SUBQ CONDITION FOR INDEX 
SUBQ CONDITION FOR INDEX is a search strategy of the SQL Optimizer.
This search strategy can be selected in the following cases:
● For the first k columns of an index, the search condition contains equality or range conditions (1 <= k <n, where n is the number of index columns).
● The search condition contains one equality condition for all columns of an index.
The values for the inversion list or index columns come from a subquery.
See also:
Search Conditions for Indexed Columns