On Mon, Jan 7, 2013 at 3:27 PM, Tom Lane <t...@sss.pgh.pa.us> wrote: > > One issue that needs some thought is that the argument for this formula > is based entirely on thinking about b-trees. I think it's probably > reasonable to apply it to gist, gin, and sp-gist as well, assuming we > can get some estimate of tree height for those, but it's obviously > hogwash for hash indexes. We could possibly just take H=0 for hash, > and still apply the log2(N) part ... not so much because that is right > as because it's likely too small to matter.
Height would be more precisely "lookup cost" (in comparisons). Most indexing structures have a well-studied lookup cost. For b-trees, it's log_b(size), for hash it's 1 + size/buckets. -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
