Sometimes, part of the predicate can be pushed down to the IXSCAN stage filter and minimise or even avoid fetching. 

Predicate: NOR [a=1, a=2 & b=1]

Sort: c_1

Index: (c_1_a_1)

Resulting query

FETCH
    filter: NOR [a=1, a=2 & b=1]
    IXSCAN:
        pattern: (c_1_a_1)

Optimal query:

FETCH
    filter: NOR [a=1, a=2 & b=1]
    IXSCAN:
        filter: NOR [a=1, a=2]
        pattern: (c_1_a_1)