In expressions, subqueries (that is, subselects) are represented by Item inherited from Item_subselect.

To hide difference in performing single SELECTs and UNIONs, Item_subselect uses two different engines, which provide uniform interface for access to underlying SELECT or UNION (subselect_single_select_engine and subselect_union_engine, both are inherited from subselect_engine).

The engine will be created at the time Item_subselect is constructed (Item_subselect::init method).

On Item_subselect::fix_fields(), engine->prepare() will be called.

Before calling any value-getting method (val, val_int, val_str, bring_value (in case of row result)) engine->exec() will be called, which executes the query or just does nothing if subquery is cacheable and has already been executed.

Inherited items have their own select_result classes. There are two types of them:

Item_subselect will never call the cleanup() procedure for JOIN. Every JOIN::cleanup will call cleanup() for inner JOINs. The uppermost JOIN::cleanup will be called by mysql_select() or mysql_union().