On Wed, Apr 15, 2020 at 10:47 PM Tomas Vondra <tomas.von...@2ndquadrant.com>
wrote:
>
> 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)
Thanks
Richard
