SUBQ CONDITION FOR INDEX 
SUBQ CONDITION FOR INDEX is a search strategy of the SQL optimizer.
When Can the System Use This Strategy?
The database system can use this search strategy if one of the following conditions is met in a subquery:
For the first k columns of an index, the search condition contains equality or range conditions (
1 ≤ k < n, n = number of indexed columns).The search condition contains one equality condition for all columns of an index.
How Does the System Access the Data?
The database system performs a sequential search on part of the index for each results row of the subquery.