I didn't follow the conversation from the begining, bu I imagine that you could improve performance using the value (work_units % (2^32) ) instead of work_units. You could even make an index on this value. Like that, the HASH function will work well. This is not a good solution, but ...
For example. create index ind1 on table1 ( work_units % (2^32) ); create index ind1 on table2 ( work_units % (2^32) ); Select * from table1 join table2 on (table1.work_units % (2^32) ) = (table2.work_units % (2^32) ) ----- Original Message ----- From: "Tom Lane" <[EMAIL PROTECTED]> To: "Jim C. Nasby" <[EMAIL PROTECTED]> Cc: <[EMAIL PROTECTED]> Sent: Saturday, April 17, 2004 6:08 PM Subject: Re: [PERFORM] Horribly slow hash join > [ resending because I fat-fingered the cc: to the list ] > > I see the problem: all the entries in your work_units column have the > low 32 bits equal to zero. > > regression=# select distinct work_units % (2^32)::bigint from Trank_work_overall; > ?column? > ---------- > 0 > (1 row) > > The hash function for int8 only takes the low word into account, so all > of the entries end up on the same hash chain, resulting in worst-case > behavior. This applies to both your hash join and hash aggregate cases. > > We could change the hash function, perhaps, but then we'd just have > different cases where there's a problem ... hashing will always fail on > *some* set of inputs. (Also, I have been harboring some notions of > supporting cross-type hash joins for integer types, which will not work > unless small int8 values hash the same as int4 etc.) > > I guess the real issue is why are you encoding work_units like that? > > regards, tom lane > > ---------------------------(end of broadcast)--------------------------- > TIP 6: Have you searched our list archives? > > http://archives.postgresql.org ---------------------------(end of broadcast)--------------------------- TIP 3: if posting/reading through Usenet, please send an appropriate subscribe-nomail command to [EMAIL PROTECTED] so that your message can get through to the mailing list cleanly
