On Mon, Mar 14, 2016 at 12:04 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> I wrote:
>> However ... one thing I was intending to mention on this thread is that
>> "get the array type over this type" isn't the only extension one might
>> wish for. Another likely desire is "get the type of field 'foo' of this
>> composite type". I don't suggest that this patch needs to implement
>> that right now; but it would be a good thing if we could see how the
>> chosen syntax could be extended in such a direction. Otherwise we might
>> be painting ourselves into a corner.
>
> To enlarge a little bit: it seems to me that what we're really wishing for
> here is a type name syntax that goes beyond simple names. If we were
> starting in a green field, we might choose a recursively-composable syntax
> like the following.
>
> type_name can be:
>
> * A simple type name, such as int8 or varchar[42].
>
> * TYPE_OF(expression), meaning that the SQL expression is parsed but never
> executed, we just take this construct as naming its result type.
>
> * ARRAY_OF(type_name), meaning the array type having type_name as its
> element type.
>
> * ELEMENT_OF(type_name), meaning the element type of the array type
> named by type_name.
>
> * ROW_OF(type_name [, type_name ...]), meaning the composite type with
> those types as columns.
>
> * FIELD_OF(type_name, foo), meaning the type of column "foo" of the
> composite type named by type_name. I'm not sure if there would be
> use-cases for selecting a column other than by a simple literal name,
> but there could be variants of this function if so.
>
> It's possible to think of other cases, for example what about range
> types? You could allow ELEMENT_OF() to apply to range types, certainly.
> I'm not sure about the other direction, because multiple range types
> could have the same element type; but it's possible to hope that some
> type-naming function along the lines of RANGE_OF(type_name, other args)
> could disambiguate. The main reason I'm thinking of a function-like
> syntax here is that it can easily handle additional arguments when
> needed.
>
> Comparing this flight of fancy to where we are today, we have
> %TYPE as a remarkably ugly and limited implementation of TYPE_OF(),
> and we have the precedent that foo[] means ARRAY_OF(foo). I'm not
> sure how we get any extensibility out of either of those things.
>
> Or in short: maybe it's time to blow up %TYPE and start fresh.
That's not a dumb idea. I think %TYPE is an Oracle-ism, and it
doesn't seem to have been their best-ever design decision.
--
Robert Haas
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