Re: strange query behavior

"Tim Jones" <TJones(at)optio(dot)com> writes:
> 18,273,008 rows in observationresults
> [ and n_distinct = 12942 ]

OK, so the estimated rowcounts are coming from those two numbers.
It's notoriously hard to get a decent n_distinct estimate from a small
sample :-(, and I would imagine the number of batteryidentifiers is
really a lot more than 12942?

What you need to do is increase the statistics target for
observationresults.batteryidentifier (see ALTER TABLE) and re-ANALYZE
and see if you get a saner n_distinct in pg_stats. I'd try 100 and
then 1000 as target. Or you could just increase the global default
target (see postgresql.conf) but that might be overkill.

It's still a bit odd that the case with two batteryidentifiers was
estimated fairly accurately when the other wasn't; I'll go look into
that. But in any case you need better stats if you want good plans.

regards, tom lane