Re: Simply join in PostrgeSQL takes too long

Vitaly Belman wrote:
> Hello pgsql-performance,
>
> I discussed the whole subject for some time in DevShed and didn't
> achieve much (as for results). I wonder if any of you guys can help
> out:
>
> http://forums.devshed.com/t136202/s.html
>

So cutting and pasting:

----- SCHEMA -----
CREATE TABLE bv_bookgenres (
book_id INT NOT NULL,
genre_id INT NOT NULL
);
CREATE TABLE bv_genre_children (
genre_id INT,
genre_child_id INT
);
-------------------

----- QUERY -----
select DISTINCT
book_id
from
bookgenres,
genre_children
WHERE
bookgenres.genre_id = genre_children.genre_child_id AND
genre_children.genre_id = 1
LIMIT 10
-----------------

----- EXPLAIN ANALYZE -----
QUERY PLAN
Limit (cost=6503.51..6503.70 rows=10 width=4) (actual
time=703.000..703.000 rows=10 loops=1)
-> Unique (cost=6503.51..6738.20 rows=12210 width=4) (actual
time=703.000..703.000 rows=10 loops=1)
-> Sort (cost=6503.51..6620.85 rows=46937 width=4) (actual
time=703.000..703.000 rows=24 loops=1)
Sort Key: bv_bookgenres.book_id
-> Merge Join (cost=582.45..2861.57 rows=46937 width=4)
(actual time=46.000..501.000 rows=45082 loops=1)
Merge Cond: ("outer".genre_id = "inner".genre_child_id)
-> Index Scan using genre_id on bv_bookgenres
(cost=0.00..1462.84 rows=45082 width=8) (actual time=0.000..158.000
rows=45082 loops=1)
-> Sort (cost=582.45..598.09 rows=6256 width=2)
(actual time=46.000..77.000 rows=49815 loops=1)
Sort Key: bv_genre_children.genre_child_id
-> Index Scan using genre_id2 on
bv_genre_children (cost=0.00..187.98 rows=6256 width=2) (actual
time=0.000..31.000 rows=6379 loops=1)
Index Cond: (genre_id = 1)
Total runtime: 703.000 ms
-------------------------------

----- CONF SETTINGS -----
shared_buffers = 1000 # min 16, at least max_connections*2, 8KB each
sort_mem = 10000
#work_mem = 1024 # min 64, size in KB
#maintenance_work_mem = 16384 # min 1024, size in KB
#max_stack_depth = 2048 # min 100, size in KB
-------------------------

Have you VACUUM ANALYZED recently. If not do that then rerun the EXPLAIN
ANALYZE.

You might wanna bump shared_buffers. You have 512MB RAM right? You
probably want to bump shared_buffers to 10000, restart PG then run a
VACUUM ANALYZE. Then rerun the EXPLAIN ANALYZE.

If that doesnt help try doing a

ALTER TABLE bv_genre_children ALTER COLUMN genre_child_id SET STATISTICS
100;

followed by a:

VACUUM ANALYZE bv_genre_children;

You might also want to be tweaking the effective_cache_size parameter in
postgresql.conf, but I am unsure how this would work on Windows. Does
Windows have a kernel disk cache anyone?

HTH

Nick