Laurenz Albe <laurenz.albe@cybertec.at> writes:
> Konstantin Knizhnik wrote:
>> I failed to support AS OF clause (as in Oracle) because of shift-reduce
>> conflicts with aliases,
>> so I have to introduce new ASOF keyword. May be yacc experts can propose
>> how to solve this conflict without introducing new keyword...
> I think it would be highly desirable to have AS OF, because that's
> the way the SQL standard has it.
Please say that's just an Oracle-ism and not SQL standard, because it's
formally ambiguous. This is required to work by spec:
regression=# select x as of from (values(1)) t(x);
of
----
1
(1 row)
so it's not possible for us ever to support an expression that includes
top-level "AS OF" (or, pretty much, "AS anything") without some rather
enormous pushups.
If we absolutely had to do it, the path to a solution would involve some
lexer-level lookahead, cf base_yylex() --- but that's messy and tends to
introduce its own set of corner case misbehaviors. I'd much rather use a
syntax that wasn't chosen with blind disregard for SQL's existing
syntactic constraints.
regards, tom lane