Scott Carey <scott@xxxxxxxxxxxxxxxxx> writes: >> On Oct 27, 2010, at 12:56 PM, Tom Lane wrote: >> Because a poorly distributed inner relation leads to long hash chains. >> In the very worst case, all the keys are on the same hash chain and it >> degenerates to a nested-loop join. > A pathological skew case (all relations with the same key), should be > _cheaper_ to probe. I think you're missing the point, which is that all the hash work is just pure overhead in such a case (and it is most definitely not zero-cost overhead). You might as well just do a nestloop join. Hashing is only beneficial to the extent that it allows a smaller subset of the inner relation to be compared to each outer-relation tuple. So I think biasing against skew-distributed inner relations is entirely appropriate. regards, tom lane -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
