SUBQ CONDITION FOR KEY 
SUBQ CONDITION FOR KEY is a search strategy of the SQL Optimizer.
This search strategy is selected if the following conditions are fulfilled:
● The table has only one primary key column.
● The values for the key column come from a subquery.
or
● The table has a primary key in several columns.
● Equality conditions are defined on all primary key columns that are located before the column with the subquery in the primary key.
● On one column of the primary key, an IN condition is defined in the form of a subquery.
For every results row of the subquery, a part of the primary key is searched sequentially.
See also:
Search Conditions for Key Columns