Re: Partial index slower than regular index

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On 06/04/11 11:31, Scott Marlowe wrote:
On Tue, Apr 5, 2011 at 4:35 PM, Thom Brown<thom@xxxxxxxxx>  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.

Mark


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