Josh Berkus <josh@agliodbs.com> writes:
> Charles,
>> Sample query to return non-uniqueness
>> SELECT A1, A2, A3, ..., An
>> FROM Table
>> GROUP BY A1, A2, A3, ..., An
>> HAVING Count(*)>1
> In order for it to be even possible to use an index (a hashaggregate
> operation, actually) on this table, you'd have to include *all* of the GROUP
> BY columns in a single, multi-column index.
> However, it would be unlikely for PG to use any kind of an index in the
> operation above, because of the number of columns, the unlikelyness of
> grouping (i.e. there will only be a minority of rows with count(*) > 1) and
> the fact that you're running this against the whole table. So any
> kind of an index is liable to be useless.
Yeah. If you are not expecting a huge number of groups, I think that it
would be more interesting to try a HashAggregate plan than a sort/group
plan. For this you need 7.4 or later and a sort_mem setting large
enough to cover whatever the planner estimates the hashtable size to be.
regards, tom lane
