Yes, i wrote this for pleasure and discusion, not for solve a real problem :).
2011/9/25, Tom Lane <tgl@sss.pgh.pa.us>:
> =?ISO-8859-2?Q?pasman_pasma=F1ski?= <pasman.p@gmail.com> writes:
>> I found second use case. Look at expression:
>> where left(str,n)='value'
>
>> function left(str,n) increase monotonically for str and n. With this
>> feature it can use index on str.
>
> Can't get excited about that, because that only works in C locale,
> and in C locale you can already get the same result with
> WHERE str LIKE '...%'
>
> Also, I think you just moved the goalposts quite a bit by introducing
> multiple-argument functions into the proposed feature. That's going
> to add even more complexity, for instance there would need to be a way
> to specify which argument(s) the function was monotonic in. The C
> versus not-C locale aspect also shows that for textual arguments,
> it might matter which locale you're talking about.
>
> In short, this is looking awfully complicated, and I gauge the probable
> level of interest by the fact that you're the first person to ask for it
> in more than a dozen years of Postgres development.
>
> regards, tom lane
>
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pasman