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.