91 lines
6.0 KiB
HTML
91 lines
6.0 KiB
HTML
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<meta name="DC.Title" content="Encoded vector indexes" />
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<meta name="abstract" content="An encoded vector index (EVI) is an index object that is used by the query optimizer and database engine to provide fast data access in decision support and query reporting environments." />
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<title>Encoded vector indexes</title>
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<body id="whatareevi"><a name="whatareevi"><!-- --></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 indexes</h1>
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<div><p>An encoded vector index (EVI) is an index object that is used by
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the query optimizer and database engine to provide fast data access in decision
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support and query reporting environments. </p>
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<p>EVIs are a complementary alternative to existing index objects (binary
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radix tree structure - logical file or SQL index) and are a variation on bitmap
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indexing. Because of their compact size and relative simplicity, EVIs provide
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for faster scans of a table that can also be processed in parallel.</p>
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<p>An EVI is a data structure that is stored as two components:
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</p>
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<ul><li>The symbol table contains statistical and descriptive information about
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each distinct key value represented in the table. Each distinct key is assigned
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a unique code, either 1, 2 or 4 bytes in size.</li>
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<li>The vector is an array of codes listed in the same ordinal position as
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the rows in the table. The vector does not contain any pointers to the actual
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rows in the table.</li>
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</ul>
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<div class="section"><p>Advantages of EVIs:</p>
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<ul><li>Require less storage</li>
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<li>May have better build times than radix, especially if the number of unique
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values in the column(s) defined for the key is relatively small.</li>
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<li>Provide more accurate statistics to the query optimizer</li>
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<li>Considerably better performance for certain grouping types of queries</li>
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<li>Good performance characteristics for decision support environments.</li>
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</ul>
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</div>
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<div class="section"><p>Disadvantages of EVIs:</p>
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<ul><li>Cannot be used in ordering</li>
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<li>Use for grouping is specialized </li>
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<li>Use with joins always done in cooperation with hash table processing</li>
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<li>Some additional maintenance idiosyncrasies</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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<ul class="ullinks">
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<li class="ulchildlink"><strong><a href="howeviworks.htm">How the EVI works</a></strong><br />
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EVIs work in different ways for costing and implementation.</li>
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<li class="ulchildlink"><strong><a href="whentouseevis.htm">When to create EVIs</a></strong><br />
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There are several instances when you should consider creating EVIs.</li>
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<li class="ulchildlink"><strong><a href="evimaintenance.htm">EVI maintenance</a></strong><br />
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There are unique challenges to maintaining EVIs. The following table shows a progression of how EVIs are maintained and the conditions under which EVIs are most effective and where EVIs are least effective based on the EVI maintenance characteristics.</li>
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<li class="ulchildlink"><strong><a href="recommendationsforevi.htm">Recommendations for EVI use</a></strong><br />
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Encoded vector indexes are a powerful tool for providing fast data access in decision support and query reporting environments, but to ensure the effective use of EVIs, you should implement EVIs with the following guidelines:</li>
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</ul>
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<div class="familylinks">
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<div class="parentlink"><strong>Parent topic:</strong> <a href="efindex.htm" title="DB2 Universal Database for iSeries provides two basic means for accessing tables: a table scan and an index-based retrieval. Index-based retrieval is typically more efficient than table scan when less than 20% of the table rows are selected.">Creating an index strategy</a></div>
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</div>
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<div class="relinfo"><strong>Related information</strong><br />
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<div><a href="../db2/rbafzmstxcindx.htm">SQL Create Index statement</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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