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https://issues.apache.org/jira/browse/DRILL-4387?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15529561#comment-15529561
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Khurram Faraaz commented on DRILL-4387:
---------------------------------------
I will track the wrong results issue separately.
As for this JIRA DRILL-4387, the fix is now verified.
with Fix on Drill 1.6.0 git commit ID: c67d070b, query takes 60.88 seconds.
{noformat}
0: jdbc:drill:schema=dfs.tmp> SELECT DISTINCT dir1 FROM `DRILL_4589`;
+-------+
| dir1 |
+-------+
| null |
| Q2 |
| Q1 |
| Q3 |
| Q4 |
+-------+
5 rows selected (60.883 seconds)
{noformat}
whereas without this fix, same query takes 106.069 seconds on Drill 1.6.0 git
commit 1d890ff9, which is one commit before the above commit.
> 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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