Re: Very strange query difference between 7.3.6 and 7.4.6 (7.3.6 kicking 7.4.6 butt)

"Joshua D. Drake" <jd(at)commandprompt(dot)com> writes:
> Let me know what you come up with. Thanks for the help.

Hmph. On my FC3 machine, 7.4 is consistently faster than 7.3 in sorting
and grouping this data --- it's about 710 vs 960 msec. (This is on a P4
1.8GHz, presumably slower than your machines.) So there's no
algorithmic change that might be biting us. It seems we have to look at
the platforms involved. At this point I can think of two hypotheses
that haven't been eliminated:
1. FC1's qsort is much faster than FC3's on this case.
2. The 64-bit build has got some kind of performance problem
that's not generic to 7.4.*.
#1 doesn't seem very probable, though it's possible. I think what you
should do next is build 7.3 on the 64-bit machine and see what performance
it's got. You might also try non-64-bit builds and see what they do.

Just FYI, you can test the behavior without loading your full database
--- just load the data you sent me and do

explain analyze
select count(*) from foo
group by
post_id,
topic_id,
topic_title,
topic_status,
topic_replies,
topic_time,
topic_type,
topic_vote,
topic_last_post_id,
forum_name,
forum_status,
forum_id,
auth_view,
auth_read,
auth_post,
auth_reply,
auth_edit,
auth_delete,
auth_sticky,
auth_announce,
auth_pollcreate,
auth_vote,
auth_attachments
;

On 7.4 and up you may have to set enable_hashagg = off to force a
Sort/GroupAggregate plan instead of HashAggregate.

regards, tom lane