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<h2><a href="https://docs.jboss.org/author/display/TEIID/Subquery+Optimization">Subquery Optimization</a></h2>
<h4>Page <b>edited</b> by <a href="https://docs.jboss.org/author/display/~shawkins">Steven Hawkins</a>
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<h4>Changes (1)</h4>
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<tr><td class="diff-snipped" >...<br></td></tr>
<tr><td class="diff-unchanged" >{code} <br> <br></td></tr>
<tr><td class="diff-changed-lines" >* The system property *org.teiid.subqueryUnnestDefault* controls whether the optimizer will by default unnest subqueries during rewrite. <span class="diff-deleted-words"style="color:#999;background-color:#fdd;text-decoration:line-through;">The default is false, since costing information is not consulted. </span> If true, then most non-negated WHERE or HAVING clause EXISTS or IN subquery predicates can be converted to a traditional join. <br></td></tr>
<tr><td class="diff-unchanged" > <br>* The planner will always convert to antijoin or semijoin variants if costing is favorable. Use a hint to override this behavior needed. <br></td></tr>
<tr><td class="diff-snipped" >...<br></td></tr>
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</div> <h4>Full Content</h4>
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<li>EXISTS subqueries are typically rewrite to "SELECT 1 FROM ..." to prevent unnecessary evaluation of SELECT expressions.</li>
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<li>Quantified compare SOME subqueries are always turned into an equivalent IN prediate or comparison against an aggregate value. e.g. col > SOME (select col1 from table) would become col > (select min(col1) from table)</li>
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<li>Uncorrelated EXISTs and scalar subquery that are not pushed to the source can be preevaluated prior to source command formation.</li>
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<li>Correlated subqueries used in DETELEs or UPDATEs that are not pushed as part of the corresponding DELETE/UPDATE will cause Teiid to perform row-by-row compensating processing. This will only happen if the affected table has a primary key. If it does not, then an exception will be thrown.</li>
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<ul>
<li>WHERE or HAVING clause IN, Quantified Comparison, Scalar Subquery Compare, and EXISTs predicates can take the MJ (merge join), DJ (dependent join), or NO_UNNEST (no unnest) hints appearing just before the subquery. The MJ hint directs the optimizer to use a traditional, semijoin, or antisemijoin merge join if possible. The DJ is the same as the MJ hint, but additional directs the optimizer to use the subquery as the independent side of a dependent join if possible. The NO_UNNEST hint, which supercedes the other hints, will direct the optimizer to leave the subquery in place.</li>
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<div class="code panel" style="border-width: 1px;"><div class="codeHeader panelHeader" style="border-bottom-width: 1px;"><b>Merge Join Hint Usage</b></div><div class="codeContent panelContent">
<pre class="theme: Confluence; brush: sql; gutter: false" style="font-size:12px; font-family: ConfluenceInstalledFont,monospace;">
SELECT col1 from tbl where col2 IN /*+ MJ*/ (SELECT col1 FROM tbl2)
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<div class="code panel" style="border-width: 1px;"><div class="codeHeader panelHeader" style="border-bottom-width: 1px;"><b>Dependent Join Hint Usage</b></div><div class="codeContent panelContent">
<pre class="theme: Confluence; brush: sql; gutter: false" style="font-size:12px; font-family: ConfluenceInstalledFont,monospace;">
SELECT col1 from tbl where col2 IN /*+ DJ */ (SELECT col1 FROM tbl2)
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<div class="code panel" style="border-width: 1px;"><div class="codeHeader panelHeader" style="border-bottom-width: 1px;"><b>No Unnest Hint Usage</b></div><div class="codeContent panelContent">
<pre class="theme: Confluence; brush: sql; gutter: false" style="font-size:12px; font-family: ConfluenceInstalledFont,monospace;">
SELECT col1 from tbl where col2 IN /*+ NO_UNNEST */ (SELECT col1 FROM tbl2)
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<ul>
<li>The system property <b>org.teiid.subqueryUnnestDefault</b> controls whether the optimizer will by default unnest subqueries during rewrite. If true, then most non-negated WHERE or HAVING clause EXISTS or IN subquery predicates can be converted to a traditional join.</li>
</ul>
<ul>
<li>The planner will always convert to antijoin or semijoin variants if costing is favorable. Use a hint to override this behavior needed.</li>
</ul>
<ul>
<li>EXISTs and scalar subqueries that are not pushed down, and not converted to merge joins, are implicitly limited to 1 and 2 result rows respectively.</li>
</ul>
<ul>
<li>Conversion of subquery predicates to nested loop joins is not yet available.</li>
</ul>
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