[jira] Commented: (HIVE-372) Nested UDFs cause _very_ high memory usage when processing query

["Zheng Shao (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HIVE-372?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12697701#action_12697701 ] 

Zheng Shao commented on HIVE-372:
---------------------------------

The reason is that "backtrack" is enabled in Hive.g. However in order to disable "backtrack" we need to do some other cleanup of the grammar to make sure it's LL(k).


> Nested UDFs cause _very_ high memory usage when processing query
> ----------------------------------------------------------------
>
>                 Key: HIVE-372
>                 URL: https://issues.apache.org/jira/browse/HIVE-372
>             Project: Hadoop Hive
>          Issue Type: Bug
>          Components: Query Processor
>         Environment: Fedora Linux, 10x Amazon EC2 (Large Instance w/ 8GB Ram)
>            Reporter: Steve Corona
>
> When nesting UDFs, the Hive Query processor takes a large amount of time+memory to process the query. For example, I ran something along the lines of:
> select trim( trim( trim(trim( trim( trim( trim( trim( trim(column))))))))) from test_table;
> This query needs 10GB+ of memory to process before it'll launch the job. The amount of memory increases exponentially with each nested UDF.
> Obviously, I am using trim() in this case as a simple example that causes the same problem to occur. In my actual use-case I had a bunch of nested regexp_replaces.

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