Re: Add RESPECT/IGNORE NULLS and FROM FIRST/LAST options

>>>>> "Tom" == Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> writes:

Tom> If you just think of recognizing FROM FIRST/LAST, you get nowhere
Tom> because that's still legal in other contexts. But if you were to
Tom> look for FROM followed by FIRST/LAST followed by
Tom> IGNORE/RESPECT/OVER, I think that could only validly happen in
Tom> this syntax.

No; you need to go four tokens ahead in total, not three. Assuming
nth_value is unreserved, then

select nth_value(x) from first ignore;

is a valid query that has nth_value(x) as an expression, "first" as a
table name and "ignore" as its alias. Only when you see NULLS after
IGNORE, or OVER after FIRST/LAST, do you know that you're looking at
a window function and not a from clause.

So FROM_LA would have to mean "FROM" followed by any of:

FIRST IGNORE NULLS
LAST IGNORE NULLS
FIRST RESPECT NULLS
LAST RESPECT NULLS
FIRST OVER
LAST OVER

Remember that while OVER is reserved, all of FIRST, LAST, RESPECT and
IGNORE are unreserved.

Tom> It'd take some work to extend base_yylex to look ahead 2 tokens
Tom> not one, but I'm sure that could be done. (You'd also need a
Tom> lookahead rule to match "IGNORE/RESPECT NULLS OVER", but that
Tom> seems just as doable.) Then the relevant productions use FROM_LA,
Tom> IGNORE_LA, RESPECT_LA instead of the corresponding bare tokens,
Tom> and the grammar no longer has an ambiguity problem.

Yeah, but at the cost of having to extend base_yylex to go 3 tokens
ahead (not 2) rather than the current single lookahead slot.

Doable, certainly (probably not much harder to do 3 than 2 actually)

--
Andrew (irc:RhodiumToad)