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From "Zheng Shao (JIRA)" <>
Subject [jira] Commented: (HIVE-372) Nested UDFs cause _very_ high memory usage when processing query
Date Fri, 10 Apr 2009 01:04:12 GMT


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:
>             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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