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https://issues.apache.org/jira/browse/HIVE-16972?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16064397#comment-16064397
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Chaozhong Yang commented on HIVE-16972:
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[~wzheng] Could you help me to review this patch？
> FetchOperator: filter out inputSplits which length is zero
> ----------------------------------------------------------
>
> Key: HIVE-16972
> URL: https://issues.apache.org/jira/browse/HIVE-16972
> Project: Hive
> Issue Type: Improvement
> Components: HiveServer2, Physical Optimizer, Query Planning
> Affects Versions: 2.1.0, 2.1.1
> Reporter: Chaozhong Yang
> Assignee: Chaozhong Yang
> Fix For: 2.1.2
>
> Attachments: HIVE-16972.patch
>
>
> * Background
> We can describe the basic work flow of common HQL query as follows:
> 1. compile and execute
> 2. fetch results
> In many cases, we don't need to worry about the issues fetching results
> from HDFS(iff there are mapreduce jobs generated in planning step). However,
> the number of results files on HDFS and data distribution will affect the
> final status of HQL query, especially for HiveServer2. We have some map-only
> queries, e.g:
> {code:sql}
> select * from myTable where date > '20170201' and date <= '20170301' and id =
> 88;
> {code}
> This query will generate more than 10,000 files on HDFS and most of those
> files are empty. Of course, they are very sparse. If we send
> TFetchResultsRequest from HiveServer2 client with some parameters(timeout:
> 90s, maxRows: 1024) , FetchOperator can not fetch 1024 rows in 90 seconds and
> our HiveServer2 client will mark this TFetchResultsRequest as timed out
> failure. Why? In fact, It's expensive to fetch results from empty file. In
> our HDFS cluster( 5000+ DataNodes) , reading data from an empty file will
> cost almost 100 ms (100ms * 1000 ==> 100s > 90s timeout). Obviously, we can
> filter out those empty files or splits to speed up the process of
> FetchResults.
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