Re: Opaque data type

Actually, that cleared things up quite a bit.  While it may be a bit
confusing, I'm no longer standing on the "ignorant" square.

Thanks!

Tom Lane wrote:

> John Burski <john.burski@911ep.com> writes:
> > Could someone clarify the concept that underlies the opaque data type?
>
> It *is* confusing, because OPAQUE is actually used for several distinct
> purposes.  If you look in the pg-hackers mail list archives, you'll find
> discussions (as yet unimplemented) to replace OPAQUE by several
> pseudo-types that have crisper meanings.
>
> Offhand I can recall these uses:
>
> * Sometimes OPAQUE means "this function doesn't return anything at all",
>   similarly to "returns void" in C and some other languages.
>
> * Sometimes OPAQUE means "this value can be any datatype at all" (eg,
>   input of the count() aggregate).
>
> * Sometimes OPAQUE means "I know exactly what this should be, but it's
>   an internal datatype with no SQL-level equivalent".  This is usually
>   what's at stake with a trigger function.
>
> * Sometimes OPAQUE means "I know exactly what this should be, but I
>   haven't declared it yet" (ie, a forward reference).  This is needed
>   to declare I/O functions of user-defined types, since the system
>   requires the functions to already exist when the type is declared.
>
> Confused yet?  Basically, OPAQUE has been used as an easy out in rather
> more places than it should have been :-(.  Someday we'll clean this up.
>
>                         regards, tom lane

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John Burski
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911EP
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