zabetak commented on code in PR #4022:
URL: https://github.com/apache/calcite/pull/4022#discussion_r1824926676
##########
core/src/main/java/org/apache/calcite/rel/rules/SubQueryRemoveRule.java:
##########
@@ -170,6 +170,13 @@ private static RexNode rewriteCollection(RexSubQuery e,
*/
private static RexNode rewriteSome(RexSubQuery e, Set<CorrelationId>
variablesSet,
RelBuilder builder, int subQueryIndex) {
+ // If the sub-query is guaranteed to return 0 row, just return
+ // FALSE.
+ final RelMetadataQuery mq = e.rel.getCluster().getMetadataQuery();
+ final Double maxRowCount = mq.getMaxRowCount(e.rel);
+ if (maxRowCount != null && maxRowCount < 1D) {
Review Comment:
We have [rule in PruneEmptyRules
](https://github.com/apache/calcite/blob/ed6c33eb1d27882b26fc16b995dde78de1709aff/core/src/main/java/org/apache/calcite/rel/rules/PruneEmptyRules.java#L649)
that does:
```java
return maxRowCount != null && maxRowCount == 0.0;
```
Given that comparisons with doubles are inherently imprecise we may need to
revisit that rule as well. Should we employ a threshold comparison?
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