Andrew Gierth <andrew@tao11.riddles.org.uk> writes:
> "Tom" == Tom Lane <tgl@sss.pgh.pa.us> writes:
> Tom> The FROM FIRST/LAST bit seems particularly badly thought through,
> Tom> because AFAICS it is flat out ambiguous with a normal FROM clause
> Tom> immediately following the window function call. The only way to
> Tom> make it not so would be to make FIRST and LAST be fully reserved,
> Tom> which is neither a good idea nor spec-compliant.
> In the actual spec syntax it's not ambiguous at all because NTH_VALUE is
> a reserved word (as are LEAD, LAG, FIRST_VALUE and LAST_VALUE), and OVER
> is a mandatory clause in its syntax, so a FROM appearing before the OVER
> must be part of a FROM FIRST/LAST and not introducing a FROM-clause.
Hmm ...
> In our syntax, if we made NTH_VALUE etc. a col_name_keyword (and thus
> not legal as a function name outside its own special syntax) it would
> also become unambiguous.
> i.e. given this token sequence (with . marking the current posision):
> select nth_value(x) . from first ignore
> if we know up front that "nth_value" is a window function and not any
> other kind of function, we know that we have to shift the "from" rather
> than reducing the select-list because we haven't seen an "over" yet.
I don't really find that to be a desirable solution, because quite aside
from the extensibility problem, it would mean that a lot of errors
become "syntax error" where we formerly gave a more useful message.
This does open up a thought about how to proceed, though. I'd been
trying to think of a way to solve this using base_yylex's ability to
do some internal lookahead and change token types based on that.
If you just think of recognizing FROM FIRST/LAST, you get nowhere
because that's still legal in other contexts. But if you were to
look for FROM followed by FIRST/LAST followed by IGNORE/RESPECT/OVER,
I think that could only validly happen in this syntax. It'd take
some work to extend base_yylex to look ahead 2 tokens not one, but
I'm sure that could be done. (You'd also need a lookahead rule to
match "IGNORE/RESPECT NULLS OVER", but that seems just as doable.)
Then the relevant productions use FROM_LA, IGNORE_LA, RESPECT_LA
instead of the corresponding bare tokens, and the grammar no longer
has an ambiguity problem.
regards, tom lane