From: Joshua D. Drake <jd@commandprompt.com>
To: Clark Slater <list@slatech.com>
Cc: pgsql-performance@postgresql.org
Subject: Re: faster search
Date: Fri, 10 Jun 2005 17:46:53 -0700
Message-ID: <42AA347D.2080407@commandprompt.com> (raw)
In-Reply-To: <20050610201740.R40688@vbp2.vbp2.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>
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
> 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);
How many rows should it return?
>>
>> 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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To: pgsql-performance@postgresql.org
Cc: jd@commandprompt.com, list@slatech.com
Subject: Re: faster search
In-Reply-To: <42AA347D.2080407@commandprompt.com>
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