HaoYang670 commented on issue #4772:
URL:
https://github.com/apache/arrow-datafusion/issues/4772#issuecomment-1404486620
Hi @Jefffrey, actually I am not sure whether we should do this. It is just
something appearing in my mind one day.
(sorry it should be the `aggregation functions` but not `empty projection`
in the title)
For `union all`, we can push down the aggregation functions to the child
plans. For example,
for a query `select count(*) from (select * from t1 union select * from
t2)`, we can optimize the logical plan to count each child plan and then sum
them together.
```
Projection: Sum(col1) as COUNT(UInt8(1))
Aggregate: groupBy=[[]], aggr=[[Sum(col1)]]
Union
Projection: COUNT(UInt8(1)) as col1
Aggregate: groupBy=[[]], aggr=[[COUNT(UInt8(1))]]
TableScan: t projection=[a]
Projection: COUNT(UInt8(1)) as col1
Aggregate: groupBy=[[]], aggr=[[COUNT(UInt8(1))]]
TableScan: t projection=[a]
```
And it is similar for min, max, avg ...
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