Re: Surjective functional indexes

Re: Konstantin Knizhnik 2017-05-29 <592BBD90(dot)3060104(at)postgrespro(dot)ru>
> On 05/27/2017 09:50 PM, Peter Eisentraut wrote:
> > On 5/25/17 12:30, Konstantin Knizhnik wrote:
> > > Functions like (info->>'name') are named "surjective" ni mathematics.
> > A surjective function is one where each value in the output type can be
> > obtained by some input value. That's not what you are after here. The
> > behavior you are describing is a not-injective function.
> >
> > I think you are right that in practice most functions are not injective.
> > But I think there is still quite some difference between a function
> > like the one you showed that selects a component from a composite data
> > structure and, for example, round(), where in practice any update is
> > likely to change the result of the function.
> >
> Thank you, I will rename "surjective" parameter to "injective" with "false" as default value.

I think the term you were looking for is "projection".

https://en.wikipedia.org/wiki/Projection_(set_theory)

Christoph