[Xapian-discuss] Making SORTAFTER useful in omega?

[Olly Betts]

On Mon, Sep 01, 2008 at 10:43:54PM +0200, Arjen van der Meijden wrote:
> According to the docs and earlier mails, omega has the 'SORTAFTER' (and 
> docid sorting) functionality to allow date-based secondary sorting. But 
> according to later mails and the documentation that's only useful if you 
> don't use the default BM25-weighting. Unfortunately you can't alter the 
> weighting scheme via Omega-calls.
> Nor does it seem to help to simply patch query.cc to use TradWeight 
> rather than BM25.

You generally need to set the parameters to BM25Weight or TradWeight to
eliminate the effect of document length on the weight (for BM25Weight:
k2 = 0 and b = 0; for TradWeight: k = 0).

It ought to be possible to set the weighting scheme used and the
parameters to it though.  I've created a ticket for this:

http://trac.xapian.org/ticket/298

Cheers,
    Olly