Re: unhelpful error message

"Kevin Grittner" <Kevin(dot)Grittner(at)wicourts(dot)gov> writes:
> It's used in an example in 34.4.2 without a lot of definition. From
> experimenting a bit, it appears that when referencing a composite data
> value, any function which can take as its only parameter an instance
> of that composite type can be used as though it were a field name.
> This includes user functions written in any language, as well as
> built-in aggregates (and presumably any other functions which accept a
> composite type as the only parameter). Is that correct?

It goes the other way too: a column name can be used as though it were
a function. You might want to look at the comments for and in
ParseFuncOrColumn in backend/parser/parse_func.c.

> Any
> restrictions or exceptions? (I assume that they are only allowed to
> retrieve values -- it doesn't seem like it would make sense to SET a
> value into such a "computed field".)

Right, this is only in places where a function call would be sensible.

> It's clearly not particular to SQL functions, so it deserves mention
> outside of the context you referenced. Chapter 4 does seem like a
> good place. Under Column References or Function Calls (or both)?

Not sure. I don't want to repeat a long spiel in both places ...

regards, tom lane