Re: unhelpful error message

Tom Lane <tgl@sss.pgh.pa.us> wrote:

> Look under "computed fields" in the index ... looks like it's
> towards the bottom of 34.4.2 in the 8.3 docs.
> http://www.postgresql.org/docs/8.3/static/xfunc-sql.html#AEN40267
>
> I had thought it was mentioned somewhere in chapter 4 as well, but
> am not seeing it there right now.

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?  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".)

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)?

-Kevin