adriangb commented on code in PR #16641:
URL: https://github.com/apache/datafusion/pull/16641#discussion_r2180504143
##########
datafusion/physical-optimizer/src/enforce_sorting/sort_pushdown.rs:
##########
@@ -216,7 +218,25 @@ fn pushdown_sorts_helper(
fn pushdown_requirement_to_children(
plan: &Arc<dyn ExecutionPlan>,
parent_required: OrderingRequirements,
+ parent_fetch: Option<usize>,
) -> Result<Option<Vec<Option<OrderingRequirements>>>> {
+ // Only attempt to push down TopK when there is an upstream LIMIT
+ if parent_fetch.is_some() {
+ // 1) Never push a new TopK below an operator that already has its own
fetch
+ if plan.fetch().is_some() {
+ return Ok(None);
+ }
+ // 2) Only allow pushdown through operators that do not increase row
count
+ // (equal cardinality). Any other operator (including joins, filter,
+ // sort-with-limit, or UDTFs that may expand rows) must stop the
pushdown.
+ let effect = plan.cardinality_effect();
+ if !matches!(effect, CardinalityEffect::Equal) {
+ return Ok(None);
+ }
+ // At this point, only single-input, non-expanding operators
+ // such as ProjectionExec, CoalesceBatchesExec, are allowed to receive
TopK.
+ }
Review Comment:
```suggestion
// If there is a limit on the parent plan we cannot push it down through
operators that change the cardinality.
// E.g. consider if LIMIT 2 is applied below a FilteExec that filters
out 1/2 of the rows we'll end up with 1 row instead of 2.
// If the LIMIT is applied after the FilterExec and the FilterExec
returns > 2 rows we'll end up with 2 rows (correct).
if parent_fetch.is_some() && !plan.supports_limit_pushdown() {
return Ok(None)
}
```
wdyt?
--
This is an automated message from the Apache Git Service.
To respond to the message, please log on to GitHub and use the
URL above to go to the specific comment.
To unsubscribe, e-mail: github-unsubscr...@datafusion.apache.org
For queries about this service, please contact Infrastructure at:
us...@infra.apache.org
---------------------------------------------------------------------
To unsubscribe, e-mail: github-unsubscr...@datafusion.apache.org
For additional commands, e-mail: github-h...@datafusion.apache.org
