Re: the LHS of OPTIONAL

Lee Feigenbaum wrote:
> (I promised Andy a note on my concerns about OPTIONAL, so here it is. 
> Opinions sought.)

Thanks!

...
> 
> {
>   { T1 } UNION { T2 } .
>   T3 .
>   OPTIONAL { T4 . }
> }
> 
> Is this:
>   Group( Optional( Group( Union(T1, T2), T3), Group(T4) ) ) )
> or
>   Group( Union(T1, T2), Optional( Group(T3), Group(T4) ) )
> 
> That is, does OPTIONAL pull in everything before it (but in the same 
> group) to form the implicit LHS group, or just the nearest bunch of triple 
> patterns and filters? I suspect the former, but find this confusing.

rq24-algebra provides a decision procedure for this (roughly, left-associativity).

A group is a sequence of Join()'s or LeftJoins() (for OPTIONAL).  There is no 
optional() as such.  There is no Group() after it is translated into the algebra.

	Andy