Re: update substring pattern matching syntax

Hello Peter,

> whereas the current standard says
>
>    SUBSTRING(text SIMILAR pattern ESCAPE escapechar)
>
> The former was in SQL99, but the latter has been there since SQL:2003.
>
> It's pretty easy to implement the second form also, so here is a patch that 
> does that.

Patches apply cleanly, compile and "make check" is ok. doc gen is ok as 
well.

Grammar cleanup is a definite improvement as it makes the grammar closer 
to the actual syntax.

I cannot say I'm a fan of this kind of keywords added for some arguments. 
I guess that it allows distinguishing between variants. I do not have the 
standard at hand: I wanted to check whether these keywords could be 
reordered, i.e. whether SUBSTRING(text ESCAPE ec SIMILAR part) was legal. 
I guess not.

Maybe the doc could advertise more systematically whether a features 
conforms fully or partially to some SQL standards, or is pg specific. The 
added documentation refers both to SQL:1999 and SQL99. I'd suggest to 
chose one, possibly the former, and use it everywhere consistently.

It seems that two instances where not updated to the new syntax, see in 
./src/backend/catalog/information_schema.sql and 
./contrib/citext/sql/citext.sql.

-- 
Fabien.