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Change-Id: I731b26fe2c225e706454f16cd3b6de697ec70fe2 Reviewed-on: http://gerrit.cloudera.org:8080/11935 Reviewed-by: Alex Rodoni <arodoni@cloudera.com> Tested-by: Impala Public Jenkins <impala-public-jenkins@cloudera.com>
84 lines
2.7 KiB
XML
84 lines
2.7 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 rev="3.1.0" id="topn_bytes_limit">
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<title>TOPN_BYTES_LIMIT Query Option (<keyword keyref="impala31_full"/> or higher only)</title>
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<titlealts audience="PDF">
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<navtitle>TOPN_BYTES_LIMIT</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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<data name="Category" value="Querying"/>
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<data name="Category" value="Developers"/>
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<data name="Category" value="Data Analysts"/>
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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>TOPN_BYTES_LIMIT</codeph> query option places a limit on the amount of
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estimated memory that Impala can process for <i>top-N</i> queries.
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</p>
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<p>
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<i>Top-N</i> queries are the queries that include both <codeph>ORDER BY</codeph> and
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<codeph>LIMIT</codeph> clauses. <i>Top-N</i> queries don't spill to disk so they have to
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keep all rows they process in memory, and those queries can cause out-of-memory issues
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when running with a large limit and an offset. If the Impala planner estimates that a
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<i>top-N</i> operator will process more bytes than the <codeph>TOPN_BYTES_LIMIT</codeph>
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value, it will replace the <i>top-N</i> operator with the <i>sort</i> operator. Switching
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to the <i>sort</i> operator allows Impala to spill to disk, thus requiring less memory
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than <i>top-N</i>, but potentially with performance penalties.
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</p>
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<p>
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The option has no effect when set to 0 or -1.
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</p>
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<p>
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<b>Syntax:</b>
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</p>
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<codeblock>SET TOPN_BYTES_LIMIT=<varname>limit</varname></codeblock>
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<p>
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<b>Type:</b> Number
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</p>
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<p>
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<b>Default:</b> 536870912 (512 MB)
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</p>
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<p>
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<b>Added in:</b> <keyword keyref="impala31"/>
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</p>
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</conbody>
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</concept>
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