Why Indexing the Sky is Desirable

Indexing the sky is a database-oriented term to indicate a partition-ing scheme of the celestial sphere in order to achieve better performance in queries involving finding close neighbours. Several schemes have been proposed: HTM, HEALPix, “IDT: iso-declination tiles”, Quadrilateral-ized Spherical Cube, etc., but their use has not become widespread. The scientific value of the internal indexation files is much higher though, as they keep track of the source density of catalogues and hence allow us to answer a family of questions not easily handled by a standard DB system and providing an unusual visual aid: a snapshot of the location of sources listed in any catalog. The pros and cons of adopting an VO-oriented in-dexation scheme are analyzed. 1