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 20:52:17 -0400 (EDT)
Message-ID: <20050610205113.V40688@vbp2.vbp2.com> (raw)
In-Reply-To: <42AA34E7.3060006@commandprompt.com>
References: <20050610132812.R94573@vbp2.vbp2.com>
<42A9D556.7070108@arbash-meinel.com>
<20050610200358.R40688@vbp2.vbp2.com>
<42AA2CE9.8040901@commandprompt.com>
<20050610201740.R40688@vbp2.vbp2.com>
<42AA34E7.3060006@commandprompt.com>
Query should return 132,528 rows.
vbp=# set enable_seqscan = false;
SET
vbp=# explain analyze select * from test where (productlistid=3 and typeid=9);
QUERY PLAN
------------------------------------------------------------------------
Index Scan using test_typeid on test (cost=0.00..137223.89 rows=156194
width=725) (actual time=25.999..25708.478 rows=132528
loops=1)
Index Cond: (typeid = 9)
Filter: (productlistid = 3)
Total runtime: 25757.679 ms
(4 rows)
On Fri, 10 Jun 2005, Joshua D. Drake wrote:
> Clark Slater wrote:
>> 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
>
> Hello,
>
> Also what happens if you:
>
> set enable_seqscan = false;
> explain analyze query....
>
> Sincerely,
>
> Joshua D. Drake
>
>
>
>> 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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>>>
>>>
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>
>
> --
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To: pgsql-performance@postgresql.org
Cc: list@slatech.com, jd@commandprompt.com
Subject: Re: faster search
In-Reply-To: <20050610205113.V40688@vbp2.vbp2.com>
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