PG17 compatibility: fix plan diffs in multi_explain

[colm-mchugh]

Regress test multi_explain has two queries that have a different query plan with PG17. Here is part of the plan diff for the query labelled Union and left join subquery pushdown in multi_explain.sql (for the complete diff, search for multi_explain here):

                                       ->  Sort
                                             Sort Key: ((users.composite_id).tenant_id), ((users.composite_id).user_id), subquery_2.hasdone, events.event_time
-                                            ->  Hash Left Join
-                                                  Hash Cond: (users.composite_id = subquery_2.composite_id)
-                                                  ->  HashAggregate
-                                                        Group Key: ((users.composite_id).tenant_id), ((users.composite_id).user_id), users.composite_id, ('action=>1'::text), events.event_time
+                                            ->  Nested Loop Left Join
+                                                  Join Filter: (users.composite_id = subquery_2.composite_id)
+                                                  ->  Unique
+                                                        ->  Sort
+                                                              Sort Key: ((users.composite_id).tenant_id), ((users.composite_id).user_id), users.composite_id, ('action=>1'::text), events.event_time
                                                               ->  Append

The change is the same in both queries; a hash left join with subquery_1 on the outer and subquery_2 on the inner side of the join is now a nested loop left join with subquery_1 on the outer and subquery_2 on the inner; additionally, the chosen method of uniquifying the UNION in subquery_1 has changed from hashed grouping to sort followed by unique, as shown in the diff above.

The PG17 commit that caused this plan change is likely Fix MergeAppend to more accurately compute the number of rows that need to be sorted because it impacts the estimated rows counts of UNION paths. Comparing a costed plan of the query between PG16 and PG17 I noticed that with PG16 the rows estimate for the UNION in subquery_1 is 4, whereas with PG17 the rows estimate is 2. A lower rows estimate in the outer side of the join may result in nested loop looking cheaper than hash join for the left outer join, hence the plan change in the two queries where there is a UNION on the outer side of a left outer join.

The proposed fix achieves a consistent plan across all supported postgres versions by temporarily disabling nested loop join and sort for the two impacted queries; the postgres optimizer selects hash join for the outer left join and hashed aggregation for the UNION operation. I investigated tweaking the queries, but was not able to arrive at a consistent plan, and I believe the SQL operator (e.g. join, group by, union) implementations are orthogonal to the intent of the test, so this should be a satisfactory solution, particularly as it avoids introducing a second alternative output file for multi_explain.

[codecov]

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[naisila]

Nice fix, given that we mostly care about the complex subquery being pushed down in these queries.
Yet again avoiding the alternative test output, so great!

Reminder to rebase to release-13.0, and drop the enable configure commit before merging.