In parse_func.c there are routines argtype_inherit() and
gen_cross_product() that date from Berkeley days. The comments
explain their reason for existence thus:
* This function is used to handle resolution of function calls when
* there is no match to the given argument types, but there might be
* matches based on considering complex types as members of their
* superclass types (parent classes).
*
* It takes an array of input type ids. For each type id in the array
* that's a complex type (a class), it walks up the inheritance tree,
* finding all superclasses of that type. A vector of new Oid type
* arrays is returned to the caller, listing possible alternative
* interpretations of the input typeids as members of their superclasses
* rather than the actually given argument types. The vector is
* terminated by a NULL pointer.
*
* The order of this vector is as follows: all superclasses of the
* rightmost complex class are explored first. The exploration
* continues from right to left. This policy means that we favor
* keeping the leftmost argument type as low in the inheritance tree
* as possible. This is intentional; it is exactly what we need to
* do for method dispatch.
*
* The vector does not include the case where no complex classes have
* been promoted, since that was already tried before this routine
* got called.
I realized that this is effectively dead code: although it can be
executed, it can never produce any useful results. The reason is that
can_coerce_type() already knows that inherited rowtypes can be promoted
to their parent rowtypes, and it considers that a legal implicit
coercion. This means that any possible function matches based on
promoting child rowtypes to ancestors were found in func_get_detail()'s
first pass. If there is exactly one match then it will be taken as
the correct answer and returned without calling argtype_inherit().
If there is more than one match then func_get_detail() will fail
(return FUNCDETAIL_MULTIPLE), again without calling argtype_inherit().
The only way to reach argtype_inherit() is if there are *no* ancestor
matches, which means that the function is a very expensive no-op that
we execute just before throwing an error.
I'm strongly tempted to just rip out argtype_inherit() and
gen_cross_product(). Even if we suppose that we might want to resurrect
the claimed functionality someday, I don't think it could be made to
work this way. You'd have to put the knowledge into
func_select_candidate() instead, else there'd be very weird interactions
with the heuristics for resolving non-complex input argument types.
Thoughts?
regards, tom lane
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