RANGE CONDITION FOR INDEX 
RANGE CONDITION FOR INDEX is a search strategy of the SQL Optimizer.
This search strategy can be selected if one of the following conditions is fulfilled:
● For the first k columns of an index, the search condition contains equality conditions (1 <= k <n, where n is the number of index columns).
● The first index column was specified with a range condition.
● The values of an IN condition are very close together.
The system uses the inversion lists in this range to access the corresponding table rows directly. So only part of the index is searched, and this part is read sequentially.
See also:
Search Conditions for Indexed Columns