WIP: The Skyline Operator

The skyline operator is a non-standard extension (even SQL:2008 does
not include it).  Queries of that type can be expressed in standard
SQL (although a bit clumsy and the execution is slow).  Others have
also worked on extending PostgreSQL with the skyline operator, see:

http://archives.postgresql.org/pgsql-hackers/2007-03/thrd9.php#00188

Nevertheless we find it interesting and did our own implementation of
BNL and SFS.  It must be considered as WIP (work in progress).  We
highly welcome your feedback.

Through a web frontend it is possible to test-drive the
implementation.  Sample queries are given, see:

http://skyline.dbai.tuwien.ac.at/

The source code (patch) is available at:

http://skyline.dbai.tuwien.ac.at/skyline-of-200811131.patch.tar.gz

This patch applies against 8.3.5.  In case of interest a patch against
-head can be provided.

References:
[Borzsonyi2001] Börzsönyi, S.; Kossmann, D. & Stocker, K.:
The Skyline Operator, In ICDE, 2001, 421--432

Best,
Hannes