zhuqi-lucas commented on code in PR #16641:
URL: https://github.com/apache/datafusion/pull/16641#discussion_r2181454354
##########
datafusion/physical-optimizer/src/enforce_sorting/sort_pushdown.rs:
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@@ -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:
Thank you @adriangb , exactly more concise and accurate solution! Merged
your suggestion now.
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