[jira] [Commented] (TAJO-1010) Improve multiple DISTINCT aggregation.

["ASF GitHub Bot (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/TAJO-1010?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14128089#comment-14128089 ] 

ASF GitHub Bot commented on TAJO-1010:
--------------------------------------

Github user hyunsik commented on the pull request:

    https://github.com/apache/tajo/pull/126#issuecomment-55072740
  
    Is it ready to be reviewed?


> Improve multiple DISTINCT aggregation.
> --------------------------------------
>
>                 Key: TAJO-1010
>                 URL: https://issues.apache.org/jira/browse/TAJO-1010
>             Project: Tajo
>          Issue Type: Improvement
>          Components: planner/optimizer
>            Reporter: Jaehwa Jung
>            Assignee: Jaehwa Jung
>
> Currently, tajo provides three stage for optimizing distinct query aggregation. But it just supports one column for distinct aggregation as follows:
> {code:title=Query1|borderStyle=solid}
> select a.flag, count(distinct a.id) as cnt, sum(distinct a.id) as total
> from table1
> group by a.flag
> {code}
> If you write two more columns for distinct aggregation, you can't apply optimized distinct aggregation as follows:
> {code:title=Query2|borderStyle=solid}
> select a.flag, count(distinct a.id) as cnt, sum(distinct a.id) as total
> , count(distinct a.name) as cnt2, count(distinct a.code) as cnt3
> from table1
> group by a.flag
> {code}
> In this case, you may see low performance for your query. Thus, we need to improve multiple DISTINCT aggregation. Correctly, we should support three stage for multiple DISTINCT aggregation.



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