Message-ID: <49DA13C1.2030200@megafon.hr>
Date: Mon, 06 Apr 2009 16:37:53 +0200
From: Mario Splivalo <mario.splivalo@megafon.hr>
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To: Scott Marlowe <scott.marlowe@gmail.com>
CC: Tom Lane <tgl@sss.pgh.pa.us>, pgsql-performance@postgresql.org
Subject: Re: Forcing seq_scan off for large table joined with tiny
	table yeilds improved performance
References: <49D0D237.7020308@megafon.hr> <3028.1238422571@sss.pgh.pa.us>	
	<49D0E687.50605@megafon.hr>	
	<dcc563d10903300925m88a73c7kf90084b5e088a445@mail.gmail.com>	
	<49D9F39F.40106@megafon.hr>
	<dcc563d10904060658v621cbd3drbb75b6428d770b04@mail.gmail.com>
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Scott Marlowe wrote:
> On Mon, Apr 6, 2009 at 6:20 AM, Mario Splivalo
> <mario.splivalo@megafon.hr> wrote:
>> Scott Marlowe wrote:
>>> It's not really solved, it's just a happy coincidence that the current
>>> plan runs well.  In order to keep the query planner making good
>>> choices you need to increase stats target for the field in the index
>>> above.  The easiest way to do so is to do this:
>>>
>>> alter database mydb set default_statistics_target=100;
>>>
>>> and run analyze again:
>>>
>>> analyze;
>> So, i removed the index on field_name, set default_default_statistics_target
>> to 100, analyzed, and the results are the same:
> 
> Why did you remove the index?
> 

Because I already have index on that column, index needed to enforce PK 
constraint. Here is the original DDL for the table:

CREATE TABLE photo_info_data
(
   photo_id integer NOT NULL,
   field_name character varying NOT NULL,
   field_value character varying,
   CONSTRAINT photo_info_data_pk PRIMARY KEY (user_id, field_name)
)

CREATE INDEX photo_info_data_ix_field_value
   ON user_info_data USING btree (field_value);

So, there is index on (user_id, field_name). Postgres is using index for 
user_id (...WHERE user_id = 12345) but not on field-name (...WHERE 
field_name = 'f-spot'). When I add extra index on field name:

CREATE INDEX photo_info_data_ix__field_name
   ON user_info_data USING btree (field_name);

Then that index is used.

	Mike