[DRILL-4387] Improve execution side when it handles skipAll query

• Type: Bug
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 1.6.0
• Assignee: Jinfeng Ni
• Reporter: Jinfeng Ni
• Reviewer: Khurram Faraaz
• Created: 16/Feb/16 16:56
• Updated: 14/Apr/21 05:57
• Resolved: 22/Feb/16 21:02

Description

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:

   SELECT DISTINCT substring(dir1, 5) from  dfs.`/Path/To/ParquetTable`;  

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.

Issue Links

• Dependent: ZOOKEEPER-704 GSoC 2010: Read-Only Mode