On Tue, Jul 25, 2006 at 12:36:42AM -0400, Tom Lane wrote:
> mark@mark.mielke.cc writes:
> > Reading 1/4, for a larger table, has a good chance of being faster than
> > reading 4/4 of the table. :-)
> Really?
>
> If you have to hit one tuple out of four, it's pretty much guaranteed
> that you will need to fetch every heap page. So using an index provides
> zero I/O savings on the heap side, and any fetches needed to read the
> index are pure cost. Now you have to demonstrate that the CPU costs
> involved in processing the index are significantly cheaper than the cost
> of just testing the WHERE qual at every heap tuple --- not a bet that's
> likely to win at a one-in-four ratio.
Haha. Of course - but that's assuming uniform spread of the values.
Next I would try clustering the table on the bitmap index... :-)
My databases aren't as large as many of yours. Most or all of them
will fit in 1 Gbytes of RAM. The I/O cost isn't substantial for these,
but the WHERE clause might be.
But yeah - we don't know. Waste of code or performance boost.
Cheers,
mark
--
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