[jira] [Created] (FLINK-7144) Optimize multiple LogicalAggregate into one

["Jark Wu (JIRA)" <ji...@apache.org>]

Jark Wu created FLINK-7144:
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             Summary: Optimize multiple LogicalAggregate into one
                 Key: FLINK-7144
                 URL: https://issues.apache.org/jira/browse/FLINK-7144
             Project: Flink
          Issue Type: Improvement
          Components: Table API & SQL
            Reporter: Jark Wu



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 , 
{code}
SELECT a FROM (SELECT a,b,c FROM MyTable GROUP BY a, b, c) GROUP BY a
{code}
should be optimized into 
{code}
DataStreamGroupAggregate(groupBy=[a], select=[a])
DataStreamCalc(select=[a])
DataStreamScan(table=[[_DataStreamTable_0]])
{code}

but get:
{code}
DataStreamGroupAggregate(groupBy=[a], select=[a])
DataStreamCalc(select=[a])
DataStreamGroupAggregate(groupBy=[a, b, c], select=[a, b, c])
DataStreamScan(table=[[_DataStreamTable_0]])
{code}

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. 



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