Re: [patch] bit XOR aggregate functions

On Sat, Mar 06, 2021 at 09:03:25PM +0100, Vik Fearing wrote:
> On 3/6/21 9:00 PM, David Fetter wrote:
> > On Sat, Mar 06, 2021 at 08:57:46PM +0100, Vik Fearing wrote:
> >> On 3/6/21 8:55 PM, David Fetter wrote:
> >>> On Wed, Mar 03, 2021 at 03:30:15PM +0100, Peter Eisentraut wrote:
> >>>> On 10.02.21 06:42, Kyotaro Horiguchi wrote:
> >>>>> We already had CREATE AGGREATE at the time, so BIT_XOR can be
> >>>>> thought as it falls into the same category with BIT_AND and
> >>>>> BIT_OR, that is, we may have BIT_XOR as an intrinsic aggregation?
> >>>>
> >>>> I think the use of BIT_XOR is quite separate from BIT_AND and
> >>>> BIT_OR. The latter give you an "all" or "any" result of the bits
> >>>> set. BIT_XOR will return 1 or true if an odd number of inputs are 1
> >>>> or true, which isn't useful by itself. But it can be used as a
> >>>> checksum, so it seems pretty reasonable to me to add it. Perhaps
> >>>> the use case could be pointed out in the documentation.
> >>>
> >>> If this is the only use case, is there some way to refuse to execute
> >>> it if it doesn't contain an unambiguous ORDER BY, as illustrated
> >>> below?
> >>>
> >>> SELECT BIT_XOR(b ORDER BY a, c)... /* works */
> >>> SELECT BIT_XOR(b) OVER (ORDER BY a, c)... /* works */
> >>> SELECT BIT_XOR(b) FROM... /* errors out */
> >>
> >>
> >> Why would such an error be necessary, or even desirable?
> >
> > Because there is no way to ensure that the results remain consistent
> > from one execution to the next without such a guarantee.
>
> I think one of us is forgetting how XOR works.

Oops. You're right.

Best,
David.
--
David Fetter <david(at)fetter(dot)org> http://fetter.org/
Phone: +1 415 235 3778

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