122 lines
7.6 KiB
HTML
122 lines
7.6 KiB
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<meta name="DC.Title" content="Encoded vector index symbol table scan" />
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<meta name="abstract" content="An encoded vector index symbol table scan operation is used to retrieve the entries from the symbol table portion of the index." />
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<title>Encoded vector index symbol table scan</title>
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</head>
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<body id="rzajqeviscan"><a name="rzajqeviscan"><!-- --></a>
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<img src="./delta.gif" alt="Start of change" /><!-- Java sync-link --><script language="Javascript" src="../rzahg/synch.js" type="text/javascript"></script>
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<h1 class="topictitle1">Encoded vector index symbol table scan</h1>
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<div><p>An encoded vector index symbol table scan operation is used to
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retrieve the entries from the symbol table portion of the index. </p>
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<div class="section"><p>All entries (symbols) in the symbol table will be sequentially
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scanned, though the sequence of the resulting entries is not in any guaranteed
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order. The symbol table can be used by the optimizer to satisfy group by or
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distinct portions of a query request. Any selection is applied to every entry
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in the symbol table. All entries are retrieved directly from the symbol table
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portion of the index without any access to the vector portion of the index
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nor any access to the records in the associated table over which the EVI is
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built.</p>
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<div class="tablenoborder"><table cellpadding="4" cellspacing="0" summary="" frame="border" border="1" rules="all"><caption>Table 1. Encoded vector index symbol table scan attributes</caption><thead align="left"><tr><th valign="top" width="30.76923076923077%" id="d0e40">Data access method</th>
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<th valign="top" width="69.23076923076923%" id="d0e42">Encoded vector index symbol table scan</th>
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</tr>
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</thead>
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<tbody><tr><td valign="top" width="30.76923076923077%" headers="d0e40 "><strong>Description</strong></td>
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<td valign="top" width="69.23076923076923%" headers="d0e42 ">Sequentially scan and process all of the symbol table
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entries associated with the index. Any selection is applied to every entry
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in the symbol table. Selected entries are retrieved directly without any access
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to the vector or the associated table</td>
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</tr>
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<tr><td valign="top" width="30.76923076923077%" headers="d0e40 "><strong>Advantages</strong></td>
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<td valign="top" width="69.23076923076923%" headers="d0e42 "><ul><li>Pre-summarized results are readily available</li>
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<li>Only processes the unique values in the symbol table, avoiding processing
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table records.</li>
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<li>Extract all of the data from the index unique key values, thus eliminating
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the need for a Table Probe or vector scan.</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="30.76923076923077%" headers="d0e40 "><strong>Considerations</strong></td>
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<td valign="top" width="69.23076923076923%" headers="d0e42 ">Dramatic performance improvement for grouping queries
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where the resulting number of groups is relatively small compared to the number
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of records in the underlying table. Can perform poorly when there are a large
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number of groups involved such that the symbol table is very large, especially
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if a large portion of symbol table has been put into the overflow area.</td>
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</tr>
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<tr><td valign="top" width="30.76923076923077%" headers="d0e40 "><strong>Likely to be used</strong></td>
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<td valign="top" width="69.23076923076923%" headers="d0e42 "><ul><li>When asking for GROUP BY, DISTINCT, COUNT or COUNT DISTINCT from a single
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table and the referenced column(s) are in the key definition </li>
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<li>When the number of unique values in the column(s) of the key definition
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is small relative to the number of records in the underlying table.</li>
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<li>When there is no selection (Where clause) within the query or the selection
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does not reduce the result set very much.</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="30.76923076923077%" headers="d0e40 "><strong>Example SQL statement</strong></td>
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<td valign="top" width="69.23076923076923%" headers="d0e42 "><pre>CREATE ENCODED VECTOR INDEX EVI1 ON Sales (Region) </pre>
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Example 1<pre>SELECT Region, count(*)
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FROM Sales
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GROUP BY Region
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OPTIMIZE FOR ALL ROWS</pre>
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Example 2<pre>SELECT DISTINCT Region
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FROM Sales
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OPTIMIZE FOR ALL ROWS</pre>
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Example 3<pre>SELECT COUNT(DISTINCT Region)
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FROM Sales </pre>
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</td>
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</tr>
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<tr><td valign="top" width="30.76923076923077%" headers="d0e40 "><strong>Messages indicating use</strong></td>
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<td valign="top" width="69.23076923076923%" headers="d0e42 "><ul><li>Optimizer Debug: <pre>CPI4328 -- Access path of file EVI1 was used by query. </pre>
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</li>
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<li>PRTSQLINF: <pre>SQL4008 -- Index EVI1 used for table 1.SQL4010</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="30.76923076923077%" headers="d0e40 "><strong>SMP parallel enabled</strong></td>
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<td valign="top" width="69.23076923076923%" headers="d0e42 ">No. Typically not critical as the 'grouping'
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has already been performed during the index build.</td>
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</tr>
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<tr><td valign="top" width="30.76923076923077%" headers="d0e40 "><strong>Also referred to as</strong></td>
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<td valign="top" width="69.23076923076923%" headers="d0e42 "> Encoded Vector Index table scan, Preload</td>
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</tr>
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<tr><td valign="top" width="30.76923076923077%" headers="d0e40 "><strong>Visual Explain icon</strong></td>
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<td valign="top" width="69.23076923076923%" headers="d0e42 "><br /><img src="rzajq557.gif" alt="Encoded vector index probe 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>
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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="perf21c.htm" title="An encoded vector index is a permanent object that provides access to a table by assigning codes to distinct key values and then representing those values in a vector.">Encoded vector index</a></div>
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</div>
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</div>
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<img src="./deltaend.gif" alt="End of change" /></body>
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