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Re: Very slow catalog query

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: "Just Someone" <just(dot)some(at)gmail(dot)com>
Cc: "pgsql-general(at)postgresql(dot)org" <pgsql-general(at)postgresql(dot)org>
Subject: Re: Very slow catalog query
Date: 2008-03-31 17:27:25
Message-ID: 14972.1206984445@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-general
"Just Someone" <just(dot)some(at)gmail(dot)com> writes:
> Here is the result of explain analyze (though this one took 1500ms and
> not 169000):

Well, it's hard to be sure what the problem is when you're not showing
us a problem case ...  but I notice that this indexscan is estimated
awfully high:

>                            ->  Index Scan using
> pg_depend_reference_index on pg_depend dep  (cost=0.00..64942.17
> rows=247 width=12) (actual time=396.542..1547.172 rows=22 loops=1)
>                                  Index Cond: (refobjid = 30375069::oid)

The reason is not far to seek: the scan is checking only the second
index key, meaning that it has to scan the entire index.  (I am
surprised it didn't use a seqscan instead.  Are you using enable_seqscan
= off?  Not a great idea.)  Since you know you are looking for a table,
you could improve matters by adding a constraint on refclassid:

	dep.refclassid = 'pg_class'::regclass

			regards, tom lane

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