Re: Sub-optimal plan chosen

> default_statistics_target = 100 (tried with 500, no change). Vacuum
> analyzed
> before initial query, and after each change to default_statistics_target.

Modifying the statistics target is useful only if the estimates are
seriously off, which is not your case - so it won't help, at least not
reliably.

> The same query, with a different "ofid", will occasionally get the more
> optimal plan -- I assume that the distribution of data is the
> differentiator
> there.

Yes, the difference between costs of the two plans is quite small (11796
vs. 13153) so it's very sensible to data distribution.

> Is there any other data I can provide to shed some light on this?

You may try to play with the 'cost' constants - see this:

http://www.postgresql.org/docs/8.4/interactive/runtime-config-query.html#RUNTIME-CONFIG-QUERY-CONSTANTS

You just need to modify them so that the bitmap index scan / bitmap heap
scan is prefered to plain index scan.

Just be careful - if set in the postgresql.conf, it affects all the
queries and may cause serious problems with other queries. So it deserves
proper testing ...

regards
Tomas