Re: hash_create API changes (was Re: speedup tidbitmap patch: hash BlockNumber)

I wrote:
> Here's a proposed patch along this line.  I left in oid_hash (in the
> form of a macro) so that this does not cause any API break for existing
> third-party modules.  However, no callers in our own code directly
> refer to tag_hash or oid_hash anymore.

Committed that version after some further comment wordsmithing.

On Teodor's original test cases, I see about 8% speedup compared to
the 4%-ish numbers he originally reported.  This may be random variation
or it might mean that we got a win someplace else besides tidbitmap.c.
I've not tried to sleuth it down exactly.  I am wondering though if
this suggests that it'd be worth our while to add a similar fast path
for 8-byte hash keys.  That would be quite painless to add now (with
the exception of actually coding the fast hash function, perhaps).
        regards, tom lane