Re: [PERFORM] Hash Anti Join performance degradation

Robert Haas <robertmhaas@gmail.com> writes:
> On Wed, Jun 1, 2011 at 4:25 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> Because of the way that a bitmap heap scan works, the rows are
>> guaranteed to be loaded into the hash table in physical order, which
>> means (in the fast case) that the row with the largest "id" value gets
>> loaded last. And because ExecHashTableInsert pushes each row onto the
>> front of its hash chain, that row ends up at the front of the hash
>> chain. Net result: for all the outer rows that aren't the one with
>> maximum id, we get a joinqual match at the very first entry in the hash
>> chain. Since it's an antijoin, we then reject that outer row and go
>> on to the next. The join thus ends up costing us only O(N) time.

> Ah!  Make sense.  If I'm reading your explanation right, this means
> that we could have hit a similar pathological case on a nestloop as
> well, just with a data ordering that is the reverse of what we have
> here?

Yeah.  It's just chance that this particular data set, with this
particular ordering, happens to work well with a nestloop version
of the query.  On average I'd expect nestloop to suck even more,
because of more per-inner-tuple overhead.

			regards, tom lane