On Fri, Apr 13, 2012 at 10:01 PM, Tom Lane <t...@sss.pgh.pa.us> wrote: > Greg Stark <st...@mit.edu> writes: >> On Fri, Apr 13, 2012 at 8:15 PM, Robert Haas <robertmh...@gmail.com> wrote: >>> That's probably true, but I'm not sure it's worth worrying about - >>> one-in-four-billion is a pretty small probability. > >> Is this not subject to the birthday paradox? If you have a given hash >> you're worried about a collision with then you have a >> one-in-four-billion chance. But if you have a collection of hashes and >> you're worried about any collisions then it only takes about 64k >> before there's likely a collision. > > ... so, if pg_stat_statements.max were set as high as 64k, you would > need to worry.
Well... at 64k, you'd be very likely to have a collision. But the whole birthday paradox thing means that there's a non-trivial collision probability even at lower numbers of entries. Seems like maybe we ought to be using 64 bits here... > Realistically, I'm more worried about collisions due to inadequacies in > the jumble calculation logic (Peter already pointed out some risk > factors in that regard). ...especially if collisions are even more frequent than random chance would suggest. -- 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
