[jira] [Commented] (PHOENIX-846) Select DISTINCT with LIMIT does full scans

["James Taylor (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/PHOENIX-846?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13933892#comment-13933892 ] 

James Taylor commented on PHOENIX-846:
--------------------------------------

This can be improved, provided there's no ORDER BY on the query. We can't use a PageFilter, as DISTINCT boils down to doing a GROUP BY, but can stop the scan once we've found the LIMIT for the groups we're collecting.

I'll take a stab at this. Thanks for reporting it.

> Select DISTINCT with LIMIT does full scans
> ------------------------------------------
>
>                 Key: PHOENIX-846
>                 URL: https://issues.apache.org/jira/browse/PHOENIX-846
>             Project: Phoenix
>          Issue Type: Bug
>    Affects Versions: 3.0.0, 4.0.0
>            Reporter: alex
>            Priority: Critical
>
> When running SELECT DISTINCT with LIMIT it does full scan and aggregation (no pageFilter/limit used on server side), 
> this severely affects performance  (query returns in 20sec vs 300ms without DISTINCT)
> : jdbc:phoenix:localhost> explain select DISTINCT ROWKEY from TEST_1M LIMIT 100;
> +------------+
> |    PLAN    |
> +------------+
> | CLIENT PARALLEL 30-WAY FULL SCAN OVER TEST_1M |
> |     SERVER FILTER BY FIRST KEY ONLY |
> |     SERVER AGGREGATE INTO ORDERED DISTINCT ROWS BY [ROWKEY] |
> | CLIENT MERGE SORT |
> | CLIENT 100 ROW LIMIT |
> +------------+
> -------------------------------------------------
> for comparison SELECT without  DISTINCT uses a limit PageFilter=100 on server side and doesn't do full scan (query returns in 300ms)
> explain select ROWKEY from TEST_1M LIMIT 100;
> +------------+
> |    PLAN    |
> +------------+
> | CLIENT PARALLEL 30-WAY FULL SCAN OVER TEST_1M |
> |     SERVER FILTER BY FIRST KEY ONLY AND PageFilter 100 |
> | CLIENT MERGE SORT |
> | CLIENT 100 ROW LIMIT |
> +------------+



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