forked from docs/doc-exports
Lu, Huayi
e6fa411af0
Reviewed-by: Pruthi, Vineet <vineet.pruthi@t-systems.com> Co-authored-by: Lu, Huayi <luhuayi@huawei.com> Co-committed-by: Lu, Huayi <luhuayi@huawei.com>
183 lines
16 KiB
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183 lines
16 KiB
HTML
<a name="EN-US_TOPIC_0000001188642164"></a><a name="EN-US_TOPIC_0000001188642164"></a>
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<h1 class="topictitle1">Configuration Parameter Hints</h1>
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<div id="body1608276293763"><div class="section" id="EN-US_TOPIC_0000001188642164__section6314183673614"><h4 class="sectiontitle">Function</h4><p id="EN-US_TOPIC_0000001188642164__p1890738133110">A hint, or a GUC hint, specifies a configuration parameter value when a plan is generated.</p>
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</div>
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<div class="section" id="EN-US_TOPIC_0000001188642164__section16613144810377"><h4 class="sectiontitle">Syntax</h4><pre class="screen" id="EN-US_TOPIC_0000001188642164__screen186956873815">set [global](guc_name guc_value)</pre>
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</div>
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<div class="section" id="EN-US_TOPIC_0000001188642164__section2332184743911"><h4 class="sectiontitle">Parameters</h4><ul id="EN-US_TOPIC_0000001188642164__ul721921684014"><li id="EN-US_TOPIC_0000001188642164__li169711246184711"><strong id="EN-US_TOPIC_0000001188642164__b7657141994918">global</strong> indicates that the parameter set by hint takes effect at the statement level. If <strong id="EN-US_TOPIC_0000001188642164__b266481984911">global</strong> is not specified, the parameter takes effect only in the subquery where the hint is located.</li><li id="EN-US_TOPIC_0000001188642164__li821910163408"><strong id="EN-US_TOPIC_0000001188642164__b725081172615">guc_name</strong> indicates the name of the configuration parameter specified by hint.</li></ul>
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<ul id="EN-US_TOPIC_0000001188642164__ul99552335403"><li id="EN-US_TOPIC_0000001188642164__li1195543315403"><strong id="EN-US_TOPIC_0000001188642164__b68648332610">guc_value</strong> indicates the value of a configuration parameter specified by hint.</li></ul>
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<div class="note" id="EN-US_TOPIC_0000001188642164__note2495444194116"><img src="public_sys-resources/note_3.0-en-us.png"><span class="notetitle"> </span><div class="notebody"><ul id="EN-US_TOPIC_0000001188642164__ul34026593410"><li id="EN-US_TOPIC_0000001188642164__li2071723511504">If a parameter set by hint takes effect at the statement level, the hint must be written to the top-level query instead of the subquery. For <strong id="EN-US_TOPIC_0000001188642164__b1426113610367">UNION</strong>, <strong id="EN-US_TOPIC_0000001188642164__b12353163920361">INTERSECT</strong>, <strong id="EN-US_TOPIC_0000001188642164__b740210416369">EXCEPT</strong>, and <strong id="EN-US_TOPIC_0000001188642164__b9688543113616">MINUS</strong> statements, you can write the GUC hint at the statement level to any <strong id="EN-US_TOPIC_0000001188642164__b8209155319366">SELECT</strong> clause that participates in the set operation. The configuration parameters set by the GUC hint take effect on each <strong id="EN-US_TOPIC_0000001188642164__b57201510123710">SELECT</strong> clause that participates in the set operation.</li><li id="EN-US_TOPIC_0000001188642164__li39421582428">When a subquery is pulled up, all GUC hints on the subquery are discarded.</li><li id="EN-US_TOPIC_0000001188642164__li234519247266">If a parameter is set by both the statement-level GUC hint and the subquery-level GUC hint, the subquery-level GUC hint takes effect in the corresponding subquery, and the statement-level GUC hint takes effect in other subqueries of the statement.</li></ul>
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</div></div>
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</div>
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<p id="EN-US_TOPIC_0000001188642164__p15317337123314">Currently, GUC hints support only some configuration parameters. Some parameters cannot be configured at the subquery level and can only be configured at the statement level. The following table lists the supported parameters.</p>
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<div class="tablenoborder"><table cellpadding="4" cellspacing="0" summary="" id="EN-US_TOPIC_0000001188642164__table9619938173819" frame="border" border="1" rules="all"><caption><b>Table 1 </b>Configuration parameters supported by GUC hints</caption><thead align="left"><tr id="EN-US_TOPIC_0000001188642164__row19619163818387"><th align="left" class="cellrowborder" valign="top" width="52.21%" id="mcps1.3.5.2.3.1.1"><p id="EN-US_TOPIC_0000001188642164__p16619173823819">Parameter</p>
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</th>
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<th align="left" class="cellrowborder" valign="top" width="47.79%" id="mcps1.3.5.2.3.1.2"><p id="EN-US_TOPIC_0000001188642164__p461915388381">Configured at the Subquery Level (Yes/No)</p>
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</th>
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</tr>
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</thead>
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<tbody><tr id="EN-US_TOPIC_0000001188642164__row86192389384"><td class="cellrowborder" valign="top" width="52.21%" headers="mcps1.3.5.2.3.1.1 "><p id="EN-US_TOPIC_0000001188642164__p16195382383">agg_redistribute_enhancement</p>
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</td>
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<td class="cellrowborder" valign="top" width="47.79%" headers="mcps1.3.5.2.3.1.2 "><p id="EN-US_TOPIC_0000001188642164__p1619338163814">Yes</p>
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</td>
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</tr>
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<tr id="EN-US_TOPIC_0000001188642164__row1961933819387"><td class="cellrowborder" valign="top" width="52.21%" headers="mcps1.3.5.2.3.1.1 "><p id="EN-US_TOPIC_0000001188642164__p10619238143816">best_agg_plan</p>
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</td>
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<td class="cellrowborder" valign="top" width="47.79%" headers="mcps1.3.5.2.3.1.2 "><p id="EN-US_TOPIC_0000001188642164__p186191938163817">Yes</p>
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</td>
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</tr>
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<tr id="EN-US_TOPIC_0000001188642164__row2619133816388"><td class="cellrowborder" valign="top" width="52.21%" headers="mcps1.3.5.2.3.1.1 "><p id="EN-US_TOPIC_0000001188642164__p1619173811386">cost_model_version</p>
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</td>
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<td class="cellrowborder" valign="top" width="47.79%" headers="mcps1.3.5.2.3.1.2 "><p id="EN-US_TOPIC_0000001188642164__p18619143883813">No</p>
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</td>
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</tr>
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<tr id="EN-US_TOPIC_0000001188642164__row461914389385"><td class="cellrowborder" valign="top" width="52.21%" headers="mcps1.3.5.2.3.1.1 "><p id="EN-US_TOPIC_0000001188642164__p561913823820">cost_param</p>
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</td>
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<td class="cellrowborder" valign="top" width="47.79%" headers="mcps1.3.5.2.3.1.2 "><p id="EN-US_TOPIC_0000001188642164__p146192383388">No</p>
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</td>
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</tr>
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<tr id="EN-US_TOPIC_0000001188642164__row661953816388"><td class="cellrowborder" valign="top" width="52.21%" headers="mcps1.3.5.2.3.1.1 "><p id="EN-US_TOPIC_0000001188642164__p26191938123810">enable_bitmapscan</p>
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</td>
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<td class="cellrowborder" valign="top" width="47.79%" headers="mcps1.3.5.2.3.1.2 "><p id="EN-US_TOPIC_0000001188642164__p3619438163810">Yes</p>
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</td>
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</tr>
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<tr id="EN-US_TOPIC_0000001188642164__row196191238153814"><td class="cellrowborder" valign="top" width="52.21%" headers="mcps1.3.5.2.3.1.1 "><p id="EN-US_TOPIC_0000001188642164__p1561993810384">enable_broadcast</p>
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</td>
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<td class="cellrowborder" valign="top" width="47.79%" headers="mcps1.3.5.2.3.1.2 "><p id="EN-US_TOPIC_0000001188642164__p196191938193810">Yes</p>
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</td>
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</tr>
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<tr id="EN-US_TOPIC_0000001188642164__row11619123815383"><td class="cellrowborder" valign="top" width="52.21%" headers="mcps1.3.5.2.3.1.1 "><p id="EN-US_TOPIC_0000001188642164__p196191638163814">enable_extrapolation_stats</p>
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</td>
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<td class="cellrowborder" valign="top" width="47.79%" headers="mcps1.3.5.2.3.1.2 "><p id="EN-US_TOPIC_0000001188642164__p12619103873810">Yes</p>
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</td>
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</tr>
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<tr id="EN-US_TOPIC_0000001188642164__row1161983816382"><td class="cellrowborder" valign="top" width="52.21%" headers="mcps1.3.5.2.3.1.1 "><p id="EN-US_TOPIC_0000001188642164__p1161993893812">enable_fast_query_shipping</p>
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</td>
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<td class="cellrowborder" valign="top" width="47.79%" headers="mcps1.3.5.2.3.1.2 "><p id="EN-US_TOPIC_0000001188642164__p2619193843817">No</p>
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</td>
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</tr>
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<tr id="EN-US_TOPIC_0000001188642164__row7619153812382"><td class="cellrowborder" valign="top" width="52.21%" headers="mcps1.3.5.2.3.1.1 "><p id="EN-US_TOPIC_0000001188642164__p9619153819388">enable_force_vector_engine</p>
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</td>
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<td class="cellrowborder" valign="top" width="47.79%" headers="mcps1.3.5.2.3.1.2 "><p id="EN-US_TOPIC_0000001188642164__p36192388388">No</p>
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</td>
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</tr>
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<tr id="EN-US_TOPIC_0000001188642164__row19619203843819"><td class="cellrowborder" valign="top" width="52.21%" headers="mcps1.3.5.2.3.1.1 "><p id="EN-US_TOPIC_0000001188642164__p361943819389">enable_hashagg</p>
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</td>
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<td class="cellrowborder" valign="top" width="47.79%" headers="mcps1.3.5.2.3.1.2 "><p id="EN-US_TOPIC_0000001188642164__p18619193817386">Yes</p>
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</td>
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</tr>
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<tr id="EN-US_TOPIC_0000001188642164__row16191338103818"><td class="cellrowborder" valign="top" width="52.21%" headers="mcps1.3.5.2.3.1.1 "><p id="EN-US_TOPIC_0000001188642164__p9620143820386">enable_hashjoin</p>
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</td>
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<td class="cellrowborder" valign="top" width="47.79%" headers="mcps1.3.5.2.3.1.2 "><p id="EN-US_TOPIC_0000001188642164__p1562010384383">Yes</p>
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</td>
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</tr>
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<tr id="EN-US_TOPIC_0000001188642164__row18620133813383"><td class="cellrowborder" valign="top" width="52.21%" headers="mcps1.3.5.2.3.1.1 "><p id="EN-US_TOPIC_0000001188642164__p5620153823816">enable_index_nestloop</p>
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</td>
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<td class="cellrowborder" valign="top" width="47.79%" headers="mcps1.3.5.2.3.1.2 "><p id="EN-US_TOPIC_0000001188642164__p1062023833813">Yes</p>
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</td>
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</tr>
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<tr id="EN-US_TOPIC_0000001188642164__row12620238103810"><td class="cellrowborder" valign="top" width="52.21%" headers="mcps1.3.5.2.3.1.1 "><p id="EN-US_TOPIC_0000001188642164__p1562033814382">enable_indexscan</p>
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</td>
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<td class="cellrowborder" valign="top" width="47.79%" headers="mcps1.3.5.2.3.1.2 "><p id="EN-US_TOPIC_0000001188642164__p86208383386">Yes</p>
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</td>
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</tr>
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<tr id="EN-US_TOPIC_0000001188642164__row862063813815"><td class="cellrowborder" valign="top" width="52.21%" headers="mcps1.3.5.2.3.1.1 "><p id="EN-US_TOPIC_0000001188642164__p166207381381">enable_join_pseudoconst</p>
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</td>
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<td class="cellrowborder" valign="top" width="47.79%" headers="mcps1.3.5.2.3.1.2 "><p id="EN-US_TOPIC_0000001188642164__p11620173893818">Yes</p>
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</td>
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</tr>
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<tr id="EN-US_TOPIC_0000001188642164__row106201638163812"><td class="cellrowborder" valign="top" width="52.21%" headers="mcps1.3.5.2.3.1.1 "><p id="EN-US_TOPIC_0000001188642164__p1562043816381">enable_nestloop</p>
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</td>
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<td class="cellrowborder" valign="top" width="47.79%" headers="mcps1.3.5.2.3.1.2 "><p id="EN-US_TOPIC_0000001188642164__p136201138193816">Yes</p>
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</td>
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</tr>
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<tr id="EN-US_TOPIC_0000001188642164__row3620113817386"><td class="cellrowborder" valign="top" width="52.21%" headers="mcps1.3.5.2.3.1.1 "><p id="EN-US_TOPIC_0000001188642164__p13620238143814">enable_nodegroup_debug</p>
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</td>
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<td class="cellrowborder" valign="top" width="47.79%" headers="mcps1.3.5.2.3.1.2 "><p id="EN-US_TOPIC_0000001188642164__p7620163853812">No</p>
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</td>
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</tr>
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<tr id="EN-US_TOPIC_0000001188642164__row1962093863817"><td class="cellrowborder" valign="top" width="52.21%" headers="mcps1.3.5.2.3.1.1 "><p id="EN-US_TOPIC_0000001188642164__p16620638193814">enable_partition_dynamic_pruning</p>
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</td>
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<td class="cellrowborder" valign="top" width="47.79%" headers="mcps1.3.5.2.3.1.2 "><p id="EN-US_TOPIC_0000001188642164__p106201938183816">Yes</p>
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</td>
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</tr>
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<tr id="EN-US_TOPIC_0000001188642164__row106201538153815"><td class="cellrowborder" valign="top" width="52.21%" headers="mcps1.3.5.2.3.1.1 "><p id="EN-US_TOPIC_0000001188642164__p76201938113818">enable_sort</p>
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</td>
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<td class="cellrowborder" valign="top" width="47.79%" headers="mcps1.3.5.2.3.1.2 "><p id="EN-US_TOPIC_0000001188642164__p5620143843815">Yes</p>
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</td>
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</tr>
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<tr id="EN-US_TOPIC_0000001188642164__row20620038113814"><td class="cellrowborder" valign="top" width="52.21%" headers="mcps1.3.5.2.3.1.1 "><p id="EN-US_TOPIC_0000001188642164__p5620183823814">enable_vector_engine</p>
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</td>
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<td class="cellrowborder" valign="top" width="47.79%" headers="mcps1.3.5.2.3.1.2 "><p id="EN-US_TOPIC_0000001188642164__p10620123883817">No</p>
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</td>
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</tr>
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<tr id="EN-US_TOPIC_0000001188642164__row11620183816381"><td class="cellrowborder" valign="top" width="52.21%" headers="mcps1.3.5.2.3.1.1 "><p id="EN-US_TOPIC_0000001188642164__p762018388381">expected_computing_nodegroup</p>
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</td>
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<td class="cellrowborder" valign="top" width="47.79%" headers="mcps1.3.5.2.3.1.2 "><p id="EN-US_TOPIC_0000001188642164__p3620143819382">No</p>
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</td>
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</tr>
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<tr id="EN-US_TOPIC_0000001188642164__row2620738183813"><td class="cellrowborder" valign="top" width="52.21%" headers="mcps1.3.5.2.3.1.1 "><p id="EN-US_TOPIC_0000001188642164__p15620183812385">force_bitmapand</p>
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</td>
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<td class="cellrowborder" valign="top" width="47.79%" headers="mcps1.3.5.2.3.1.2 "><p id="EN-US_TOPIC_0000001188642164__p10620123814384">Yes</p>
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</td>
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</tr>
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<tr id="EN-US_TOPIC_0000001188642164__row206201938203818"><td class="cellrowborder" valign="top" width="52.21%" headers="mcps1.3.5.2.3.1.1 "><p id="EN-US_TOPIC_0000001188642164__p19620638193816">from_collapse_limit</p>
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</td>
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<td class="cellrowborder" valign="top" width="47.79%" headers="mcps1.3.5.2.3.1.2 "><p id="EN-US_TOPIC_0000001188642164__p17620103873818">Yes</p>
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</td>
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</tr>
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<tr id="EN-US_TOPIC_0000001188642164__row1862023816386"><td class="cellrowborder" valign="top" width="52.21%" headers="mcps1.3.5.2.3.1.1 "><p id="EN-US_TOPIC_0000001188642164__p106201638133815">join_collapse_limit</p>
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</td>
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<td class="cellrowborder" valign="top" width="47.79%" headers="mcps1.3.5.2.3.1.2 "><p id="EN-US_TOPIC_0000001188642164__p6620738103813">Yes</p>
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</td>
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</tr>
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<tr id="EN-US_TOPIC_0000001188642164__row3620173811382"><td class="cellrowborder" valign="top" width="52.21%" headers="mcps1.3.5.2.3.1.1 "><p id="EN-US_TOPIC_0000001188642164__p136206382388">join_num_distinct</p>
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</td>
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<td class="cellrowborder" valign="top" width="47.79%" headers="mcps1.3.5.2.3.1.2 "><p id="EN-US_TOPIC_0000001188642164__p762043816383">Yes</p>
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</td>
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</tr>
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<tr id="EN-US_TOPIC_0000001188642164__row186206382385"><td class="cellrowborder" valign="top" width="52.21%" headers="mcps1.3.5.2.3.1.1 "><p id="EN-US_TOPIC_0000001188642164__p462053811380">qrw_inlist2join_optmode</p>
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</td>
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<td class="cellrowborder" valign="top" width="47.79%" headers="mcps1.3.5.2.3.1.2 "><p id="EN-US_TOPIC_0000001188642164__p1162016387382">Yes</p>
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</td>
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</tr>
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<tr id="EN-US_TOPIC_0000001188642164__row662073883817"><td class="cellrowborder" valign="top" width="52.21%" headers="mcps1.3.5.2.3.1.1 "><p id="EN-US_TOPIC_0000001188642164__p1262063813812">qual_num_distinct</p>
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</td>
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<td class="cellrowborder" valign="top" width="47.79%" headers="mcps1.3.5.2.3.1.2 "><p id="EN-US_TOPIC_0000001188642164__p7620103818383">Yes</p>
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</td>
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</tr>
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<tr id="EN-US_TOPIC_0000001188642164__row2620938173816"><td class="cellrowborder" valign="top" width="52.21%" headers="mcps1.3.5.2.3.1.1 "><p id="EN-US_TOPIC_0000001188642164__p14620203817387">query_dop</p>
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</td>
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<td class="cellrowborder" valign="top" width="47.79%" headers="mcps1.3.5.2.3.1.2 "><p id="EN-US_TOPIC_0000001188642164__p962023812382">No</p>
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</td>
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</tr>
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<tr id="EN-US_TOPIC_0000001188642164__row2621173814389"><td class="cellrowborder" valign="top" width="52.21%" headers="mcps1.3.5.2.3.1.1 "><p id="EN-US_TOPIC_0000001188642164__p362173873812">rewrite_rule</p>
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</td>
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<td class="cellrowborder" valign="top" width="47.79%" headers="mcps1.3.5.2.3.1.2 "><p id="EN-US_TOPIC_0000001188642164__p196211538143813">No</p>
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</td>
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</tr>
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<tr id="EN-US_TOPIC_0000001188642164__row56211738123814"><td class="cellrowborder" valign="top" width="52.21%" headers="mcps1.3.5.2.3.1.1 "><p id="EN-US_TOPIC_0000001188642164__p1562193814384">skew_option</p>
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</td>
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<td class="cellrowborder" valign="top" width="47.79%" headers="mcps1.3.5.2.3.1.2 "><p id="EN-US_TOPIC_0000001188642164__p462113811386">Yes</p>
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</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 class="section" id="EN-US_TOPIC_0000001188642164__section10533624154212"><h4 class="sectiontitle">Examples</h4><p id="EN-US_TOPIC_0000001188642164__p24781341174213">Hint the query plan in <a href="dws_04_0455.html#EN-US_TOPIC_0000001188642062__section671421102912">Examples</a> as follows:</p>
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<pre class="screen" id="EN-US_TOPIC_0000001188642164__screen1769012916445">explain
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select /*+ set global(query_dop 0) */ i_product_name product_name
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...</pre>
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<p id="EN-US_TOPIC_0000001188642164__p67181253164414">This hint indicates that the <strong id="EN-US_TOPIC_0000001188642164__b1177215233263">query_dop</strong> parameter is set to <strong id="EN-US_TOPIC_0000001188642164__b13778142382611">0</strong> when the plan for a statement is generated, which means the SMP adaptation function is enabled. The generated plan is as follows:</p>
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<p id="EN-US_TOPIC_0000001188642164__p58781332164513"><span><img id="EN-US_TOPIC_0000001188642164__image1287819329457" src="figure/en-us_image_0000001188642274.png"></span></p>
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<div class="familylinks">
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<div class="parentlink"><strong>Parent topic:</strong> <a href="dws_04_0454.html">Hint-based Tuning</a></div>
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