mohammadnaqvi04 commented on code in PR #25391:
URL: https://github.com/apache/datafusion/pull/25391#discussion_r4032166345
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datafusion/optimizer/src/decorrelate.rs:
##########
@@ -383,6 +454,7 @@ impl TreeNodeRewriter for PullUpCorrelatedExpr {
// Correlated exist subquery, remove the limit(so that
correlated expressions can pull up)
(true, false) => Transformed::yes(match
limit.get_fetch_type()? {
FetchType::Literal(Some(0)) => {
+ self.forces_empty_result = true;
Review Comment:
`LIMIT 0` on the subquery collapses it to an `EmptyRelation` before the
count-bug join ever sees it, so without this flag, join-compensation has no way
of knowing that the subquery is unconditionally empty and defaults every row to
matched. For
```sql
SELECT t1.t1_int FROM t1 WHERE EXISTS (
SELECT count(*) FROM t2 WHERE t1.t1_int = t2.t2_int LIMIT 0
);
```
`t1_int` should never appear in the result, since `LIMIT 0` empties the
subquery regardless of whether `t2` has a matching row. Setting
`forces_empty_result` is what makes `build_join_with_count_bug` return `false`
unconditionally instead of falling back to its usual "matched" default.
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