Jan --

You wrote....

This can't be totally new, but sofar I failed to find good literature on
this. In the end this is a database query optimalisation problem but I
hope (and think) the Prolog and especially RDF context allow for a good
and not very complicated solution.

Does anyone know some good pointers?

A key paper, the one that started it all I think, goes back to 1979.  The reference is appended below.

HTH,  -- Adrian Walker

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Access Path Selection in a Relational Database Management System.

Patricia G. Selinger, Morton M. Astrahan, Donald D. Chamberlin, Raymond A. Lorie, Thomas G. Price: Access Path Selection in a Relational Database Management System. SIGMOD Conference 1979: 23-34
  author    = {Patricia G. Selinger and
               Morton M. Astrahan and
               Donald D. Chamberlin and
               Raymond A. Lorie and
               Thomas G. Price},
  editor    = {Philip A. Bernstein},
  title     = {Access Path Selection in a Relational Database Management System},
  booktitle = {Proceedings of the 1979 ACM SIGMOD International Conference on
               Management of Data, Boston, Massachusetts, May 30 - June 1},
  publisher = {ACM},
  year      = {1979},
  isbn      = {0-89791-001-X},
  pages     = {23-34},
  ee        = {db/conf/sigmod/SelingerACLP79.html},
  crossref  = {DBLP:conf/sigmod/79},
  bibsource = {DBLP, http://dblp.uni-trier.de}


In a high level query and data manipulation language such as SQL, requests are stated non-procedurally, without reference to access paths. This paper describes how System R chooses access paths for both simple (single relation) and complex queries (such as joins), given a user specification of desired data as a boolean expression of predicates. System R is an experimental database management system developed to carry out research on the relational model of data. System R was designed and built by members of the IBM San Jose Research'Laboratory.