adriangb commented on code in PR #16641:
URL: https://github.com/apache/datafusion/pull/16641#discussion_r2180419834
##########
datafusion/physical-optimizer/src/enforce_sorting/sort_pushdown.rs:
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@@ -216,7 +218,28 @@ 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 or lower-equal cardinality). Any other operator
(including joins,
+ // sort-with-limit, or UDTFs that may expand rows) must stop the
pushdown.
+ let effect = plan.cardinality_effect();
+ if !matches!(
+ effect,
+ CardinalityEffect::Equal | CardinalityEffect::LowerEqual
+ ) {
+ return Ok(None);
+ }
Review Comment:
Hmm but if `FilterExec` is `LowerEqual` aren't you saying "if the
cardinality effect is not one of Equal or LowerEqual allow pushdown" which is
equivalent to "if the cardinality effect is either of Equal or LowerEqual do
allow pushdown", which would be the opposite of what we want? I think we *only*
want to allow pushdown if the cardinality effect is `Equal`
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