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From "Namit Jain (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HIVE-707) add group_concat
Date Wed, 29 Jul 2009 16:24:14 GMT

    [ https://issues.apache.org/jira/browse/HIVE-707?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12736689#action_12736689

Namit Jain commented on HIVE-707:

I am assuming you mean: group_concat() in mysql

2.  should not be a probelm:

>From MySQL documentation,

The result is truncated to the maximum length that is given by the group_concat_max_len  system
variable, which has a default value of 1024. The value can be set higher, although the effective
maximum length of the return value is constrained by the value of max_allowed_packet. The
syntax to change the value of group_concat_max_len at runtime is as follows, where val  is
an unsigned integer:

SET [GLOBAL | SESSION] group_concat_max_len = val

We can also add a new configuration parameter for the maximum length

> add group_concat
> ----------------
>                 Key: HIVE-707
>                 URL: https://issues.apache.org/jira/browse/HIVE-707
>             Project: Hadoop Hive
>          Issue Type: New Feature
>          Components: Query Processor
>            Reporter: Namit Jain
>            Assignee: Min Zhou
> Moving the discussion to a new jira:
> I've implemented group_cat() in a rush, and found something difficult to slove:
> 1. function group_cat() has a internal order by clause, currently, we can't implement
such an aggregation in hive.
> 2. when the strings will be group concated are too large, in another words, if data skew
appears, there is often not enough memory to store such a big result.

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