>>>>> "Tom" == Tom Lane <t...@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. 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. (Neither "first" nor "ignore" are reserved, so "select foo(x) from first ignore;" is a valid and complete query, and without reserving the function name we'd need at least four tokens of lookahead to decide otherwise.) This is why I think the col_name_keyword option needs to be given serious consideration - it still doesn't reserve the names as strongly as the spec does, but enough to make the standard syntax work without needing any dubious hacks. -- Andrew (irc:RhodiumToad)
