Re: Help with a seq scan on multi-million row table

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(at)sss(dot)pgh(dot)pa(dot)us>
To: ogjunk-pgjedan(at)yahoo(dot)com
Cc: pgsql-sql(at)postgresql(dot)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(at)yahoo(dot)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

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TIP 6: explain analyze is your friend