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From "Khurram Faraaz (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (DRILL-4387) Improve execution side when it handles skipAll query
Date Tue, 21 Jun 2016 19:01:57 GMT

    [ https://issues.apache.org/jira/browse/DRILL-4387?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15342463#comment-15342463
] 

Khurram Faraaz commented on DRILL-4387:
---------------------------------------

The below queries return wrong results. (the problem seems to be there for quite some time)

{noformat}
Directory structure is

[root@centos-01 DRILL_4589]# ls
1990  1992  1994  1996  1998  2000  2002  2004  2006  2008  2010  2012  2014
1991  1993  1995  1997  1999  2001  2003  2005  2007  2009  2011  2013  2015
[root@centos-01 DRILL_4589]# cd 1990
[root@centos-01 1990]# ls
Q1  Q2  Q3  Q4
and so on...

Below two queries return 0, I don't think the results are correct, please review

0: jdbc:drill:schema=dfs.tmp> select count(dir0) from `DRILL_4589`;
+---------+
| EXPR$0  |
+---------+
| 0       |
+---------+
1 row selected (9.117 seconds)
0: jdbc:drill:schema=dfs.tmp> select count(dir1) from `DRILL_4589`;
+---------+
| EXPR$0  |
+---------+
| 0       |
+---------+
1 row selected (8.97 seconds)

0: jdbc:drill:schema=dfs.tmp> explain plan for select count(dir0) from `DRILL_4589`;
+------+------+
| text | json |
+------+------+
| 00-00    Screen
00-01      Project(EXPR$0=[$0])
00-02        Project(EXPR$0=[$0])
00-03          Scan(groupscan=[org.apache.drill.exec.store.pojo.PojoRecordReader@5275c59a[columns
= null, isStarQuery = false, isSkipQuery = false]])


0: jdbc:drill:schema=dfs.tmp> explain plan for select count(dir1) from `DRILL_4589`;
+------+------+
| text | json |
+------+------+
| 00-00    Screen
00-01      Project(EXPR$0=[$0])
00-02        Project(EXPR$0=[$0])
00-03          Scan(groupscan=[org.apache.drill.exec.store.pojo.PojoRecordReader@337121ac[columns
= null, isStarQuery = false, isSkipQuery = false]])
{noformat}

> Improve execution side when it handles skipAll query
> ----------------------------------------------------
>
>                 Key: DRILL-4387
>                 URL: https://issues.apache.org/jira/browse/DRILL-4387
>             Project: Apache Drill
>          Issue Type: Bug
>            Reporter: Jinfeng Ni
>            Assignee: Jinfeng Ni
>             Fix For: 1.6.0
>
>
> DRILL-4279 changes the planner side and the RecordReader in the execution side when they
handles skipAll query. However, it seems there are other places in the codebase that do not
handle skipAll query efficiently. In particular, in GroupScan or ScanBatchCreator, we will
replace a NULL or empty column list with star column. This essentially will force the execution
side (RecordReader) to fetch all the columns for data source. Such behavior will lead to big
performance overhead for the SCAN operator.
> To improve Drill's performance, we should change those places as well, as a follow-up
work after DRILL-4279.
> One simple example of this problem is:
> {code}
>    SELECT DISTINCT substring(dir1, 5) from  dfs.`/Path/To/ParquetTable`;  
> {code}
> The query does not require any regular column from the parquet file. However, ParquetRowGroupScan
and ParquetScanBatchCreator will put star column as the column list. In case table has dozens
or hundreds of columns, this will make SCAN operator much more expensive than necessary. 



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