Re: why hash on the primary key?

> What's strange about it?  A probe into an in-memory hashtable is a lot
> cheaper than a probe into an index, so this type of plan makes plenty
> of sense if the hashtable will fit in RAM and there are going to be a
> lot of probes.  (Where "a lot" means "enough to amortize the cost of
> building the hashtable", of course.)

Hmm...  it didn't occur to me that the index probe itself might be
more expensive than a hash probe.  Is that due to concurrency control,
or are you talking about the need to possibly read index pages in from
disk?

>> Experimentation shows this is actually about 25% faster.
>
> Well, that just says your cost parameters need a bit of adjustment
> if you'd like the planner to get the crossover point exactly right.

Any sense of which ones might be worth fiddling with?

...Robert