RANGE CONDITION FOR KEY 
RANGE CONDITION FOR KEY is a search strategy of the SQL Optimizer.
Only part of the base table is searched. What are known as start and stop keys are used to restrict the range.
This search strategy is selected if one of the following conditions is fulfilled:
● For the first k columns of the primary key, equality conditions are specified (1 <= k <n, where n is the number of key columns). All the other key columns either cannot be specified or can be specified with a range condition.
● The first key column is specified with a range condition.
● The values of an IN condition are very close together.
The system performs a sequential search of the specified range of the table.
See also:
Search Conditions for Key Columns