On 2014-12-19 22:03:55 -0600, Jim Nasby wrote:
> I'm not suggesting we change BufferTag or BufferLookupEnt; clearly we
> can't simply throw away any of the fields I was talking about (well,
> except possibly tablespace ID. AFAICT that's completely redundant for
> searching because relid is UNIQUE).
It's actually not. BufferTag's contain relnodes via RelFileNode - that's
not the relation's oid, but the filenode. And that's *not* guranteed
unique across database unfortunately.
> What I am thinking is not using all of those fields in their raw form to calculate the hash value. IE: something
analogousto:
>
> hash_any(SharedBufHash, (rot(forkNum, 2) | dbNode) ^ relNode) << 32 | blockNum)
>
> perhaps that actual code wouldn't work, but I don't see why we couldn't do something similar... am I missing
something?
I don't think that'd improve anything. Jenkin's hash does have a quite
mixing properties, I don't believe that the above would improve the
quality of the hash.
Greetings,
Andres Freund
-- Andres Freund http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training &
Services
