Who do we send optimizer questions to?
For example, I have a logbook which is ordered by a logtime stamp. Here's
my explain output (and by the way, that was very helpful, I hadn't noticed
that command before):
EXPLAIN select * from logbook order by logtime desc limit 25;
NOTICE: QUERY PLAN:
Sort (cost=3671.28 rows=76766 width=80) -> Seq Scan on logbook (cost=3671.28 rows=76766 width=80)
Now I wonder why it's doing a sequential scan. If it's ordered by logtime
and a limit is specified can't it determine that it should just take the
bottom 25 entries off the list?
_____
Justin Long
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-----Original Message-----
From: Peter Eisentraut [SMTP:e99re41@DoCS.UU.SE]
Sent: Monday, March 06, 2000 2:22 AM
To: David Celjuska
Cc: pgsql-sql@postgreSQL.org
Subject: Re: [SQL] database with 1000000 rows is very slow
On Sun, 5 Mar 2000, David Celjuska wrote:
> CREATE UNIQUE INDEX "article_pkey" on "article" using btree ( "id"
> "varchar_ops" );
> this database store. But I think that select * from article where id
> like 'something%' is very slow (some minutes) and query as: select *
> from article where id='something' is very slow too. I don't know where
> is a problem a I would like optimalise this, but how can I do it?
If you haven't run vacuum analyze lately then you should do that.
> When I use hash except btree, query as: select * from article where
> id='something' is fast but select * from article where id='something%'
> is very slow.
Yup. That's because hashes only work on exact matches and btrees can do
ordering (like 'somethink' is surely "larger" than 'something%').
> Or postgresql make indexes automaticly?
No, you have to make them, but you did that right.
> How can I see that postgres use/or no use index on some query? It is
> possible?
Yup. EXPLAIN SELECT ....
In fact, if you can't make any progress you should always accompany any
optimizer issues with the EXPLAIN output. That will help our optimizer
gurus. :)
--
Peter Eisentraut Sernanders vag 10:115
peter_e@gmx.net 75262 Uppsala
http://yi.org/peter-e/ Sweden
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