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134 lines
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<meta name="DC.Title" content="Buffer scan" />
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<meta name="abstract" content="The buffer scan operation is used when a query is processed using DB2 UDB Symmetric Multiprocessing, yet a portion of the query is not enabled to be processed under parallelism. The buffer scan acts as a gateway to control access to rows between the parallel enabled portions of the query and the non-parallel portions." />
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<meta name="DC.subject" content="definitions, buffer scan access method, temporary buffer, scan access method, buffer, access method, buffer scan" />
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<title>Buffer scan</title>
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<body id="rzajqbuffscan"><a name="rzajqbuffscan"><!-- --></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">Buffer scan</h1>
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<div><p>The buffer scan operation is used when a query is processed using DB2<sup>®</sup> UDB
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Symmetric Multiprocessing, yet a portion of the query is not enabled to be
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processed under parallelism. The buffer scan acts as a gateway to control
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access to rows between the parallel enabled portions of the query and the
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non-parallel portions.</p>
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<div class="section"><p>Multiple threads can be used to fetch the selected rows from the
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buffer, allowing the query to perform any remaining processing in parallel.
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However, the buffer will be populated in a non-parallel manner.</p>
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</div>
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<div class="section"><p>A buffer scan operation is identical to the list scan operation
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that is performed upon the temporary list object. The main difference is that
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a buffer does not need to be fully populated before the start of the scan
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operation. A temporary list requires that the list is fully populated before
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fetching any rows.</p>
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</div>
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<div class="section">
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<div class="tablenoborder"><table cellpadding="4" cellspacing="0" summary="" frame="border" border="1" rules="all"><caption>Table 1. Buffer scan attributes</caption><thead align="left"><tr><th valign="top" width="30.96446700507614%" id="d0e47">Data access method</th>
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<th valign="top" width="69.03553299492386%" id="d0e49">Buffer scan</th>
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</tr>
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</thead>
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<tbody><tr><td valign="top" width="30.96446700507614%" headers="d0e47 "><strong>Description</strong></td>
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<td valign="top" width="69.03553299492386%" headers="d0e49 ">Sequentially scan and process all of the rows in the
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temporary buffer. Enables SMP parallelism to be performed over a non-parallel
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portion of the query.</td>
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</tr>
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<tr><td valign="top" width="30.96446700507614%" headers="d0e47 "><strong>Advantages</strong></td>
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<td valign="top" width="69.03553299492386%" headers="d0e49 "><ul><li>The temporary buffer can be used to enable parallelism over a portion
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of a query that is non-parallel</li>
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<li>The temporary buffer does not need to be fully populated in order to start
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fetching rows</li>
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</ul>
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</td>
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</tr>
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<tr><td valign="top" width="30.96446700507614%" headers="d0e47 "><strong>Considerations</strong></td>
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<td valign="top" width="69.03553299492386%" headers="d0e49 ">Generally used to prevent portions of the query from
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being processed multiple times when no key columns are required to satisfy
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the request. </td>
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</tr>
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<tr><td valign="top" width="30.96446700507614%" headers="d0e47 "><strong>Likely to be used</strong></td>
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<td valign="top" width="69.03553299492386%" headers="d0e49 "><ul><li>When the query is attempting to take advantage of DB2 UDB Symmetric Multiprocessing</li>
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<li>When a portion of the query cannot be performed in parallel (for example,
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index scan or index probe)</li>
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</ul>
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</td>
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</tr>
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<tr><td valign="top" width="30.96446700507614%" headers="d0e47 "><strong>Example SQL statement</strong></td>
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<td valign="top" width="69.03553299492386%" headers="d0e49 "><pre>CHGQRYA DEGREE(*OPTIMIZE)
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CREATE INDEX X1 ON
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Employee (LastName, WorkDept)
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SELECT * FROM Employee
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WHERE WorkDept BETWEEN 'A01' AND 'E01'
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AND LastName IN ('Smith', 'Jones', 'Peterson')
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OPTIMIZE FOR ALL ROWS</pre>
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</td>
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</tr>
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<tr><td valign="top" width="30.96446700507614%" headers="d0e47 "><strong>Messages indicating use</strong></td>
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<td valign="top" width="69.03553299492386%" headers="d0e49 "><ul><li>Optimizer Debug: <pre>CPI4328 -- Access path of file X1 was used by query.
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CPI4330 -- 8 tasks used for parallel index scan
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of file EMPLOYEE.</pre>
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</li>
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<li>PRTSQLINF: <pre>SQL4027 -- Access plan was saved with DB2 UDB
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SMP installed on the system.
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SQL4008 -- Index X1 used for table 1.
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SQL4011 -- Index scan-key row positioning
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used on table 1.
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SQL4030 -- 8 tasks specified for parallel scan
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on table 1.</pre>
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</li>
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</ul>
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</td>
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</tr>
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<tr><td valign="top" width="30.96446700507614%" headers="d0e47 "><strong>SMP parallel enabled</strong></td>
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<td valign="top" width="69.03553299492386%" headers="d0e49 ">Yes</td>
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</tr>
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<tr><td valign="top" width="30.96446700507614%" headers="d0e47 "><strong>Also referred to as</strong></td>
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<td valign="top" width="69.03553299492386%" headers="d0e49 ">Not applicable</td>
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</tr>
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<tr><td valign="top" width="30.96446700507614%" headers="d0e47 "><strong>Visual Explain icon</strong></td>
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<td valign="top" width="69.03553299492386%" headers="d0e49 "><br /><img src="rzajq538.gif" alt="Buffer scan icon" /><br /></td>
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</tr>
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</tbody>
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</table>
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</div>
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</div>
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<div class="section"><p>Using the example above, the optimizer chose to use the existing
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index X1 to perform an index probe operation against the table. In order to
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speed up the remaining processing for this query (for example, the Table Probe
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operation), DB2 Symmetric
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Multiprocessing will be used to perform the random probe into the table. Since
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the index probe operation is not SMP parallel enabled for the SQL Query Engine,
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that portion of the query is placed within a temporary buffer to control access
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to the selected index entries. </p>
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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="rzajqtempbuff.htm" title="The temporary buffer is a temporary object that is used to help facilitate operations such as parallelism. It is an unsorted data structure that is used to store intermediate rows of a query. The main difference between a temporary buffer and a temporary list is that the buffer does not need to be fully populated in order to allow its results to be processed.">Temporary buffer</a></div>
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</div>
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</div>
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