[DRAFT] Questions about REDUCED, LeftJoin, Join

To-be-to: Jiri Dokulil <Jiri.Dokulil@mff.cuni.cz>

/me just addressing the first one.

* Jiri Dokulil <Jiri.Dokulil@mff.cuni.cz> [2007-04-24 09:22+0200]
> 
> Hello all,
> 
> 1.
> I'm confused by REDUCED in the current version of the draft. Is
> REDUCED going to be removed or not? Because it is only described in
> the 9.4 REDUCED section and is mostly missing in other parts of the
> draft, eg. list of keywords in A.8 Grammar, or list of modifiers in
> 12.2 SPARQL Query.
> So it seems "half-removed" to me.

added REDUCED to 12.2 and A.8 in revision 1.87

> 2.
> In section 12.4 SPARQL Algebra after definition of LeftJoin list is a
> "Written in full that is:" part, but it seems to me, that it is not
> equivalent to the definition.
> 
> Shouldn't
> { μ1 | μ1 in Ω1and μ2 in Ω2, and μ1 and μ2 are not compatible }
> { μ1 | μ1 in Ω1and μ2 in Ω2, and μ1 and μ2 are compatible and
> expr(merge(μ1, μ2)) is false }
> correspond to
> { μ | μ in Ω1 such that for all μ′ in Ω2, μ and μ′ are not 
> compatible }
> { μ | μ in Ω1 such that for all μ′ in Ω2, μ and μ' are compatible 
> and
> expr(merge(μ, μ')) is false }
> from the definition of Diff?
> 
> 3.
> My last question has probably already been discussed and answered, so
> could someone point me to some good explanation?
> What is the reason for Join(Ω1,Ω2) being defined in a way that is
> different from relational algebra when it comes to 'NULL values'?
> I do understand that unlike relational algebra, SPARQL algebra does
> not define schema for results of algebra operations, so at this time
> it is impossible to define Join to disallow joining of two mappings
> that 'contain NULLs in common columns'.
> But is this desired?
> 
> In my opinion this feature makes evaluation of SPARQL queries more
> complicated and I can't see the benefits it brings. What is the
> use-case behind this decision?
> 
> Best regards,
> Jiri Dokulil
> 

-- 
-eric

office: +1.617.258.5741 NE43-344, MIT, Cambridge, MA 02144 USA
mobile: +1.617.599.3509

(eric@w3.org)
Feel free to forward this message to any list for any purpose other than
email address distribution.