On Thu, Jan 13, 2011 at 2:05 PM, Tom Lane <t...@sss.pgh.pa.us> wrote: > Jon Nelson <jnelson+pg...@jamponi.net> writes: >> Your comment regarding "each individual de-duplication looked like it >> would fit in work_mem" doesn't really make sense, exactly. Maybe I'm >> misunderstanding you. > > Yeah. What I was suggesting was to NOT add the DISTINCT's, but instead > raise work_mem high enough so you get just one HashAggregation step at > the top level. (Although I think this only works in 8.4 and up.) > That should be faster than two levels of de-duplication.
Gave it a try -- performance either way doesn't seem to change - although the final set that has to undergo de-duplication is rather larger (WITHOUT DISTINCT) so I still run the risk of not getting Hash Aggregation. Since having the DISTINCT doesn't seem to hurt, and it avoids (potential) significant pain, I'll keep it. I still think that having UNION do de-duplication of each contributory relation is a beneficial thing to consider -- especially if postgresql thinks the uniqueness is not very high. Thanks! -- Jon -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
