Re: Query not using the index

Mark Halliwell <mark@transportservices.com.au> writes:
> The majority of records (about 6.8 million) have computer = 8 with sequence
> starting at 2200000 and incrementing by 1.
> There are about 497000 records with computer = 3 with the sequence starting at
> 1 and also incrementing by 1.
> There are only a few records with other computer numbers.

You aren't going to find any non-kluge solution, because Postgres keeps
no cross-column statistics and thus is quite unaware that there's any
correlation between the computer and sequence fields.  So in a query
like

> select * from replicate where computer = 3 and sequence >= 490000;

the sequence constraint looks extremely unselective to the planner, and
you get a seqscan, even though *in the domain of computer = 3* it's a
reasonably selective constraint.

> that if a specify an upper limit for sequence (a value which I cannot always
> easily predict), it also uses the index.

I would think that it'd be sufficient to say

  select * from replicate where computer = 3 and sequence >= 490000
  and sequence < 2200000;

If it's not, try increasing the statistics target for the sequence
column so that ANALYZE gathers a finer-grain histogram for that column.

            regards, tom lane