[inciampati]

Succinct full text indexes can be substantially smaller than the source text. It depends on the zero order entropy of the text. If things are highly repetitive, a very small index might be feasible. Usually lookup times are linearly proportional to query size, with logarithmic factors in database size.

[tmikaeld]

I've yet to see such a system (in production) except for Sonic, but sonic doesn't allow for full-text search only search on a key-by-key basis.