Re: Setting Statistics on Functional Indexes

Shaun Thomas <sthomas@optionshouse.com> writes:
> On 10/24/2012 02:31 PM, Shaun Thomas wrote:
>> The main flaw with my example is that it's random. But I swear I'm not
>> making it up! :)

> And then I find a way to make it non-random. Hooray:

I can't reproduce this.  In 9.1 for instance, I get

 Sort  (cost=9.83..9.83 rows=1 width=23) (actual time=0.029..0.029 rows=0 loops=1)
   Sort Key: id, action_date
   Sort Method: quicksort  Memory: 25kB
   ->  Index Scan using idx_date_test_col1_col2 on date_test  (cost=0.00..9.82 rows=1 width=23) (actual
time=0.021..0.021rows=0 loops=1) 
         Index Cond: (((col1)::text = 'S:96'::text) AND (col2 = 657::numeric))
         Filter: (date_trunc('day'::text, action_date) >= '2012-10-24 00:00:00'::timestamp without time zone)
 Total runtime: 0.086 ms

and those estimates don't change materially with the stats adjustments.
If I drop that index altogether, it goes over to this:

 Sort  (cost=521.83..521.83 rows=1 width=23) (actual time=2.544..2.544 rows=0 loops=1)
   Sort Key: id, action_date
   Sort Method: quicksort  Memory: 25kB
   ->  Index Scan using idx_date_test_action_date_trunc_col1 on date_test  (cost=0.00..521.82 rows=1 width=23) (actual
time=2.536..2.536rows=0 loops=1) 
         Index Cond: ((date_trunc('day'::text, action_date) >= '2012-10-24 00:00:00'::timestamp without time zone) AND
((col1)::text= 'S:96'::text)) 
         Filter: (col2 = 657::numeric)
 Total runtime: 2.600 ms

So the planner's conclusions look fairly sane from here.  I get about
the same results from HEAD, 9.2 branch tip, or 9.1 branch tip.

So I'm wondering exactly what "9.1" version you're using, and also
whether you've got any nondefault planner cost parameters.

            regards, tom lane