Message-ID: <42AA2CE9.8040901@commandprompt.com>
Date: Fri, 10 Jun 2005 17:14:33 -0700
From: "Joshua D. Drake" <jd@commandprompt.com>
Organization: Command Prompt, Inc.
User-Agent: Mozilla Thunderbird 1.0.2 (X11/20050404)
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To: Clark Slater <list@slatech.com>
Cc: pgsql-performance@postgresql.org
Subject: Re: faster search
References: <20050610132812.R94573@vbp2.vbp2.com>
	<42A9D556.7070108@arbash-meinel.com>
	<20050610200358.R40688@vbp2.vbp2.com>
In-Reply-To: <20050610200358.R40688@vbp2.vbp2.com>
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Clark Slater wrote:
> hmm, i'm baffled.  i simplified the query
> and it is still taking forever...

What happens if you:

alter table test alter column productlistid set statistics 150;
alter table test alter column typeid set statistics 150;
explain analyze select * from test where (productlistid=3 and typeid=9);

Sincerely,

Joshua D. Drake


> 
> 
>           test
> -------------------------
>  id            | integer
>  partnumber    | character varying(32)
>  productlistid | integer
>  typeid        | integer
> 
> 
> Indexes:
> "test_productlistid" btree (productlistid)
> "test_typeid" btree (typeid)
> "test_productlistid_typeid" btree (productlistid, typeid)
> 
> 
> explain analyze select * from test where (productlistid=3 and typeid=9);
> 
>                               QUERY PLAN
> -----------------------------------------------------------------------
>  Seq Scan on test  (cost=0.00..96458.27 rows=156194 width=725) (actual
> time=516.459..41930.250 rows=132528 loops=1)
>    Filter: ((productlistid = 3) AND (typeid = 9))
>  Total runtime: 41975.154 ms
> (3 rows)
> 
> 
> System specs:
> PostgreSQL 7.4.2 on RedHat 9
> dual AMD Athlon 2GHz processors
> 1 gig memory
> mirrored 7200 RPM IDE disks
> 
> 
> On Fri, 10 Jun 2005, John A Meinel wrote:
> 
>> Clark Slater wrote:
>>
>>> Hi-
>>>
>>> Would someone please enlighten me as
>>> to why I'm not seeing a faster execution
>>> time on the simple scenario below?
>>>
>>> there are 412,485 rows in the table and the
>>> query matches on 132,528 rows, taking
>>> almost a minute to execute.  vaccuum
>>> analyze was just run.
>>
>>
>> Well, if you are matching 130k out of 400k rows, then a sequential scan
>> is certainly prefered to an index scan. And then you have to sort those
>> 130k rows by partnumber. This *might* be spilling to disk depending on
>> what your workmem/sortmem is set to.
>>
>> I would also say that what you would really want is some way to get the
>> whole thing from an index. And I think the way to do that is:
>>
>> CREATE INDEX test_partnum_listid_typeid_idx ON
>>     test(partnumber, productlistid, typeid);
>>
>> VACUUM ANALYZE test;
>>
>> EXPLAIN ANALYZE SELECT * FROM test
>>     WHERE productlistid=3 AND typeid=9
>>     ORDER BY partnumber, productlistid, typeid
>>     LIMIT 15
>> ;
>>
>> The trick is that you have to match the order by exactly with the index,
>> so the planner realizes it can do an indexed lookup to get the 
>> information.
>>
>> You could also just create an index on partnumber, and see how that
>> affects your original query. I think the planner could use an index
>> lookup on partnumber to get the ordering correct. But it will have to do
>> filtering after the fact based on productlistid and typeid.
>> With my extended index, I think the planner can be smarter and lookup
>> all 3 by the index.
>>
>>>
>>> Thanks!
>>> Clark
>>
>>
>> Good luck,
>> John
>> =:->
>>
> 
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