On 03/04/2018 01:14 AM, Tomas Vondra wrote:
> ...
>
> The one overflow issue I found in the patch is that the numeric
> "distance" function does this:
>
> d = DirectFunctionCall2(numeric_sub, a2, a1); /* a2 - a1 */
>
> PG_RETURN_FLOAT8(DirectFunctionCall1(numeric_float8, d));
>
> which can overflow, of course. But that is not fatal - the index may get
> inefficient due to non-optimal merging of ranges, but it will still
> return correct results. But I think this can be easily improved by
> passing not only the two values, but also minimum and maximum, and use
> that to normalize the values to [0,1].
>
Attached is an updated patch series, addressing this possible overflow
the way I proposed - by computing (a2 - a1) / (b2 - b1), which is
guaranteed to produce a value between 0 and 1.
The two new arguments are ignored for most "distance" functions, because
those can't overflow or underflow in double precision AFAICS.
regards
--
Tomas Vondra http://www.2ndQuadrant.com
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