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eadd35f6d5
The recent documentation formatting changes introduced the navigation panel on the left. However, due to the length of the query options navigation title these could overlap with the documentation paragraphs. This commit removes the underscores from the navigation titles of the query options, so browsers can break them into multiple lines. Additionally, the "SET" and "Query Options for the SET Statement" pages are merged to save some more space for the query option navigation titles. Testing: - Built the documentation and tested manually Change-Id: Icec787d7a2af848aaaff65be2ecf311a5ce8fe7f Reviewed-on: http://gerrit.cloudera.org:8080/20556 Tested-by: Impala Public Jenkins <impala-public-jenkins@cloudera.com> Reviewed-by: Jason Fehr <jfehr@cloudera.com> Reviewed-by: Peter Rozsa <prozsa@cloudera.com> Reviewed-by: Tamas Mate <tmater@apache.org>
96 lines
3.9 KiB
XML
96 lines
3.9 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="2.10.0" id="disable_codegen_rows_threshold">
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<title>DISABLE_CODEGEN_ROWS_THRESHOLD Query Option (<keyword keyref="impala210_full"/> or higher only)</title>
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<titlealts audience="PDF"><navtitle>DISABLE CODEGEN ROWS THRESHOLD</navtitle></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="Scalability"/>
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<data name="Category" value="Performance"/>
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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 rev="2.0.0"> This setting controls the cutoff point (in terms of number
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of rows processed per Impala daemon) below which Impala disables native
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code generation for the whole query. Native code generation is very
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beneficial for queries that process many rows because it reduces the time
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taken to process of each row. However, generating the native code adds
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latency to query startup. Therefore, automatically disabling codegen for
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queries that process relatively small amounts of data can improve query
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response time. </p>
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<p conref="../shared/impala_common.xml#common/syntax_blurb"/>
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<codeblock>SET DISABLE_CODEGEN_ROWS_THRESHOLD=<varname>number_of_rows</varname></codeblock>
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<p>
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<b>Type:</b> numeric
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</p>
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<p>
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<b>Default:</b> 50000
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</p>
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<p>
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<b>Usage notes:</b> Typically, you increase the default value to make this optimization apply to more queries.
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If incorrect or corrupted table and column statistics cause Impala to apply this optimization incorrectly to
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queries that actually involve substantial work, you might see the queries being slower as a result of codegen
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being disabled. In that case, recompute statistics with the <codeph>COMPUTE STATS</codeph> or
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<codeph>COMPUTE INCREMENTAL STATS</codeph> statement. If there is a problem collecting accurate statistics,
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you can turn this feature off by setting the value to 0.
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</p>
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<p conref="../shared/impala_common.xml#common/internals_blurb"/>
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<p>
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This setting applies to queries where the number of rows processed can be accurately
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determined, either through table and column statistics, or by the presence of a
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<codeph>LIMIT</codeph> clause. If Impala cannot accurately estimate the number of rows,
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then this setting does not apply.
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</p>
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<p rev="2.3.0">
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If a query uses the complex data types <codeph>STRUCT</codeph>, <codeph>ARRAY</codeph>,
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or <codeph>MAP</codeph>, then codegen is never automatically disabled regardless of the
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<codeph>DISABLE_CODEGEN_ROWS_THRESHOLD</codeph> setting.
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</p>
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<p conref="../shared/impala_common.xml#common/added_in_2100"/>
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<!-- Don't have any other places that tie into this particular optimization technique yet.
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Potentially: conceptual topics about code generation, distributed queries
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<p conref="../shared/impala_common.xml#common/related_info"/>
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<p>
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</p>
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-->
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</conbody>
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</concept>
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