Re: new correlation metric

On Sun, Oct 26, 2008 at 01:38:02AM -0700, Jeff Davis wrote:
> I worked with Nathan Boley to come up with what we think is a better
> metric for measuring this cost. It is based on the number of times in
> the ordered sample that you have to physically backtrack (i.e. the data
> value increases, but the physical position is earlier).
>
> For example, if the table's physical order is
>
> 6 7 8 9 10 1 2 3 4 5

How does it deal with a case like the following:

1 6 2 7 3 8 4 9 5 10  (interleaving)

ISTM that your code will overestimate the cost whereas the old code
wouldn't have done too badly.

I think the code is in the right direction, but I think want you want
is some kind of estimate of "given I've looked for tuple X, how many
tuples in the next k pages are near this one". Unfortunatly I don't see
a way of calculating it other than a full simulation.

Have a nice day,
--
Martijn van Oosterhout   <kleptog@svana.org>   http://svana.org/kleptog/
> Please line up in a tree and maintain the heap invariant while
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