mohammadnaqvi04 commented on code in PR #25391:
URL: https://github.com/apache/datafusion/pull/25391#discussion_r4032117255
##########
datafusion/optimizer/src/decorrelate_predicate_subquery.rs:
##########
@@ -616,6 +659,118 @@ fn build_join(
Ok(Some(new_plan))
}
+/// Builds the join for a correlated `EXISTS` subquery whose groupless
+/// aggregate requires count-bug compensation.
Review Comment:
This decorrelates a groupless-aggregate subquery to a `LEFT JOIN`, grouped
by the correlated column, with the unmatched-row `NULL` replaced by the
aggregate's empty-input default (e.g. `count(*)` → `0`, `sum(x)` -> `NULL`).
For
```sql
SELECT t1.t1_int FROM t1 WHERE EXISTS (
SELECT count(*) FROM t2 WHERE t1.t1_int = t2.t2_int
);
```
this is equivalent to
```sql
SELECT t1.t1_int
FROM t1
LEFT JOIN (SELECT t2_int, count(*) AS cnt FROM t2 GROUP BY t2_int) AS sq
ON t1.t1_int = sq.t2_int;
```
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