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f3820e6205
For history and tracking purposes, there are many instances of rev="CDH-1234" for various CDH- JIRA numbers. This produces no visible output, it's just FYI for the person editing the source. Removing all these now from the upstream doc source, so as not to have "CDH" all through the doc source files. Change-Id: I29089e5a31cd72e876b2ccb8375d1c10693c6aba Reviewed-on: http://gerrit.cloudera.org:8080/6349 Reviewed-by: Ambreen Kazi <ambreen.kazi@cloudera.com> Reviewed-by: John Russell <jrussell@cloudera.com> Tested-by: Impala Public Jenkins
74 lines
2.7 KiB
XML
74 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 id="max_scan_range_length">
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<title>MAX_SCAN_RANGE_LENGTH Query Option</title>
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<titlealts audience="PDF"><navtitle>MAX_SCAN_RANGE_LENGTH</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="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>
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<indexterm audience="hidden">MAX_SCAN_RANGE_LENGTH query option</indexterm>
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Maximum length of the scan range. Interacts with the number of HDFS blocks in the table to determine how many
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CPU cores across the cluster are involved with the processing for a query. (Each core processes one scan
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range.)
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</p>
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<p>
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Lowering the value can sometimes increase parallelism if you have unused CPU capacity, but a too-small value
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can limit query performance because each scan range involves extra overhead.
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</p>
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<p>
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Only applicable to HDFS tables. Has no effect on Parquet tables. Unspecified or 0 indicates backend default,
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which is the same as the HDFS block size for each table.
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</p>
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<p>
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Although the scan range can be arbitrarily long, Impala internally uses an 8 MB read buffer so that it can
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query tables with huge block sizes without allocating equivalent blocks of memory.
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</p>
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<p>
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<b>Type:</b> numeric
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</p>
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<p rev="IMPALA-3799">
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In <keyword keyref="impala27_full"/> and higher, the argument value can include unit specifiers,
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such as <codeph>100m</codeph> or <codeph>100mb</codeph>. In previous versions,
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Impala interpreted such formatted values as 0, leading to query failures.
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</p>
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<p>
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<b>Default:</b> 0
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</p>
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</conbody>
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</concept>
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