how to ignore position in indexing?

[Li Li]

hi all
     in lucene, we can only store tf of a term's invert list. in my
application, I only provide dismax query with boolean query and don't
support queries which need position info such as phrase query. So I
don't want to store position info in prx file. How to turn off it? And
if I turn off it, will search become quicker?