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From "Eric Chu (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HIVE-6134) Merging small files based on file size only works for CTAS queries
Date Mon, 06 Jan 2014 22:23:53 GMT

    [ https://issues.apache.org/jira/browse/HIVE-6134?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13863512#comment-13863512
] 

Eric Chu commented on HIVE-6134:
--------------------------------

[~xuefuz] Regarding when to merge, isn't that precisely what hive.merge.mapfiles, hive.merge.mapredfiles,
and hive.merge.smallfiles.avgsize are for? I never propose to merge files for every query.
Rather, I'm proposing to honor these properties for queries without move tasks as well. Users
won't need to decide when to merge; it'd be decided based on the configuration and the avg
output file size, just like queries that result in a new table.


> Merging small files based on file size only works for CTAS queries
> ------------------------------------------------------------------
>
>                 Key: HIVE-6134
>                 URL: https://issues.apache.org/jira/browse/HIVE-6134
>             Project: Hive
>          Issue Type: Bug
>    Affects Versions: 0.8.0, 0.10.0, 0.11.0, 0.12.0
>            Reporter: Eric Chu
>
> According to the documentation, if we set hive.merge.mapfiles to true, Hive will launch
an additional MR job to merge the small output files at the end of a map-only job when the
average output file size is smaller than hive.merge.smallfiles.avgsize. Similarly, by setting
hive.merge.mapredfiles to true, Hive will merge the output files of a map-reduce job. 
> My expectation is that this is true for all MR queries. However, my observation is that
this is only true for CTAS queries. In GenMRFileSink1.java, HIVEMERGEMAPFILES and HIVEMERGEMAPREDFILES
are only used if ((ctx.getMvTask() != null) && (!ctx.getMvTask().isEmpty())). So,
for a regular SELECT query that doesn't have move tasks, these properties are not used.
> Is my understanding correct and if so, what's the reasoning behind the logic of not supporting
this for regular SELECT queries? It seems to me that this should be supported for regular
SELECT queries as well. One scenario where this hits us hard is when users try to download
the result in HUE, and HUE times out b/c there are thousands of output files. The workaround
is to re-run the query as CTAS, but it's a significant time sink.



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