Jon Nelson <jnelson+pgsql@xxxxxxxxxxx> writes: > On Thu, Jan 13, 2011 at 5:05 PM, Tom Lane <tgl@xxxxxxxxxxxxx> wrote: >> If you have enough memory to de-dup them individually, you surely have >> enough to de-dup all at once. > If everything were available up-front, sure. > However, and please correct me if I'm wrong, but doesn't postgresql > work in a fairly linear fashion, moving from table to table performing > a series of operations on each? Doing a single sort+uniq works like that. But the alternate plan you are proposing we should consider involves building all the lower hashtables, and then reading from them to fill the upper hashtable. Max memory consumption *is* worst case here. Remember HashAggregate is incapable of swapping to disk (and if it did, you wouldn't be nearly as pleased with its performance). regards, tom lane -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
