rubenada opened a new pull request #1870: [CALCITE-3846] EnumerableMergeJoin: wrong comparison of composite key with null values URL: https://github.com/apache/calcite/pull/1870 Jira: [CALCITE-3846](https://issues.apache.org/jira/browse/CALCITE-3846) EnumerableMergeJoin implementation expects its inputs to be sorted in ascending order, nulls last (see EnumerableMergeJoinRule). In case of a composite key, EnumerableMergeJoin will represent keys as `JavaRowFormat.LIST`, which is a comparable list, whose comparison is implemented via `FlatLists.ComparableListImpl#compare`. This method will compare both lists, item by item, but in will consider that a null item is less-than a non-null item. This is a de-facto nulls-first collation, which contradicts the pre-requisite of the mergeJoin algorithm. Proposed solution: merge join algorithm should not compare keys using compareTo method, instead an ad-hoc Comparator should be generated by EnumerableMergeJoin, similar to EnumerableSort approach (i.e. this Comparator could be null, in which case compareTo method will be used).
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