racevedoo commented on code in PR #4144:
URL: https://github.com/apache/calcite/pull/4144#discussion_r1914691721
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core/src/main/java/org/apache/calcite/rel/rules/SubQueryRemoveRule.java:
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@@ -392,21 +392,22 @@ private static RexNode rewriteSome(RexSubQuery e,
Set<CorrelationId> variablesSe
// then false // sub-query is empty for corresponding corr value
// when q.c = 0 then false // sub-query is empty
// when e.deptno is null then unknown
- // when q.c <> q.d && q.d <= 1
+ // when q.c <> q.d && q.dd <= 1
Review Comment:
the `q.c <> q.d` comparison identifies if there are any null values, since
`count(*)` counts null values and `count(deptno)` does not.
the `q.dd <= 1` part (previously `q.d <= 1`) remains the with the same
semantics (true if there is a single non-null value), since in this patch `dd`
is `count(distinct deptno)`, which is equivalent to `d` in the main branch.
so this `when` clause means "if there are any null values and there is a
single non-null value".
in `main`, c = `count(distinct *)` and d = `count(distinct deptno)`, which
means that when there are null values and a single non-null value, `c = d` and
this clause evaluates to false.
I know it's a tricky case, so let me know if additional clarifications are
needed 😄
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