> On Feb 25, 2020, at 2:14 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>
> Paul Jungwirth <pj@illuminatedcomputing.com> writes:
>> Not that this is necessarily fatal, but you'd need to avoid parsing
>> trouble with the other EXCEPT, e.g.
>> SELECT 1 EXCEPT SELECT 1;
>
> Yeah, it doesn't sound like much consideration has been given to
> that ambiguity, but it's a big problem if you want to use a syntax
> like this.
>
>> Google Big Query was mentioned upthread. I see they require parens, e.g.
>> SELECT ... EXCEPT (...). I don't think that actually fixes the ambiguity
>> though.
>
> Indeed it doesn't, because you can parenthesize an EXCEPT's sub-queries:
>
> regression=# select 1 except (select 2);
> ?column?
> ----------
> 1
> (1 row)
>
> In principle, once you got to the SELECT keyword you could tell things
> apart, but I'm afraid that might be too late for a Bison-based parser.
>
>> So it seems they require at least one `*` in the SELECT target list. In
>> fact the `*` must be the very last thing. Personally I think it should
>> be as general as possible and work even without a `*` (let alone caring
>> about its position).
>
> I wonder if they aren't thinking of the EXCEPT as annotating the '*'
> rather than the whole SELECT list. That seems potentially more flexible,
> not less so. Consider
>
> SELECT t1.* EXCEPT (foo, bar), t2.* EXCEPT (baz) ... FROM t1, t2, ...
>
> This doesn't have any problem with ambiguity if t2 has a "foo" column,
> or if t1 has a "baz" column; which indeed would be cases where this
> sort of ability would be pretty useful, since otherwise you end up
> with painful-to-rename duplicate output column names. And certainly
> there is no particular need for this construct if you didn't write
> a "*".
>
> regards, tom lane
>
OMIT rather than EXCEPT?
>