12.7 Subqueries

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:

  • select_singlerow_subselect , to store values of given rows in Item_singlerow_subselect cache on send_data() call, and report error if Item_subselect has 'assigned' attribute.

  • select_exists_subselect just store 1 as value of Item_exists_subselect on send_data() call. Since Item_in_subselect and Item_allany_subselect are inherited from Item_exists_subselect, they use the same select_result class.

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().

User Comments
User comments in this section are, as the name implies, provided by MySQL users. The MySQL documentation team is not responsible for, nor do they endorse, any of the information provided here.
Sign Up Login You must be logged in to post a comment.