Re: multi-column index

David Brown <time@bigpond.net.au> writes:
> Actually, I'm surprised the planner came up with such a low cost for the
> single column index, unless ... perhaps correlation statistics aren't
> used when determining costs for multi-column indexes?

The correlation calculation for multi-column indexes is pretty whacked
out pre-8.0.  I don't think it's that great in 8.0 either --- we really
need to make ANALYZE calculate the correlation explicitly for each
index, probably, rather than trying to use per-column correlations.

            regards, tom lane