“Derived tables” is the internal name for subqueries in the FROM clause.
The processing of derived tables is now included in the table opening process
(open_and_lock_tables() call). Routine of execution derived tables and substituting temporary table instead of it (mysql_handle_derived()) will be called just after opening and locking all real tables used in query (including tables used in derived table query).
If lex->derived_tables flag is present, all SELECT_LEX structures will be scanned (there is a list of all SELECT_LEX structures in reverse order named lex->all_selects_list, the first SELECT in the query will be last in this list).
There is a pointer for the derived table, SELECT_LEX_UNIT stored in the TABLE_LIST structure (TABLE_LIST::derived). For any table that has this pointer, mysql_derived() will be called. mysql_derived():
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A Guided Tour Of The Mysql Source Code
Important Algorithms And Structures
How Mysql Performs Different Selects
How Mysql Transforms Subqueries
Mysql Client/server Protocol
Prepared Statements And Stored Routines
Myisam Storage Engine
Innodb Storage Engine
Writing A Custom Storage Engine
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