Message-ID: <4D9BCA09.5020307@catalyst.net.nz>
Date: Wed, 06 Apr 2011 14:03:53 +1200
From: Mark Kirkwood <mark.kirkwood@catalyst.net.nz>
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To: Scott Marlowe <scott.marlowe@gmail.com>
CC: Thom Brown <thom@linux.com>,
	pgsql-performance <pgsql-performance@postgresql.org>
Subject: Re: Partial index slower than regular index
References: <BANLkTimFZjYve4GDOXQJ6VjRO1pOQEwp4w@mail.gmail.com>
	<BANLkTi=izsG5SVwMW_28Dg2-0M__kSTg=A@mail.gmail.com>
	<4D9BA87B.4090500@catalyst.net.nz>
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On 06/04/11 11:40, Mark Kirkwood wrote:
> On 06/04/11 11:31, Scott Marlowe wrote:
>> On Tue, Apr 5, 2011 at 4:35 PM, Thom Brown<thom@linux.com>  wrote:
>>> I'm using 9.1dev.
>> SNIP
>>
>>> DROP INDEX indextest_stuff;
>>>
>>> CREATE INDEX indextest_stuff ON indextest(stuff) WHERE stuff = 'bark';
>>>
>>> postgres=# explain analyze select * from indextest where stuff = 
>>> 'bark';
>>>                                                     QUERY PLAN
>>> ------------------------------------------------------------------------------------------------------------------- 
>>>
>>>   Seq Scan on indextest  (cost=0.00..143386.48 rows=5606 width=9)
>>> (actual time=164.321..1299.794 rows=8000 loops=1)
>>>    Filter: (stuff = 'bark'::text)
>>>   Total runtime: 1300.267 ms
>>> (3 rows)
>>>
>>> The index doesn't get used.  There's probably a logical explanation,
>>> which is what I'm curious about.
>> Works fine for me:
>>
>> explain analyze select * from indextest where stuff = 'bark';
>>                                                               QUERY
>> PLAN
>> ------------------------------------------------------------------------------------------------------------------------------------ 
>>
>>   Index Scan using indextest_stuff on indextest  (cost=0.00..837.01
>> rows=13347 width=9) (actual time=0.226..6.073 rows=8000 loops=1)
>>     Index Cond: (stuff = 'bark'::text)
>>   Total runtime: 7.527 ms
>>
>> Even with a random_page_cost = 4 it works.  Running 8.3.13 btw.
>>
>
> I reproduce what Thom sees - using 9.1dev with default config 
> settings. Even cranking up effective_cache_size does not encourage the 
> partial index to be used.
>
>


However trying with 9.0 gives me the (expected) same 8.3 behaviour:


test=# CREATE INDEX indextest_stuff ON indextest(stuff) WHERE stuff = 
'bark';
CREATE INDEX

test=# explain analyze select * from indextest where stuff = 'bark';
                                                             QUERY PLAN
--------------------------------------------------------------------------------------------
  Index Scan using indextest_stuff on indextest  (cost=0.00..284.20 
rows=5873 width=9)
                                                 (actual 
time=0.276..9.621 rows=8000 loops=1)
    Index Cond: (stuff = 'bark'::text)
  Total runtime: 16.621 ms
(3 rows)


regards

Mark