On Sat, Oct 30, 2010 at 10:01 AM, Tom Lane <t...@sss.pgh.pa.us> wrote: > marcin mank <marcin.m...@gmail.com> writes: >> On Sat, Oct 30, 2010 at 6:21 PM, Tom Lane <t...@sss.pgh.pa.us> wrote: >>> 3. To hash, apply the element type's hash function to each array >>> element. Combine these values by rotating the accumulator left >>> one bit at each step and xor'ing in the next element's hash value. >>> >>> Thoughts? In particular, is anyone aware of a better method >>> for combining the element hash values? > >> This would make the hash the same for arrays with elements 32 apart swapped. > > Well, there are *always* going to be cases where you get the same hash > value for two different inputs; it's unavoidable given that you have to > combine N 32-bit hash values into only one 32-bit output.
Sure. The goal is to make those hard to predict, though. I think "multiply by 31 and add the next value" is a fairly standard way of getting that behavior. It mixes up the bits a lot more than just left-shifting by a variable offset. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
