I happened to notice that currently MemoizePath cannot be generated for partitionwise join. I traced that and have found the problem. One condition we need to meet to generate MemoizePath is that inner path's ppi_clauses should have the form "outer op inner" or "inner op outer", as checked by clause_sides_match_join in paraminfo_get_equal_hashops.
Note that when are at this check, the inner path has not got the chance to be re-parameterized (that is done later in try_nestloop_path), so if it is parameterized, it is parameterized by the topmost parent of the outer rel, not the outer rel itself. Thus this check performed by clause_sides_match_join could not succeed if we are joining two child rels. The fix is straightforward, just to use outerrel->top_parent if it is not null and leave the reparameterization work to try_nestloop_path. In addition, I believe when reparameterizing MemoizePath we need to adjust its param_exprs too. Attach the patch for fix. Thanks Richard
v1-0001-Fix-MemoizePath-for-partitionwise-join.patch
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