Re: Why don't we support external input/output functions for the composite types

On Fri, Apr 26, 2024 at 12:54 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> But that's exactly the point: we DON'T consider the initial identifier
> of a qualified name "as a unit", and neither does the standard.
> We have to figure out how many of the identifiers name an object
> (column or table) referenced in the query, and that is not clear
> up-front.  SQL99's rules don't make that any better.  The parens in
> our current notation serve to separate the object name from any field
> selection(s) done on the object.  There's still some ambiguity,
> because we allow you to write either "(table.column).field" or
> "(table).column.field", but we've dealt with that for ages.

I agree that this is exactly the point.

No other programming language that I know of, and no other database
that I know of, looks at x.y.z and says "ok, well first we have to
figure out whether the object is named x or x.y or x.y.z, and then
after that, we'll use whatever is left over as a field selector."
Instead, they have a top-level namespace where x refers to one and
only one thing, and then they look for something called y inside of
that, and if that's a valid object then they look inside of that for
z.

JavaScript is probably the purest example of this. Everything is an
object, and x.y just looks up 'x' in the object that is the current
namespace. Assuming that returns an object rather than nothing, we
then try to find 'y' inside of that object.

I'm not an Oracle expert, but I am under the impression that the way
that Oracle works is closer to that than it is to our
read-the-tea-leaves approach.

I'm almost positive you're about to tell me that there's no way in the
infernal regions that we could make a semantics change of this
magnitude, and maybe you're right. But I think our current approach is
deeply unsatisfying and seriously counterintuitive. People do not get
an error about x.y and think "oh, right, I need to write (x).y so that
the parser understands that the name is x rather than x.y and the .y
part is field-selection rather than a part of the name itself." They
get an error about x.y and say "crap, I guess this syntax isn't
supported" and then when you show them that "(x).y" fixes it, they say
"why in the world does that fix it?" or "wow, that's dumb."

Imagine if we made _ perform string concatenation but also continued
to allow it as an identifier character. When we saw a_b without
spaces, we'd test for whether there's an a_b variable, and/or whether
there are a and b variables, to guess which interpretation was meant.
I hope we would all agree that this would be insane language design.
Yet that's essentially what we've done with period, and I don't think
we can blame that on the SQL standard, because I don't think other
systems have this problem. I wonder if anyone knows of another system
that works like PostgreSQL in this regard (without sharing code).

--
Robert Haas
EDB: http://www.enterprisedb.com