Re: Partial index slower than regular index

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(at)linux(dot)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