Comment 4 for bug 636996

The metadata available is the same as you see in the .desktop files for your apps. So look fx. at /usr/share/applications/gedit.desktop and/or /usr/share/applications/totem.desktop.

For searching and indexing we currently use Xapian. The relevancy algorithm is BM25 described here http://xapian.org/docs/bm25.html. But note that the only place we actually use that algorithm is in the Top Results group when searching in the Files Place. Otherwise we sort apps alphabetically (as per IX spec) and files based on time.

Back to relevancy ranking in general; It must be noted that we also have a slightly harder time getting the most out of the relevancy ranking for a few reasons:

 a) We combine more than one source - the menu structure and the software-center index. *Normally* it is not even theoretically possible to do strictly correct relevancy ranking merged on two disparate source (you can apply hacks, but not correct solutions). We are in luck, however, since these two sources both comes as local Xapian indexes and Xapian comes with an API to do exactly this.

 b) We do prefix matching an not full term matching. Eg searching for 'ged' matches gedit. This is not the case in traditional searching (eg. online searches with google where you'd match only if you type 'gedit' completely)