When Postgres is fast, it is really fast. I love it. My biggest problem
is when/how it chooses best path, it seems to me that relatively few
records with a high duplication destroy performance. I can't stress
enough that this is a serious problem in the real world.
Take these two queries:
cdinfo=# explain select trackid, song, title from zsong, ztitles where
ztitles.performer2 like 'Van Halen' and ztitles.muzenbr= zsong.muzenbr
and contains(song, 'panama', 10)>0;
NOTICE: QUERY PLAN:
Merge Join (cost=6012.55..182151.93 rows=10902 width=36)
-> Sort (cost=6012.55..6012.55 rows=3130 width=16)
-> Index Scan using ztitles_performer2_ndx on ztitles
(cost=0.00..5830.80 rows=3130 width=16)
-> Index Scan using zsong_muzenbr_ndx on zsong (cost=0.00..166961.86
rows=731071 width=20)
cdinfo=# explain select trackid, song, title from zsong, ztitles where
ztitles.title like 'Van Halen' and ztitles.muzenbr = zsong.muzenbr and
contains(song, 'panama', 10)>0;
NOTICE: QUERY PLAN:
Nested Loop (cost=0.00..93.45 rows=4 width=36)
-> Index Scan using ztitles_title_ndx on ztitles (cost=0.00..7.08
rows=1 width=16)
-> Index Scan using zsong_muzenbr_ndx on zsong (cost=0.00..78.43
rows=7 width=20)
They are fundamentally the same query, each with an index, each doing
about the same thing. Except that the performer2 field has a high number
of duplicate records ala "Various Artists"
Now we have had some small debates about how to fix this, and perhaps I
am over simplifying it, but the current statistics are broken, they do
not work reliably and produce unreliable results. I think this is a must
for 7.1. A simple hack, such as discarding the upper and lower %5-10%
should be able to fix this behavior, without too many side effects (if
any). While I agree it is not the "right" way to do something, it would
be a "better" way of doing something that is currently wrong.
With the exception of this problem, I love postgres, but this problem
really goes a long way to make it look REAL bad.
BTW anyone know a way around this?
--
http://www.mohawksoft.com
