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Message-ID: <20060511034711.46266.qmail@web50305.mail.yahoo.com>
Date: Wed, 10 May 2006 20:47:11 -0700 (PDT)
From: <ogjunk-pgjedan@yahoo.com>
Subject: Re: Help with a seq scan on multi-million row table
To: pgsql-sql@postgresql.org
In-Reply-To: <26129.1147312429@sss.pgh.pa.us>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii

Not sure if I'm showing you what you asked for, but here it is:

select * from pg_stats  where tablename='user_url_tag' and attname='user_url_id';
 schemaname |  tablename   |   attname   | null_frac | avg_width | n_distinct |                            most_common_vals                           |                                                 most_common_freqs                   |                                 histogram_bounds                                 | correlation
------------+--------------+-------------+-----------+-----------+------------+------------------------------------------------------------------------+--------------------------------------------------------------------------------------------------------------------+----------------------------------------------------------------------------------+-------------
 public     | user_url_tag | user_url_id |         0 |         4 |      60825 | {458321,1485346,16304,68027,125417,153465,182503,201175,202973,218423} | {0.00133333,0.001,0.000666667,0.000666667,0.000666667,0.000666667,0.000666667,0.000666667,0.000666667,0.000666667} | {195,195993,325311,480323,647778,782598,1014527,1201726,1424822,1614712,1853719} |    0.795521

You asked if the table has been analyzed recently.
I think so - I run ANALYZE on the whole DB every night, like this:

$ psql -U me -c "ANALYZE;" mydb

For a good measure, I just analyzed the table now: $ psql -U me -c "ANALYZE user_url_tag;" mydb
Then I set the enable_hashjoin back to ON and re-run the EXPLAIN ANALYZE.
I still get the sequential scan, even after analyzing the table :(

I'm not sure which numbers you are referring to when you said the estimate is off, but here are some numbers:
  The whole table has 6-7 M rows.
  That query matches about 2500 rows.

If there are other things I can play with and help narrow this down, please let me know.

Thanks,
Otis


----- Original Message ----
From: Tom Lane <tgl@sss.pgh.pa.us>
To: ogjunk-pgjedan@yahoo.com
Cc: pgsql-sql@postgresql.org
Sent: Wednesday, May 10, 2006 9:53:49 PM
Subject: Re: [SQL] Help with a seq scan on multi-million row table 

<ogjunk-pgjedan@yahoo.com> writes:
> Aha!  set hashjoin=off did the trick.

>                ->  Index Scan using ix_user_url_tag_user_url_id on user_url_tag userurltag0_  (cost=0.00..157.34 rows=103 width=14) (actual time=1.223..1.281 rows=5 loops=1666)
>                      Index Cond: (userurltag0_.user_url_id = "outer".id)

This seems to be the problem right here: the estimate of matching rows
is off by a factor of 20, and that inflates the overall cost estimate
for this plan about the same, causing the planner to think the other way
is cheaper.

What does the pg_stats row for user_url_tag.user_url_id contain?
Have you analyzed that table recently?

            regards, tom lane

---------------------------(end of broadcast)---------------------------
TIP 6: explain analyze is your friend