Re: [HACKERS] All things equal, we are still alot slower then MySQL?

Okay, after playing around with this some more tonight, and playing with
the PGOPTIONS you've presented...I've gotten the query to be faster then
with mysql :)  The error of my ways: not enough indices *sigh* I created a
few more on the fields that were being used on the query, and have:

SELECT c.id, c.name, c.url
FROM aecCategory c
WHERE EXISTS (
SELECT a.status
FROM aecEntMain a, aecWebEntry b
WHERE a.status LIKE 'active:ALL%'
AND a.representation LIKE '%:ALL%'
AND b.status LIKE 'active:ALL%'
AND b.indid='000001'
AND b.divid='100016'
AND ((a.id,a.mid) = (b.id,b.mid))
AND ((b.catid,b.indid,b.divid) = (c.id,c.ppid,c.pid)));

==========
Seq Scan on aeccategory c  (cost=69.61 rows=1170 width=36) SubPlan   ->  Nested Loop  (cost=4.10 rows=1 width=60)
 ->  Index Scan using aecwebentry_divid on aecwebentry b  (cost=2.03 rows=1 width=24)         ->  Index Scan using
aecentmain_primaryon aecentmain a  (cost=2.07 rows=480 width=36)
 
===========

producing the results I need in 1.26seconds, using 1.5% of the CPU.

Now, something does bother me here, and I'm not sure if its a problem we
need to address, or if its expected, but if I remove the index
aecwebentry_divid, it reverts to using aecwebentry_primary and increases
the query time to 12secs, which is:

create unique index aecWebEntry_primary on aecWebEntry ( indid,divid,catid,id,mid);

Should it do that?

On Wed, 22 Sep 1999, Tom Lane wrote:

> The Hermit Hacker <scrappy@hub.org> writes:
> > Anyone get a chance to look into this?
> 
> Only just now, but I do have a couple of thoughts.
> 
> For the query
> 
>         SELECT distinct b.indid, b.divid, b.catid, a.id, a.mid \
>           FROM aecEntMain a, aecWebEntry b \
>          WHERE (a.id=b.id AND a.mid=b.mid) \
>            AND (a.status like 'active%' and b.status like 'active%')
>            AND (a.status like '%active:ALL%' and b.status like '%active:ALL%')
>            AND (a.representation like '%:ALL%')
>            AND (b.indid=? and b.divid=? and b.catid=?)";
> 
> you're showing a plan of 
> 
> Unique  (cost=1271.15 rows=5 width=84)
>   ->  Sort  (cost=1271.15 rows=5 width=84)
>         ->  Nested Loop  (cost=1271.15 rows=5 width=84)
>               ->  Index Scan using aecwebentry_primary on aecwebentry b  (cost=1269.08 rows=1 width=60)
>               ->  Index Scan using aecentmain_primary on aecentmain a  (cost=2.07 rows=16560 width=24)
> 
> which indicates that the optimizer is guessing only one match in
> aecwebentry and is therefore putting it on the outside of the nested
> loop (so that the inner scan over aecentmain would only have to be
> done once, if it's guessing right).  But in a later message you
> say that the actual number of hits is more like 39 for aecwebentry
> and one for aecentmain.  Which means that the nested loop would go
> faster if it were done the other way round, aecentmain on the outside.
> I'm not sure of a way to force the system to try it that way, though.
> 
> The other question is why is it using a nested loop at all, rather
> than something more intelligent like merge or hash join.  Presumably
> the optimizer thinks those would be more expensive, but it might be
> wrong.
> 
> You could try forcing selection of merge and hash joins for this
> query and see (a) what kind of plan do you get, (b) how long does
> it really take?  To do that, start psql with PGOPTIONS environment
> variable set:
> 
> PGOPTIONS="-fn -fh"    # forbid nestloop and hash, ie, force mergejoin
> 
> PGOPTIONS="-fn -fm"    # forbid nestloop and merge, ie, force hashjoin
> 
> Also, I don't think you ever mentioned exactly what the available
> indexes are on these tables?
> 
>             regards, tom lane
> 

Marc G. Fournier                   ICQ#7615664               IRC Nick: Scrappy
Systems Administrator @ hub.org 
primary: scrappy@hub.org           secondary: scrappy@{freebsd|postgresql}.org