Re: Slow query: table iteration (8.3)

On Fri, Jan 29, 2010 at 10:49 PM, Glenn Maynard <glenn(at)zewt(dot)org> wrote:
> Hitting a performance issues that I'm not sure how to diagnose.
>
> SELECT highscores_for_steps_and_card(s.id, 591, 1) FROM stomp_steps s;
> Seq Scan on stomp_steps s  (cost=0.00..793.52 rows=2902 width=4)
> (actual time=26509.919..26509.919 rows=0 loops=1)
> Total runtime: 26509.972 ms
>
> The inner function looks like this:
>
> CREATE FUNCTION highscores_for_steps_and_card(steps_id int, card_id
> int, count int) RETURNS SETOF INTEGER LANGUAGE SQL AS $$
>        SELECT r.id FROM stomp_round r
>        WHERE ($1 IS NULL OR r.steps_id = $1) AND ($2 IS NULL OR
> r.user_card_id = $2)
>        ORDER BY r.score DESC LIMIT $3
> $$
>
>  Limit  (cost=13.12..13.12 rows=1 width=8) (actual time=0.054..0.054
> rows=0 loops=1)
>   ->  Sort  (cost=13.12..13.12 rows=1 width=8) (actual
> time=0.051..0.051 rows=0 loops=1)
>         Sort Key: score
>         Sort Method:  quicksort  Memory: 17kB
>         ->  Bitmap Heap Scan on stomp_round r  (cost=9.09..13.11
> rows=1 width=8) (actual time=0.036..0.036 rows=0 loops=1)
>               Recheck Cond: ((280 = steps_id) AND (user_card_id = 591))
>               ->  BitmapAnd  (cost=9.09..9.09 rows=1 width=0) (actual
> time=0.032..0.032 rows=0 loops=1)
>                     ->  Bitmap Index Scan on stomp_round_steps_id
> (cost=0.00..4.40 rows=20 width=0) (actual time=0.030..0.030 rows=0
> loops=1)
>                           Index Cond: (280 = steps_id)
>                     ->  Bitmap Index Scan on stomp_round_user_card_id
>  (cost=0.00..4.44 rows=25 width=0) (never executed)
>                           Index Cond: (user_card_id = 591)
>  Total runtime: 0.153 ms
> (12 rows)
>
> stomp_steps has about 1500 rows, so it finds 1500 high scores, one for
> each stage.
>
> I expected scalability issues from this on a regular drive, since
> it'll be doing a ton of index seeking when not working out of cache,
> so I expected to need to change to an SSD at some point (when it no
> longer easily fits in cache).  However, I/O doesn't seem to be the
> bottleneck yet.  If I run it several times, it consistently takes 26
> seconds.  The entire database is in OS cache (find | xargs cat:
> 250ms).
>
> I'm not sure why the full query (26s) is orders of magnitude slower
> than 1500*0.150ms (225ms).  It's not a very complex query, and I'd
> hope it's not being re-planned every iteration through the loop.  Any
> thoughts?  Using SELECT to iterate over a table like this is very
> useful (and I don't know any practical alternative), but it's
> difficult to profile since it doesn't play nice with EXPLAIN ANALYZE.

I believe that the time for the seq-scan node doesn't include the time
to generate the outputs, which is where all the function calls are.
As a general rule, I have found that function calls are reaaaaally
slow, and that calling a function in a loop is almost always a bad
idea. You didn't mention what PG version you're running, but I
believe that with a sufficiently new version (8.4?) it'll actually
inline SQL functions into the invoking query, which will probably be
lots faster. If not, you can inline it manually.

Rewriting it as a join will likely be faster still:

SELECT r.id FROM stomp_steps s, stomp_round r WHERE (s.id IS NULL OR
r.steps_id = s.id) AND ($1 IS NULL OR r.user_card_id = $1) ORDER BY
r.score DESC LIMIT $2

You might even break it into two cases:

SELECT r.id FROM stomp_steps s, stomp_round r WHERE r.steps_id = s.id
AND ($1 IS NULL OR r.user_card_id = $1)
UNION ALL
SELECT r.id FROM stomp_steps s, stomp_round r WHERE s.id IS NULL AND
($1 IS NULL OR r.user_card_id = $1)
ORDER BY r.score DESC LIMIT $2

Or if s.id can't really be NULL:

SELECT r.id FROM stomp_steps s, stomp_round r WHERE r.steps_id = s.id
AND ($1 IS NULL OR r.user_card_id = $1)
ORDER BY r.score DESC LIMIT $2

These kinds of rewrites allow the query planner progressively more
flexibility - to use a hash or merge join, for example, instead of a
nested loop. And they eliminate overhead. You'll have to play around
with it and see what works best in your particular environment, but in
general, I find it pays big dividends to avoid wrapping these kinds of
logic bits inside a function.

...Robert