I continue to be stumped by this. You are right that I should have listed the estimates provided by explain... basically for the select where bar = 41, it's estimating there will be 40,000 rows instead of 7, out of what's actuallly 5 million records in the table.

So far I've tried increase statistics for the bar column from the default 10 to 100 (vacuum analyzing after) and the explain-plan hasn't changed. I also notice that afterward, the pg_stats record for the bar column still only lists the top 5 values of bar (out of 68 unique values in the table). Are there any other settings I could try to improve the detail of the statistics?

By the way, I think I do have a workaround for this particular query:
select * from (select * from foo where barId = 412 order by id desc) as tempview limit 25;
This query uses the bar index and completes instantly. However, I feel like I should find the heart of the problem, since bad statistics could end up affecting other plans, right?

 - Mike

On Mon, 7 Mar 2005 23:03:43 -0800 (PST), Stephan Szabo <[EMAIL PROTECTED]> wrote:

On Mon, 7 Mar 2005, Michael McFarland wrote:

   I'm trying to understand why a particular query is slow, and it seems
like the optimizer is choosing a strange plan.  See this summary:

* I have a large table, with an index on the primary key 'id' and on a field 'foo'. > select count(*) from foo; 1,000,000 > select count(*) from foo where bar = 41; 7

* This query happens very quickly.
> explain select * from foo where barId = 412 order by id desc;
Sort ()
   Sort key= id
  ->   Index scan using bar_index on foo ()
     Index cond: barId = 412

But this query takes forever

> explain select * from foo where barId = 412 order by id desc limit 25;
Limit ()
  ->   Index scan backward using primarykey_index
   Filter:  barID = 412

You didn't show the row estimates, but I'd guess that it's expecting either that ther are more rows that match barId=412 than there actually are (which may be solvable by raising the statistics target on the column and re-analyzing) such that going backwards on id in order to make 25 matching rows isn't a bad plan or that barId and id are correlated which is unfortunately not going to be recognized right now.

-- Using Opera's revolutionary e-mail client: http://www.opera.com/mail/

---------------------------(end of broadcast)---------------------------
TIP 5: Have you checked our extensive FAQ?


Reply via email to