</body>
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</html>
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