[FLINK-7144] Optimize multiple LogicalAggregate into one

• Type: Improvement
• Status: Closed
• Priority: Minor
• Resolution: Won't Fix
• Component/s: Table SQL / Legacy Planner
• Labels: auto-deprioritized-major auto-unassigned
• Assignee: Unassigned
• Reporter: Jark Wu
• Created: 11/Jul/17 03:52
• Updated: 05/Jul/21 13:21
• Resolved: 05/Jul/21 13:21

Description

When applying multiple GROUP BY, and no aggregates or expression in the first GROUP BY, and the second GROUP fields is a subset of first GROUP fields. Then the first GROUP BY can be removed.

Such as the following SQL ,

SELECT a FROM (SELECT a,b,c FROM MyTable GROUP BY a, b, c) GROUP BY a

should be optimized into

DataStreamGroupAggregate(groupBy=[a], select=[a])
DataStreamCalc(select=[a])
DataStreamScan(table=[[_DataStreamTable_0]])

but get:

DataStreamGroupAggregate(groupBy=[a], select=[a])
DataStreamCalc(select=[a])
DataStreamGroupAggregate(groupBy=[a, b, c], select=[a, b, c])
DataStreamScan(table=[[_DataStreamTable_0]])

I looked for the Calcite built-in rules, but can't find a match one. So maybe we should implement one , and maybe we should implement it in Calcite.