From: Clark Slater <list@slatech.com>
To: Joshua D. Drake <jd@commandprompt.com>
Cc: pgsql-performance@postgresql.org
Subject: Re: faster search
Date: Fri, 10 Jun 2005 20:20:25 -0400 (EDT)
Message-ID: <20050610201740.R40688@vbp2.vbp2.com> (raw)
In-Reply-To: <42AA2CE9.8040901@commandprompt.com>
References: <20050610132812.R94573@vbp2.vbp2.com>
<42A9D556.7070108@arbash-meinel.com>
<20050610200358.R40688@vbp2.vbp2.com>
<42AA2CE9.8040901@commandprompt.com>
thanks for your suggestion.
a small improvement. still pretty slow...
vbp=# alter table test alter column productlistid set statistics 150;
ALTER TABLE
vbp=# alter table test alter column typeid set statistics 150;
ALTER TABLE
vbp=# 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=525.617..36802.556 rows=132528 loops=1)
Filter: ((productlistid = 3) AND (typeid = 9))
Total runtime: 36847.754 ms
(3 rows)
Time: 36850.719 ms
On Fri, 10 Jun 2005, Joshua D. Drake wrote:
> 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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To: pgsql-performance@postgresql.org
Cc: list@slatech.com, jd@commandprompt.com
Subject: Re: faster search
In-Reply-To: <20050610201740.R40688@vbp2.vbp2.com>
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