Well, yeah. The problem is the Unique simply compares the columns in the
order it sees them, and it does not match the column order desired by
incremental sort. But we don't push down this information at all :-(
This is a nice optimization better to have. Since the 'Sort and Unique'
would unique-ify the result of a UNION by sorting on all columns, why
not we adjust the sort order trying to match parse->sortClause so that
we can avoid the final sort node?
Doing that we can transform plan from:
# explain (costs off) select * from foo union select * from foo order by 1,3;
QUERY PLAN
-----------------------------------------------
Incremental Sort
Sort Key: foo.a, foo.c
Presorted Key: foo.a
-> Unique
-> Sort
Sort Key: foo.a, foo.b, foo.c
-> Append
-> Seq Scan on foo
-> Seq Scan on foo foo_1
(9 rows)
To:
# explain (costs off) select * from foo union select * from foo order by 1,3;
QUERY PLAN
-----------------------------------------
Unique
-> Sort
Sort Key: foo.a, foo.c, foo.b
-> Append
-> Seq Scan on foo
-> Seq Scan on foo foo_1
(6 rows)