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https://issues.apache.org/jira/browse/DRILL-4387?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15152820#comment-15152820
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ASF GitHub Bot commented on DRILL-4387:
---------------------------------------
Github user jinfengni commented on a diff in the pull request:
https://github.com/apache/drill/pull/379#discussion_r53360482
--- Diff:
exec/java-exec/src/main/java/org/apache/drill/exec/physical/base/GroupScan.java
---
@@ -35,6 +35,8 @@
public interface GroupScan extends Scan, HasAffinity{
public static final List<SchemaPath> ALL_COLUMNS =
ImmutableList.of(SchemaPath.getSimplePath("*"));
+ public static final List<SchemaPath> EMPTY_COLUMNS = ImmutableList.of();
--- End diff --
Nice catch. It's no longer needed. (Originally, I intended to convert NULL
to empty_columns. But not it's not necessary). I'll remove that. thx.
> 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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