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Re: Missing LET (Assignment) in SPARQL 1.1

From: Holger Knublauch <yahoo@knublauch.com>
Date: Tue, 27 Oct 2009 10:08:51 -0700
To: SPARQL Working Group Comments <public-rdf-dawg-comments@w3.org>
Message-Id: <729829C2-3712-4B9F-B7C6-01CF446AAB7A@knublauch.com>
Hi Danny,

> But as Holger points out, LET could likely be order-dependent, but -
> it doesn't have to be a drain. CONSTANT, no? The first assignation is
> the only assignation to care about.

Whenever the right hand side of the LET contains variables and these  
variables change their binding, then the LET would need to be  
recomputed. So just computing them once won't work.

However, there might be a nice compromise here that also preserves the  
declarative purity of SPARQL: only compute LETs on demand. In this  
scenario, LET would need to have an unbound variable on the left, and  
any algebraic operation that refers to that variable (e.g. in a BGP)  
would first call the function on the right to compute the value.  
Engines can then easily optimize the performance and avoid duplicate  
computations as long as the input variables have not changed yet. What  
would be the problems with this approach?

Received on Tuesday, 27 October 2009 17:09:28 UTC

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