Re: [PERFORM] Hash Anti Join performance degradation

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?

-- 
Robert Haas
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