Jon Nelson <jnelson+pg...@jamponi.net> writes: > A rough summary of the patch follows:
> - a GUC variable enables or disables this capability > - in nodeAgg.c, eliding duplicate tuples is enabled if the number of > distinct columns is equal to the number of sort columns (and both are > greater than zero). > - in createplan.c, eliding duplicate tuples is enabled if we are > creating a unique plan which involves sorting first > - ditto planner.c > - all of the remaining changes are in tuplesort.c, which consist of: > + a new macro, DISCARDTUP and a new structure member, discardtup, are > both defined and operate similar to COMPARETUP, COPYTUP, etc... > + in puttuple_common, when state is TSS_BUILDRUNS, we *may* simply > throw out the new tuple if it compares as identical to the tuple at > the top of the heap. Since we're already performing this comparison, > this is essentially free. > + in mergeonerun, we may discard a tuple if it compares as identical > to the *last written tuple*. This is a comparison that did not take > place before, so it's not free, but it saves a write I/O. > + We perform the same logic in dumptuples [ raised eyebrow ... ] And what happens if the planner drops the unique step and then the sort doesn't actually go to disk? regards, tom lane -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers