jayzhan211 commented on PR #15281:
URL: https://github.com/apache/datafusion/pull/15281#issuecomment-2769195062
```
[2025-04-01T12:19:41Z DEBUG datafusion_optimizer::utils]
scalar_subquery_to_join:
Projection: e.b, __scalar_sq_1.CASE WHEN max(e2.a) > Int64(10) THEN
Utf8("a") ELSE Utf8("b") END AS CASE WHEN max(e2.a) > Int64(10) THEN Utf8("a")
ELSE Utf8("b") END
Left Join: Filter: CAST(__scalar_sq_1.b AS Int64) = CAST(e.b AS
Int64) + Int64(1)
SubqueryAlias: e
TableScan: t
SubqueryAlias: __scalar_sq_1
Projection: CASE WHEN max(e2.a) > Int32(10) THEN Utf8("a") ELSE
Utf8("b") END AS CASE WHEN max(e2.a) > Int64(10) THEN Utf8("a") ELSE Utf8("b")
END, e2.b
Aggregate: groupBy=[[e2.b]], aggr=[[max(e2.a)]]
SubqueryAlias: e2
TableScan: t2
```
In `scalar_subquery_to_join` optimization, we have left join on
`__scalar_sq_1` and `e`.
I wonder could we join the plan first and then call aggregation on top of
joined plan.
The plan I expected is something like this
```
[2025-04-01T12:19:41Z DEBUG datafusion_optimizer::utils]
scalar_subquery_to_join:
Projection: CASE WHEN max(e2.a) > Int32(10) THEN Utf8("a") ELSE Utf8("b")
END AS CASE WHEN
max(e2.a) > Int64(10) THEN Utf8("a") ELSE Utf8("b") END, e2.b
Aggregate: groupBy=[[e2.b]], aggr=[[max(e2.a)]]
Left Join: Filter: CAST(e2.b AS Int64) = CAST(e.b AS Int64) + Int64(1)
SubqueryAlias: e
TableScan: t
SubqueryAlias: e2
TableScan: t2
```
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