Re: AS OF queries

On 20.12.2017 19:26, Tom Lane 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.
> 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';

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.
But certainly it also can be fixed.

-- 
Konstantin Knizhnik
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company