On Thu, Dec 21, 2017 at 05:00:35PM +0300, Konstantin Knizhnik wrote: > On 20.12.2017 19:26, Tom Lane wrote: > >Peter Eisentraut <peter.eisentr...@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. > >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. > > > > regards, tom lane > > Thank you for suggestion with AS_LA: it really works. > Actually instead of AS_LA I just return ASOF token if next token after AS is > OF. > So now it is possible to write query in this way: > > select * from foo as of timestamp '2017-12-21 14:12:15.1867';
Thanks for your hard work so far on this! It looks really exciting. > There is still one significant difference of my prototype implementation > with SQL standard: it associates timestamp with select statement, not with > particular table. > It seems to be more difficult to support and I am not sure that joining > tables from different timelines has much sense. I can think of a use case right offhand that I suspect would be very common: comparing the state of a table at multiple times. > But certainly it also can be fixed. That would be really fantastic. Best, David. -- David Fetter <david(at)fetter(dot)org> http://fetter.org/ Phone: +1 415 235 3778 Remember to vote! Consider donating to Postgres: http://www.postgresql.org/about/donate
