Re: patch for geqo tweaks

From: Nathan Wagner <nw+pg(at)hydaspes(dot)if(dot)org>
To: pgsql-hackers(at)postgresql(dot)org
Subject: Re: patch for geqo tweaks
Date: 2015-11-06 03:14:25
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On Wed, Nov 04, 2015 at 12:51:52PM -0500, Tom Lane wrote:

> As for the second part, I had to look up Fisher-Yates ;-) but after
> having read Wikipedia's entry about it I think this is a good change.
> The code's shorter and more efficient, and it should mathematically
> provide an equally-unbiased initial shuffle. It could do with a
> better comment, and I'd be inclined to handle the first element
> outside the loop rather than uselessly computing geqo_randint(0,0),
> but those are trivial changes.

I see you committed a modified version of my patch in commit

You changed the tour[0] to be hardcoded to 1, but it should be any of
the possible gene numbers from 0 to remainder. If you want to pull the
geqo_randint(0,0) out of the loop, it would be the last element, not the
first (i.e. where remainder == 0).

We might be able to just skip the last swap, and the loop could be

for (i=0; i < num_gene-1; i++) {

but I'd need to re-read the details of the Fisher-Yates algorithm to be
sure. It may be that the last swap needs to happen for the shuffle to
be fully random. In any case, tour[0] certainly shouldn't be hardcoded
to 1.


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