Re: get_fn_expr_variadic considered harmful

Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> On Tue, Apr 1, 2014 at 12:52 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> In bug #9817 there's a complaint that the planner fails to consider
>> these expressions equivalent:
>> foo('a'::text, 'b'::text)
>> foo(variadic array['a'::text, 'b'::text])
>> when foo() is declared as taking variadic text[].

> My first reaction to this was "who cares? after all, the user should
> just write the expression the same way both times and then they won't
> have this problem". But after going and looking at the bug report I
> see that the user wrote it the first way consistently, but pg_dump
> blithely rewrote it to the second way. I'm disinclined to view that
> as a planner problem; it seems to me to be a pg_dump or ruleutils bug.
> If those two things don't have the same parse representation, then
> pg_dump has no business treating them as equivalent - even if we were
> to put enough smarts into the planner to paper over that
> non-equivalence.

The point is that they *were* equivalent before 9.3, and so ruleutils
was entirely within its rights to not worry about which way it dumped
the expression; indeed, it couldn't, because the information was not
there as to which way the call had been written originally. I do not
think it's appropriate to blame ruleutils for taking advantage of this
equivalence, because more than likely user applications have too.

Or in other words, what I wrote above is a more general statement of the
problem than what was complained of in bug #9817 ... but if we just hack
ruleutils to dump the cases differently, we will fail to fix the more
general problem. So we can still expect future bug reports about that,
because it worked as-expected for years before 9.3.

There's also the point that even if we changed ruleutils' behavior
now, this would not fix existing dump files that have considered the
two forms interchangeable ever since VARIADIC existed. And we
generally try hard to not break existing dump files. To be even
more to the point: what you propose is incapable of fixing the precise
problem stated in the bug report, because it's complaining about a
dump taken from 9.1, and there is *no* way to make 9.1 produce a
dump that only uses VARIADIC if the original call did. It hasn't
got the information. Even using a newer version of pg_dump wouldn't
help that.

regards, tom lane