I wrote:
> I feel uncomfortable about this proposal because it will compute
> different hashes for values that differ only in having different
> numbers of trailing zeroes.  Now the numeric.c code is supposed to
> suppress extra trailing zeroes on output, but that's never been a
> correctness property ... are we willing to make it one?

> There are various related cases involving unstripped leading zeroes.

> Another point is that sign = NUMERIC_NAN makes it a NAN regardless
> of any other fields; ignoring the sign does not get the right result
> here.

Something else I just remembered is that ndigits = 0 makes it a zero
regardless of the weight.

Perhaps a sufficiently robust way would be to form the hash as the
XOR of each supplied digit, circular-shifted by say 3 times the
digit's weight.  This is insensitive to leading/trailing zeroes:

        if (is NAN)
                return -1;      // or any other fixed value
        hash = 0;
        shift = 3 * weight;
        for (i = 0; i < ndigits; i++)
                thisshift = (shift & 31);
                hash |= ((uint32) digit[i]) << thisshift;
                if (thisshift > 0)
                        hash |= ((uint32) digit[i]) >> (32 - thisshift);
                shift -= 3;
        return hash;

That might look pretty ugly, but then again hash_any isn't especially

                        regards, tom lane

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