Re: subselect requires offset 0 for good performance.

Scott Marlowe <scott.marlowe@gmail.com> writes:
> I extracted all the data like so:
> select * into dba.pp_test_wide from original table;
> and get this query plan from explain analyze:
> http://explain.depesz.com/s/EPx which takes 20 minutes to run.
> If I extract it this way:
> select tree_sortkey, product_name, deleted_at into db.pp_test_3col
> from original table;
> I get this plan: http://explain.depesz.com/s/gru which gets a
> materialize in it, and suddenly takes 106 ms.

There's no reason why suppressing some unrelated columns would change the
rowcount estimates, but those two plans show different rowcount estimates.

I suspect the *actual* reason for the plan change was that autovacuum had
had a chance to update statistics for the one table, and not yet for the
other.  Please do a manual ANALYZE on both tables and see if there's
still a plan difference.

            regards, tom lane