On Wed, Dec 20, 2017 at 4:26 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
Peter Eisentraut <peter.eisentraut@2ndquadrant.com> writes: > On 12/20/17 10:29, Tom Lane wrote: >> Please say that's just an Oracle-ism and not SQL standard, because it's >> formally ambiguous.
> The SQL standard syntax appears to be something like
> "tablename" [ AS OF SYSTEM TIME 'something' ] [ [ AS ] "alias" ]
> That's not going to be fun to parse.
Examples from DB2 documentation (which may be closer to the standard):
SELECT coverage_amt
FROM policy FOR SYSTEM_TIME AS OF '2010-12-01'
WHERE id = 1111;
SELECT count(*)
FROM policy FOR SYSTEM_TIME FROM '2011-11-30'
TO '9999-12-30'
WHERE vin = 'A1111';
So besides AS .. AS , it could also be FROM .. FROM
Bleah. In principle we could look two tokens ahead so as to recognize "AS OF SYSTEM", but base_yylex is already a horrid mess with one-token lookahead; I don't much want to try to extend it to that.
Possibly the most workable compromise is to use lookahead to convert "AS OF" to "AS_LA OF", and then we could either just break using OF as an alias, or add an extra production that allows "AS_LA OF" to be treated as "AS alias" if it's not followed by the appropriate stuff.
It's a shame that the SQL committee appears to be so ignorant of standard parsing technology.