Thread: getting an index to work with partial indices ...

Try as I might, I can't seem to get it to work ... table has >9million
rows in it, I've created an index "using btree ( priority ) where priority
< 0;", where the table distribution looks like:

  priority |  count
----------+---------
        -2 |  138435
        -1 |  943250
         1 |    3416
         9 | 1134171
           | 7276960
(5 rows)

And it still won't use the index:

# explain update table set priority = -3 where priority = -1;
                             QUERY PLAN
------------------------------------------------------------------
  Seq Scan on table  (cost=0.00..400735.90 rows=993939 width=278)
    Filter: (priority = -1)
(2 rows)

But, ti will if I try 'priority = -2' ... what is teh threshhold for using
the index?  obviously 10% of the records is too high ...

thanks ...

----
Marc G. Fournier           Hub.Org Networking Services (http://www.hub.org)
Email: scrappy@hub.org           Yahoo!: yscrappy              ICQ: 7615664

"Marc G. Fournier" <scrappy@postgresql.org> writes:
> But, ti will if I try 'priority = -2' ... what is teh threshhold for using
> the index?  obviously 10% of the records is too high ...

Depends on a lot of factors, but usually somewhere between 1% and 10%.
(The new bitmap index scan code in 8.1 should be workable for higher
percentages.)  If this doesn't seem to square with reality for you,
you might try reducing random_page_cost.

            regards, tom lane