[HIVE-22454] Remove subplan under "Limit 0"

• Type: Improvement
• Status: Open
• Priority: Major
• Resolution: Unresolved
• Component/s: CBO
• Assignee: Unassigned
• Reporter: Steve Carlin
• Created: 05/Nov/19 01:44
• Updated: 05/Nov/19 16:14

Description

After the Calcite upgrade and some fixes, there are plans that have a "limit 0" but then do stuff after 0 rows are selected.

An example is in filter_union.q.out:
...

      Reduce Operator Tree:

        Group By Operator

          aggregations: count(VALUE._col0)

          keys: KEY._col0 (type: string)

          mode: mergepartial

          outputColumnNames: _col0, _col1

          Statistics: Num rows: 250 Data size: 23750 Basic stats: COMPLETE Column stats: COMPLETE

          Limit

            Number of rows: 0
...
We can simplify the plan by maybe creating a dummy table after the Limit 0.

Issue Links

• is related to: HIVE-22274 Upgrade Calcite version to 1.21.0