local predicates in a join

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To be friendly, join should permit "local predicates", i.e. conditions
involving only one input.  The interpretation would be a filter on that
input before the join.  One open question: what's the effect on "preserve"?
 Should we block local predicates on preserved inputs?

Until this is delivered, user should place such local predicates before the
join.

Example:

join i in I,
     j in J
 where i.x == j.x
   and i.local == 3
;

interpretation / workaround:

join i in (I -> filter i.local == 3),
     j in J
 where i.x == j.x
;

Original issue reported on code.google.com by [e-mail] on 27 May 2010 at 9:30