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https://issues.apache.org/jira/browse/DRILL-4387?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15150797#comment-15150797
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Jinfeng Ni commented on DRILL-4387:
-----------------------------------
I did a simple performance comparison on my mac, using TPCH SF10 lineitem
parquet table. Both numbers are measured with warm cache.
Without patch:
{code}
0: jdbc:drill:zk=local> select distinct dir1 from
dfs.`/drill/testdata/tpch-sf10/lineitem`;
+-------+
| dir1 |
+-------+
| null |
+-------+
1 row selected (18.958 seconds)
{code}
With patch:
{code}
0: jdbc:drill:zk=local> select distinct dir1 from
dfs.`/drill/testdata/tpch-sf10/lineitem`;
+-------+
| dir1 |
+-------+
| null |
+-------+
1 row selected (2.255 seconds)
{code}
Basically, the query time is reduced from 18.9 seconds to 2.2 seconds.
The profile also shows the memory for the Scan operator is reduced from 54M to
2M.
> 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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