adriangb commented on code in PR #25672:
URL: https://github.com/apache/datafusion/pull/25672#discussion_r4097872615
##########
datafusion/optimizer/src/push_down_limit.rs:
##########
@@ -171,8 +171,16 @@ fn rewrite_limit(mut limit: Limit) ->
Result<Transformed<LogicalPlan>> {
}
} else {
sort.fetch = new_fetch;
- limit.input = Arc::new(LogicalPlan::Sort(sort));
- Ok(Transformed::yes(LogicalPlan::Limit(limit)))
+ if skip > 0 {
+ limit.input = Arc::new(LogicalPlan::Sort(sort));
+ Ok(Transformed::yes(LogicalPlan::Limit(limit)))
+ } else {
+ // With `skip = 0` the Sort's fetch already caps the
output,
+ // so drop the Limit now instead of on the next pass. The
+ // Sort then replaces the Limit as the visited node and its
+ // own visit is skipped, so apply the TopK pushdown here.
+ Ok(Transformed::yes(push_topk_through_join(sort)?.data))
Review Comment:
It is intentional. At this point the `Limit` is already removed, so the plan
changed even when `push_topk_through_join` returns `Transformed::no`. Passing
that `no` through would report an unchanged plan. I made this explicit in the
code (684da9c205) and added `limit_push_down_sort_reports_transformed`, which
fails with the suggested change.
##########
datafusion/optimizer/src/push_down_limit.rs:
##########
Review Comment:
Yes, good catch. That branch has the same skipped visit, so the TopK
pushdown waited one more pass there too. In 684da9c205 both `skip == 0`
branches share one path that calls `push_topk_through_join`, and
`topk_pushed_through_limit_over_sort_with_{same,smaller}_fetch_in_one_pass`
cover it with `max_passes = 1`.
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