Re: Precedence of standard comparison operators

Kevin Grittner <kgrittn(at)ymail(dot)com> writes:
> Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> the precedence of <= >= and <> is neither sane nor standards compliant.

> I wonder whether it would be feasible to have an option to generate
> warnings (or maybe just LOG level messages?) for queries where the
> results could differ.

My guess (admittedly not yet based on much) is that warnings won't be too
necessary. If a construction is parsed differently than before, you'll
get no-such-operator gripes. The case of interest is something like

a <= b %% c

which was formerly

(a <= b) %% c

and would become

a <= (b %% c)

Now, if it worked before, %% must expect a boolean left input; but the
odds are pretty good that b is not boolean.

This argument does get a lot weaker when you consider operators that
take nearly anything, such as ||; for instance if a b c are all text
then both parsings of

a <= b || c

are type-wise acceptable. But that's something that I hope most people
would've parenthesized to begin with, because (a <= b) || c is not exactly
the intuitive expectation for what you'll get.

Anyway, to answer your question, I think that Bison knows somewhere inside
when it's making a precedence-driven choice like this, but I don't believe
it's exposed in any way that we could get at easily. Perhaps there would
be a way to produce a warning if we hand-hacked the C-code bison output,
but we're not gonna do that.

regards, tom lane