73 lines
4.9 KiB
HTML
73 lines
4.9 KiB
HTML
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<meta name="DC.Title" content="Table function considerations" />
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<meta name="abstract" content="An external table function is a UDF that delivers a table to the SQL in which it was referenced. A table function reference is only valid in a FROM clause of a SELECT statement." />
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<meta name="description" content="An external table function is a UDF that delivers a table to the SQL in which it was referenced. A table function reference is only valid in a FROM clause of a SELECT statement." />
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<title>Table function considerations</title>
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<body id="rbafytfconsider"><a name="rbafytfconsider"><!-- --></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">Table function considerations</h1>
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<div><p><span>An external table function is
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a UDF that delivers a table to the SQL in which it was referenced. A table
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function reference is only valid in a FROM clause of a SELECT statement.</span></p>
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<div class="section"><p>When using table functions, observe the following:</p>
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<ul><li>Even though a table function delivers a table, the physical interface
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between DB2<sup>®</sup> and
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the UDF is one-row-at-a-time. There are five types of calls made to a table
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function: OPEN, FETCH, CLOSE, FIRST, and FINAL. The existence of FIRST and
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FINAL calls depends on how you define the UDF. The same <em>call-type</em> mechanism
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that can be used for scalar functions is used to distinguish these calls.</li>
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<li>The standard interface used between DB2 and user-defined scalar functions is
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extended to accommodate table functions. The <em>SQL-result</em> argument repeats
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for table functions; each instance corresponding to a column to be returned
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as defined in the RETURNS TABLE clause of the CREATE FUNCTION statement. The <em>SQL-result-ind</em> argument
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likewise repeats, each instance related to the corresponding <em>SQL-result</em> instance.</li>
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<li>Not every result column defined in the RETURNS clause of the CREATE FUNCTION
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statement for the table function has to be returned. The DBINFO keyword of
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CREATE FUNCTION, and corresponding <em>dbinfo</em> argument enable the optimization
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that only those columns needed for a particular table function reference need
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be returned.</li>
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<li>The individual column values returned conform in format to the values
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returned by scalar functions.</li>
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<li>The CREATE FUNCTION statement for a table function has a CARDINALITY <em>n</em> specification.
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This specification enables the definer to inform the DB2 optimizer of the approximate size of
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the result so that the optimizer can make better decisions when the function
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is referenced. Regardless of what has been specified as the CARDINALITY of
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a table function, exercise caution against writing a function with infinite
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cardinality; that is, a function that always returns a row on a FETCH call. DB2 expects
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the <em>end-of-table</em> condition, as a catalyst within its query processing.
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So a table function that never returns the end-of-table condition (SQL-state
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value '02000') will cause an infinite processing loop.</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="rbafywudfextern.htm" title="You can write the executable code of a UDF in a language other than SQL.">Write UDFs as external functions</a></div>
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