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Change-Id: I42cb22a13b26bdc3d22585e4fc23934dfc91722a Reviewed-on: http://gerrit.cloudera.org:8080/10094 Reviewed-by: Tianyi Wang <twang@cloudera.com> Tested-by: Impala Public Jenkins <impala-public-jenkins@cloudera.com>
60 lines
1.8 KiB
XML
60 lines
1.8 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
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<!--
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Licensed to the Apache Software Foundation (ASF) under one
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or more contributor license agreements. See the NOTICE file
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distributed with this work for additional information
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regarding copyright ownership. The ASF licenses this file
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to you under the Apache License, Version 2.0 (the
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"License"); you may not use this file except in compliance
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with the License. You may obtain a copy of the License at
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http://www.apache.org/licenses/LICENSE-2.0
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Unless required by applicable law or agreed to in writing,
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software distributed under the License is distributed on an
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"AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
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KIND, either express or implied. See the License for the
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specific language governing permissions and limitations
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under the License.
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-->
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<!DOCTYPE concept PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
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<concept id="shuffle_distinct_exprs">
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<title>SHUFFLE_DISTINCT_EXPRS Query Option</title>
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<titlealts audience="PDF">
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<navtitle>SHUFFLE_DISTINCT_EXPRS</navtitle>
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</titlealts>
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<prolog>
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<metadata>
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<data name="Category" value="Impala"/>
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<data name="Category" value="Impala Query Options"/>
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</metadata>
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</prolog>
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<conbody>
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<p>
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The <codeph>SHUFFLE_DISTINCT_EXPRS</codeph> query option controls the
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shuffling behavior when a query has both grouping and distinct expressions.
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Impala can optionally include the distinct expressions in the hash exchange
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to spread the data among more nodes. However, this plan requires one more
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hash exchange phase.
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</p>
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<p>
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It is recommended that you turn off this option if the NDVs of the grouping
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expressions are high.
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</p>
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<p conref="../shared/impala_common.xml#common/type_boolean"/>
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<p conref="../shared/impala_common.xml#common/default_false"/>
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</conbody>
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</concept>
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