Re: [HACKERS] Why type coercion is not performed for parameters?

Konstantin Knizhnik <k.knizhnik@postgrespro.ru> writes:
> So my question is whether it is possible to use the same rule for type 
> coercion of parameters as for constant?

It's not possible, I think, and even if it is, we would almost certainly
reject a patch that tried to do it.

There are four different ways to spell type coercion of a literal:
cast('foo' as typename)'foo'::typenametypename_thats_a_simple_identifier('foo')typename 'foo'

The last of those is a huge syntactic PITA because it's so close to
being ambiguous against other constructs.  If you tried to allow
anything but a string literal there, it almost certainly would be
ambiguous, resulting in bison failures.  Even if you managed to
wedge it into the grammar today, I for one would vote to reject the
patch because of the near certainty that it would result in syntactic
conflicts further down the line.

The others are better for your purposes, because at least syntactically
they allow either a literal or something else as the subject of the
coercion.  But there's still an important point I think you're missing,
which is that even though these syntaxes look like type coercion (that
is, run-time conversion of values from one type to another), they are
not that when applied to a literal string.  Instead they represent
initial assignment of a type to the literal; so they feed the string
to the type's typinput function and then produce a Const with a resolved
type, not a type-coercion expression node.

Params are sort of a mess because depending on parser context, an attempt
to coerce them might result in either a runtime type coercion, or a
decision that a previously-indeterminate-type Param is now of a known
type.  The latter bears some similarities to assignment of a type to
an unknown literal, but it's not the same thing.

The code you are looking at in func_get_detail() can handle the situation
where the argument is a literal, because it knows what coerce_type() will
do in that case.  However, it does not know what coerce_type() would do
with a Param, and it can't readily find out because that information is
hidden behind a parser-hook API.  As noted in the comments in
func_get_detail, we *must not* return FUNCDETAIL_COERCION unless we know
that coerce_type will succeed, and we do not know that for the case you
are concerned with.

I could imagine extending the parser hook API, in the direction of adding
a function that can be asked "if we were to call p_coerce_param_hook on
this Param, would that be interpreted as a type assignment?".  But that
seems ugly: it's only squishily defined, and it would require a bunch of
places to supply additional hook code.

Given that you're not going to get anywhere with the "typename $1"
syntax, I don't see much point in complicating the parser hook API
to resolve the third case.  You need to think of a different way
to approach what you're trying to do.  Personally I'd think about
replacing the entire literal-with-cast construct with a Param having
already-known type.
        regards, tom lane