IN CONDITION FOR INDEX 
IN CONDITION FOR INDEX is a search strategy of the SQL Optimizer.
This search strategy can be selected in the following situation: Equality conditions are entered for the first k-1 columns of the index, and an IN condition was defined for the k-th index column (1 <= k < n, where n is the number of index columns).
This search strategy is also selected especially if the first index column is represented by an IN condition.
The individual values of an IN condition are processed as an equality condition in this search strategy.
The system uses the inversion lists to access the corresponding table rows directly.
An intermediate result is set up.
If the values in the IN condition are close together, or if many values are displayed, it can be more practical to use a range condition instead of an equality condition.
Multiple IN conditions can be considered on an index. Here it must be ensured that the second IN condition is only processed with one range.
See also:
Search Conditions for Indexed Columns