In sort_inner_and_outer, we create mergejoin join paths by explicitly sorting both relations on each possible ordering of the available mergejoin clauses. However, if there are no available mergejoin clauses, we can skip this process entirely. It seems that this is a relatively common scenario. Checking the regression tests I noticed that there are a lot of cases where we would arrive here with an empty mergeclause_list.
So I'm wondering if it's worth considering a shortcut in sort_inner_and_outer by checking if mergeclause_list is empty. This can help skip all the statements preceding select_outer_pathkeys_for_merge. In particular this may help avoid building UniquePath paths in the case of JOIN_UNIQUE_OUTER or JOIN_UNIQUE_INNER. I asked this because in the "Right Semi Join" patch [1] I wanted to exclude mergejoin from being considered for JOIN_RIGHT_SEMI. So I set mergeclause_list to NIL, but noticed that it still runs the statements in sort_inner_and_outer until no available outer pathkeys are found in select_outer_pathkeys_for_merge. Attached is a trivial patch for this. Thoughts? [1] https://www.postgresql.org/message-id/flat/CAMbWs4_X1mN%3Dic%2BSxcyymUqFx9bB8pqSLTGJ-F%3DMHy4PW3eRXw%40mail.gmail.com Thanks Richard
v1-0001-Short-circuit-sort_inner_and_outer-if-there-are-no-mergejoin-clauses.patch
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