julianhyde commented on code in PR #4022:
URL: https://github.com/apache/calcite/pull/4022#discussion_r1826318264
##########
core/src/main/java/org/apache/calcite/rel/rules/SubQueryRemoveRule.java:
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@@ -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:
Sure. If you can find a better place to put the logic at
https://github.com/apache/calcite/blob/3db4dd1ebf04abaf74d29464ae311a598ae929f7/core/src/main/java/org/apache/calcite/rel/rules/SubQueryRemoveRule.java#L456
then do it. Maybe a method `@Nullable Boolean
RelMetadataQuery.isEmpty(RelNode)` (where true means always empty, false means
always non-empty, null for everything else).
I cannot imagine a case where a provider would return a maxRowCount value
less than one where the relation was not guaranteed empty, so I wouldn't worry
too much whether the condition is `< 1D` or `== 0D`.
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