On Wed, Jun 19, 2013 at 12:49 PM, Heikki Linnakangas < [email protected]> wrote:
> On 19.06.2013 11:30, Alexander Korotkov wrote: > >> On Wed, Jun 19, 2013 at 11:48 AM, Heikki Linnakangas< >> [email protected]> wrote: >> >> On 18.06.2013 23:59, Alexander Korotkov wrote: >>> >>> I would like to illustrate that on example. Imagine you have fulltext >>>> query >>>> "rare_term& frequent_term". Frequent term has large posting tree while >>>> >>>> rare term has only small posting list containing iptr1, iptr2 and iptr3. >>>> At >>>> first we get iptr1 from posting list of rare term, then we would like to >>>> check whether we have to scan part of frequent term posting tree where >>>> iptr >>>> < iptr1. So we call pre_consistent([false, true]), because we know >>>> that >>>> rare term is not present for iptr< iptr2. pre_consistent returns >>>> false. >>>> So >>>> we can start scanning frequent term posting tree from iptr1. Similarly >>>> we >>>> can skip lags between iptr1 and iptr2, iptr2 and iptr3, from iptr3 to >>>> maximum possible pointer. >>>> >>>> >>> Thanks, now I understand the rare-term& frequent-term problem. Couldn't >>> >>> you do that with the existing consistent function? I don't see why you >>> need >>> the new pre-consistent function for this. >>> >> >> In the case of two entries I can. But in the case of n entries things >> becomes more complicated. Imagine you have "term_1& term_2& ...& >> term_n" >> >> query. When you get some item pointer from term_1 you can skip all the >> lesser item pointers from term_2, term_3 ... term_n. But if all you have >> for it is consistent function you have to call it with following check >> arguments: >> 1) [false, false, false, ... , false] >> 2) [false, true, false, ... , false] >> 3) [false, false, true, ... , false] >> 4) [false, true, true, ..., false] >> ...... >> i.e. you have to call it 2^(n-1) times. But if you know the query specific >> (i.e. in opclass) it's typically easy to calculate exactly what we need in >> single pass. That's why I introduced pre_consistent. >> > > Hmm. So how does that work with the pre-consistent function? Don't you > need to call that 2^(n-1)-1 times as well? I call pre-consistent once with [false, true, true, ..., true]. Pre-consistent knows that each true passed to it could be false positive. So, if it returns false it guarantees that consistent will be false for all possible combinations. ------ With best regards, Alexander Korotkov.
