[rbanffy]

Again, ideally, this decision should be passed down to the underlying datastore. It's only at that level that you have the reliable information on how the data is distributed among cores.

On what kinds of back-end do LINQ and PLINQ support parallelization?

[sparky]

I'm not understanding what underlying datastore you're referring to in the case of LINQ-to-Objects or LINQ-to-XML. These frameworks perform "queries" on ordinary objects or XML data stored in memory within the .Net process, not on some other machine as with LINQ-to-SQL. You use LINQ-to-Objects in place of a filtering foreach loop, and LINQ-to-XML in place of XPath et al.

PLINQ is just a parallel implementation of LINQ-to-Objects and LINQ-to-XML. It works in the same scenarios .Net programs work in, i.e. multicores or SMPs running Windows (not sure if PLINQ works with Mono).