#include <item_subselect.h>

Inheritance diagram for subselect_indexsubquery_engine:

Collaboration diagram for subselect_indexsubquery_engine:

Public Member Functions
	subselect_indexsubquery_engine (THD thd_arg, st_join_table tab_arg, Item_subselect subs, Item where, Item *having_arg, bool chk_null, bool unique_arg)
virtual bool	exec ()
virtual void	print (String *str, enum_query_type query_type)
virtual enum_engine_type	engine_type () const
virtual void	cleanup ()
virtual bool	prepare ()
virtual void	fix_length_and_dec (Item_cache **row)
virtual uint	cols () const
virtual uint8	uncacheable () const
virtual void	exclude ()
virtual table_map	upper_select_const_tables () const
virtual bool	change_result (Item_subselect si, select_result_interceptor result)
virtual bool	no_tables () const
bool	scan_table ()
void	copy_ref_key (bool require_scan, bool convert_error)
Public Member Functions inherited from subselect_engine
	subselect_engine (Item_subselect si, select_result_interceptor res)
void	set_thd_for_result ()
	Sets "thd" for 'result'. Should be called before prepare()
virtual enum Item_result	type () const
virtual enum_field_types	field_type () const
virtual bool	may_be_null () const
virtual bool	is_executed () const
const Item_subselect *	get_item () const

Protected Attributes
st_join_table *	tab
Item *	cond
Protected Attributes inherited from subselect_engine
select_result_interceptor *	result
Item_subselect *	item
enum Item_result	res_type
enum_field_types	res_field_type
bool	maybe_null

Additional Inherited Members
Public Types inherited from subselect_engine
enum	enum_engine_type { ABSTRACT_ENGINE, SINGLE_SELECT_ENGINE, UNION_ENGINE, UNIQUESUBQUERY_ENGINE, INDEXSUBQUERY_ENGINE, HASH_SJ_ENGINE }
Static Public Member Functions inherited from subselect_engine
static table_map	calc_const_tables (TABLE_LIST *)
Protected Member Functions inherited from subselect_engine
void	set_row (List< Item > &item_list, Item_cache **row)

Detailed Description

A subquery execution engine that evaluates the subquery by doing index lookups in a single table's index.

This engine is used to resolve subqueries in forms

outer_expr IN (SELECT tbl.key FROM tbl WHERE subq_where)

or, row-based:

(oe1, .. oeN) IN (SELECT key_part1, ... key_partK FROM tbl WHERE subqwhere)

i.e. the subquery is a single table SELECT without GROUP BY, aggregate functions, etc.

Definition at line 664 of file item_subselect.h.

Member Function Documentation

bool subselect_indexsubquery_engine::change_result	(	Item_subselect *	si,
		select_result_interceptor *	res
	)

virtual

change select_result emulation, never should be called.

Parameters

si	new subselect Item
res	new select_result object

Return values

FALSE	OK
TRUE	error

Implements subselect_engine.

Definition at line 3392 of file item_subselect.cc.

virtual void subselect_indexsubquery_engine::cleanup ( )

inlinevirtual

Cleanup engine after complete query execution, free all resources.

Implements subselect_engine.

Reimplemented in subselect_hash_sj_engine.

Definition at line 702 of file item_subselect.h.

void subselect_indexsubquery_engine::copy_ref_key	(	bool *	require_scan,
		bool *	convert_error
	)

Copy ref key and check for null parts in it

Construct a search tuple to be used for index lookup. If one of the key parts have a NULL value, the following logic applies:

For top level items, e.g.

"WHERE <outer_value_list> IN (SELECT <inner_value_list>...)"

where one of the outer values are NULL, the IN predicate evaluates to false/UNKNOWN (we don't care) and it's not necessary to evaluate the subquery. That shortcut is taken in Item_in_optimizer::val_int(). Thus, if a key part with a NULL value is found here, the NULL is either not outer or this subquery is not top level. Therefore we cannot shortcut subquery execution if a NULL is found here.

Thus, if one of the key parts have a NULL value there are two possibilities:

a) The NULL is from the outer_value_list. Since this is not a top level item (see above) we need to check whether this predicate evaluates to NULL or false. That is done by checking if the subquery has a row if the conditions based on outer NULL values are disabled. Index lookup cannot be used for this, so a table scan must be done.

b) The NULL is local to the subquery, e.g.:

  "WHERE ... IN (SELECT ... WHERE inner_col IS NULL)"

In this case we're looking for rows with the exact inner_col value of NULL, not rows that match if the "inner_col IS NULL" condition is disabled. Index lookup can be used for this.

See Also: subselect_indexsubquery_engine::exec(); Item_in_optimizer::val_int()

Parameters

[out]	require_scan	true if a NULL value is found that falls into category a) above, false if index lookup can be used.
[out]	convert_error	true if an error occured during conversion of values from one type to another, false otherwise.

Definition at line 2952 of file item_subselect.cc.

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