IN CONDITION FOR KEY 
IN CONDITION FOR KEY is a search strategy of the SQL Optimizer.
This search strategy is selected in the following situation: The first k-1 columns of the primary key were qualified with equality conditions, and for the k-th key column an IN condition was defined (1 <= k <= n, where n is the number of key columns).
This search strategy is also selected especially if the first key column was represented by an IN condition.
The individual values in the IN condition are processed as an equality condition in this strategy. The system accesses the corresponding table rows directly. An intermediate result is set up.
If the values of an 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. This is displayed accordingly in the execution plan.
See also:
Search Conditions for Key Columns