Comparison of various queries doing searches by position on a large collection (43000000 entries)

 Algo components
  [B] : A basic box surrounding the cone : pos_ra BETWEEN ra-size, AND ra+size)and pos_dec BETWEEN dec-size AND dec+size. Poles are not taken in account : This box cannot be used for DEC > 85 or DEC < -85.
 [D] : Computation of distance between the entry and the cone center
 [Q] : Use of QBoxes (includes a distance computation for QBoxes on the cone edge.

 The query has been tested at position 17:46:32.86-29:03:34.9 for 3 cone radius. The postmaster is setup with 200Mb of shared buffers. The source density is about 2500 per qbox in this area.

It appears that Qboxes are efficient for small cones but not for larger cones. The limit is around 100 QBoxes. The reason is that the Saada query engine uses QBox as a column. Therefore, queries on large cones contain a lot of OR operator which is painfull for the SQL engine.