On 2018-08-28 13:50:43 +1200, Thomas Munro wrote: > I'm curious about something which may qualify as a stupid question. > > What bad thing would happen if we used OIDs directly as hash values in > internal hash tables (that is, instead of uint32_hash() we'd use > uint32_identity(), or somehow optimise it away entirely, as you can > see in some C++ standard libraries for eg std::hash<int>)?
Oids are very much not equally distributed, so in all likelihood you'd get cases very you currently have a reasonably well averaged out usage of the hashtable, not be that anymore. It's also fairly cheap to hash an oid. > However, as far as I can see OIDs are expected to have an even > distribution (or at least we don't expect regular sized gaps), so the > hazard doesn't seem to apply. Huh? Oids between, say, 1 and FirstNormalObjectId, are vastly more common than the rest. And even after that, individual tables get large clusters of sequential values to the global oid counter. Greetings, Andres Freund
