In order to parse a statement, we save the currently active arena of THD and assign its members from the permanent arena of the statement. This is achieved by calling THD::set_and_backup_active_arena. This way alloc_query and yyparse operate on the permanent arena.

We don't want the garbage which is created during statement validation to be left in the permanent arena of the statement. For that, after parse but before validation of the statement, we restore the THD arena saved in (1). In other words, we use the arena of THD that was active when Prepared_statement::prepare was invoked as the runtime arena of the statement when it is validated.

Statement validation is performed in function check_prepared_statement(). This function will subsequently call st_select_lex_unit::prepare() and setup_fields() for the main LEX unit, create JOINs for every unit, and call JOIN::prepare for every join (JOINs in MySQL represents a part of the execution plan). Our prepared statement engine does not save the execution plan in a prepared statement for reuse, and ideally we should not create it at prepare stage. However, currently there is no other way to validate a statement except to call JOIN::prepare for all its units.

During validation we may perform a transformation of the parsed tree. In a clean implementation this would belong to a separate step, but in our case the majority of the server runtime was not refactored to support reexecution of statements, and a permanent transformation of the parsed tree can happen at any moment during validation. Such transformations absolutely must use the permanent arena of the prepared statement. To make this arena accessible, we save a pointer to it in thd->stmt_arena before calling check_prepared_statement.

Later, whenever we need to perform a permanent transformation, we first call THD::activate_stmt_arena_if_needed to make the permanent arena active, transform the tree, and restore the runtime arena.

Some parts of the execution do not distinguish between preparation of a prepared statement and its execution and perform destructive optimizations of the parsed tree even during validation. These changes of the parsed tree are recorded in THD::change_list using method THD::register_item_tree_change.

After the validation is done, we rollback the changes registered in THD::change_list and free new items and other memory allocated by destructive transformations.