Re: Question about indexes

Greg Stark wrote:
>
> Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> writes:
>
> > In any case, this discussion is predicated on the assumption that the
> > operations involving the bitmap are a significant fraction of the total
> > time, which I think is quite uncertain. Until we build it and profile
> > it, we won't know that.
>
> The other thought I had was that it would be difficult to tell when to follow
> this path. Since the main case where it wins is when the individual indexes
> aren't very selective but the combination is very selective, and we don't have
> inter-column correlation statistics ...

We actually have heap access cost and index access cost. You could
compare costs of looking at all of index A's heap vs. looking at index
B and then hopefully fewer heap rows.

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