opt_sum.cc File Reference

Optimising of MIN(), MAX() and COUNT(*) queries without 'group by' clause by replacing the aggregate expression with a constant. More...

#include <limits.h>
#include <stddef.h>
#include <sys/types.h>
#include "ft_global.h"
#include "my_base.h"
#include "my_bitmap.h"
#include "my_dbug.h"
#include "my_inttypes.h"
#include "my_sys.h"
#include "my_table_map.h"
#include "mysql_com.h"
#include "sql/field.h"
#include "sql/handler.h"
#include "sql/item.h"
#include "sql/item_cmpfunc.h"
#include "sql/item_func.h"
#include "sql/item_sum.h"
#include "sql/key.h"
#include "sql/sql_bitmap.h"
#include "sql/sql_class.h"
#include "sql/sql_const.h"
#include "sql/sql_error.h"
#include "sql/sql_lex.h"
#include "sql/sql_list.h"
#include "sql/sql_opt_exec_shared.h"
#include "sql/sql_select.h"
#include "sql/table.h"

Functions
static bool	find_key_for_maxmin (bool max_fl, Index_lookup *ref, Item_field *item_field, Item *cond, uint *range_fl, uint *prefix_len)
	Check whether we can get value for {max|min}(field) by using a key. More...
static bool	reckey_in_range (bool max_fl, Index_lookup *ref, Item_field *item_field, Item *cond, uint range_fl, uint prefix_len)
	Check whether found key is in range specified by conditions. More...
static bool	maxmin_in_range (bool max_fl, Item_field *item_field, Item *cond)
	Check whether {MAX|MIN}(field) is in range specified by conditions. More...
static ulonglong	get_exact_record_count (Table_ref *tables)
	Get exact count of rows in all tables. More...
static int	get_index_min_value (TABLE *table, Index_lookup *ref, Item_field *item_field, uint range_fl, uint prefix_len)
	Use index to read MIN(field) value. More...
static int	get_index_max_value (TABLE *table, Index_lookup *ref, uint range_fl)
	Use index to read MAX(field) value. More...
bool	optimize_aggregated_query (THD *thd, Query_block *select, const mem_root_deque< Item * > &fields, Item *conds, aggregate_evaluated *decision)
	Substitute constants for some COUNT(), MIN() and MAX() functions in an aggregated (implicitly grouped) query. More...
bool	is_simple_predicate (Item_func *func_item, Item **args, bool *inv_order)
	Test if the predicate compares a field with constants. More...
static bool	matching_cond (bool max_fl, Index_lookup *ref, KEY *keyinfo, KEY_PART_INFO *field_part, Item *cond, table_map map, key_part_map *key_part_used, uint *range_fl, uint *prefix_len)
	Check whether a condition matches a key to get {MAX|MIN}(field):. More...

Detailed Description

Optimising of MIN(), MAX() and COUNT(*) queries without 'group by' clause by replacing the aggregate expression with a constant.

Given a table with a compound key on columns (a,b,c), the following types of queries are optimised (assuming the table handler supports the required methods)

SELECT COUNT(*) FROM t1[,t2,t3,...]
SELECT MIN(b) FROM t1 WHERE a=const
SELECT MAX(c) FROM t1 WHERE a=const AND b=const
SELECT MAX(b) FROM t1 WHERE a=const AND b<const
SELECT MIN(b) FROM t1 WHERE a=const AND b>const
SELECT MIN(b) FROM t1 WHERE a=const AND b BETWEEN const AND const
SELECT MAX(b) FROM t1 WHERE a=const AND b BETWEEN const AND const

Instead of '<' one can use '<=', '>', '>=' and '=' as well. Instead of 'a=const' the condition 'a IS NULL' can be used.

If all selected fields are replaced then we will also remove all involved tables and return the answer without any join. Thus, the following query will be replaced with a row of two constants:

SELECT MAX(b), MIN(d) FROM t1,t2
  WHERE a=const AND b<const AND d>const

(assuming a index for column d of table t2 is defined)

Function Documentation

find_key_for_maxmin()

static bool find_key_for_maxmin ( bool  max_fl, Index_lookup *  ref, Item_field *  item_field, Item *  cond, uint *  range_fl, uint *  prefix_len  )

static

Check whether we can get value for {max|min}(field) by using a key.

If where-condition is not a conjunction of 0 or more conjuct the function returns false, otherwise it checks whether there is an index including field as its k-th component/part such that:

1. for each previous component f_i there is one and only one conjunct of the form: f_i= const_i or const_i= f_i or f_i is null
2. references to field occur only in conjucts of the form: field {<|<=|>=|>|=} const or const {<|<=|>=|>|=} field or field BETWEEN const1 AND const2
3. all references to the columns from the same table as column field occur only in conjucts mentioned above.
4. each of k first components the index is not partial, i.e. is not defined on a fixed length proper prefix of the field.

If such an index exists the function through the ref parameter returns the key value to find max/min for the field using the index, the length of first (k-1) components of the key and flags saying how to apply the key for the search max/min value. (if we have a condition field = const, prefix_len contains the length of the whole search key)

Parameters

[in]	max_fl	0 for MIN(field) / 1 for MAX(field)
[in,out]	ref	Reference to the structure we store the key value
[in]	item_field	Field used inside MIN() / MAX()
[in]	cond	WHERE condition
[out]	range_fl	Bit flags for how to search if key is ok
[out]	prefix_len	Length of prefix for the search range

Note

This function may set field->table->key_read to true, which must be reset after index is used! (This can only happen when function returns 1)

Returns

true if index can be used to optimize MIN()/MAX(), false otherwise. If true, ref, range_fl and prefix_len are updated

get_exact_record_count()

static ulonglong get_exact_record_count ( Table_ref *  tables )

static

Get exact count of rows in all tables.

This is called, when at least one of the table handlers support HA_COUNT_ROWS_INSTANT, but not HA_STATS_RECORDS_IS_EXACT (NDB is one such storage engine).

Parameters

tables

List of tables

Return values

Product

of number of rows in all tables. ULLONG_MAX for error.

get_index_max_value()

static int get_index_max_value ( TABLE *  table, Index_lookup *  ref, uint  range_fl  )

static

Use index to read MAX(field) value.

Parameters

table	Table object
ref	Reference to the structure where we store the key value
range_fl	Whether range endpoint is strict greater than

Return values

0	No errors HA_ERR_... Otherwise

get_index_min_value()

static int get_index_min_value ( TABLE *  table, Index_lookup *  ref, Item_field *  item_field, uint  range_fl, uint  prefix_len  )

static

Use index to read MIN(field) value.

Parameters

table	Table object
ref	Reference to the structure where we store the key value
item_field	Field used in MIN()
range_fl	Whether range endpoint is strict less than
prefix_len	Length of common key part for the range

Return values

0	No errors HA_ERR_... Otherwise

is_simple_predicate()

bool is_simple_predicate	(	Item_func *	func_item,
		Item **	args,
		bool *	inv_order
	)

Test if the predicate compares a field with constants.

Parameters

	func_item	Predicate item
[out]	args	Here we store the field followed by constants
[out]	inv_order	Is set to true if the predicate is of the form 'const op field'

Returns

true if function is a simple predicate, false otherwise.

matching_cond()

static bool matching_cond ( bool  max_fl, Index_lookup *  ref, KEY *  keyinfo, KEY_PART_INFO *  field_part, Item *  cond, table_map  map, key_part_map *  key_part_used, uint *  range_fl, uint *  prefix_len  )

static

Check whether a condition matches a key to get {MAX|MIN}(field):.

For the index specified by the keyinfo parameter and an index that contains the field as its component (field_part), the function checks whether

• the condition cond is a conjunction,
• all of its conjuncts refer to columns of the same table, and
• each conjunct is on one of the following forms:
  • f_i = const_i or const_i = f_i or f_i IS NULL, where f_i is part of the index
  • field {<|<=|>=|>|=} const
  • const {<|<=|>=|>|=} field
  • field BETWEEN const_1 AND const_2

As a side-effect, the key value to be used for looking up the MIN/MAX value is actually stored inside the Field object. An interesting feature is that the function will find the most restrictive endpoint by over-eager evaluation of the WHERE condition. It continually stores the current endpoint inside the Field object. For a query such as

SELECT MIN(a) FROM t1 WHERE a > 3 AND a > 5;

the algorithm will recurse over the conjunction, storing first a 3 in the field. In the next recursive invocation the expression a > 5 is evaluated as 3 > 5 (Due to the dual nature of Field objects as value carriers and field identifiers), which will obviously fail, leading to 5 being stored in the Field object.

Parameters

[in]	max_fl	Set to true if we are optimizing MAX(), false means we are optimizing MIN()
[in,out]	ref	Reference to the structure where the function stores the key value
[in]	keyinfo	Reference to the key info
[in]	field_part	Pointer to the key part for the field
[in]	cond	WHERE condition
[in]	map	Table map for the key
[in,out]	key_part_used	Map of matchings parts. The function will output the set of key parts actually being matched in this set, yet it relies on the caller to initialize the value to zero. This is due to the fact that this value is passed recursively.
[in,out]	range_fl	Says whether endpoints use strict greater/less than.
[out]	prefix_len	Length of common key part for the range where MAX/MIN is searched for

Return values

false	Index can't be used.
true	We can use the index to get MIN/MAX value

maxmin_in_range()

static bool maxmin_in_range ( bool  max_fl, Item_field *  item_field, Item *  cond  )

static

Check whether {MAX|MIN}(field) is in range specified by conditions.

Parameters

[in]	max_fl	false for MIN(field) / true for MAX(field)
[in]	item_field	Item representing field used in MIN/MAX expression
[in]	cond	Condition to check

Returns

true if condition is not true for the found row, otherwise false.

optimize_aggregated_query()

bool optimize_aggregated_query	(	THD *	thd,
		Query_block *	select,
		const mem_root_deque< Item * > &	fields,
		Item *	conds,
		aggregate_evaluated *	decision
	)

Substitute constants for some COUNT(), MIN() and MAX() functions in an aggregated (implicitly grouped) query.

Parameters

[in]	thd	thread handler
[in]	select	query block
[in]	fields	All fields to be returned
[in]	conds	WHERE clause
[out]	decision	outcome for successful execution = AGGR_REGULAR regular execution required = AGGR_COMPLETE values available = AGGR_DELAYED execution with ha_records() required = AGGR_EMPTY source tables empty, aggregates are NULL or zero (for COUNT)

Returns

false if success, true if error

This function is called for queries with aggregate functions and no GROUP BY, thus the result set will contain a single row only.

First, the function will analyze the source tables and WHERE clause to see whether the query qualifies for optimization. If not, the decision AGGR_REGULAR is returned.

Second, the function walks over all expressions in the SELECT list. If the expression can be optimized with a storage engine operation that is O(1) (MIN or MAX) or O(0) (instant COUNT), the value is looked up and inserted in the value buffer, and the corresponding Item is marked as being const. If the expression is a COUNT operation that can be evaluated efficiently by the storage manager (but still O(N)), indicated with HA_COUNT_ROWS_INSTANT, it will be marked as such.

When all SELECT list expressions have been processed, there are four possible outcomes:

• An empty result from the source tables is indicated, and the output state is AGGR_EMPTY. Notice that the result of aggregation is still one row, filled with zero for COUNT operations and NULL values for all other expressions.
• All expressions have been given values, indicated with output state AGGR_COMPLETE.
• All expressions have been given values, except for one or more COUNT operations that will be evaluated in execution. This is indicated with AGGR_DELAYED.
• Some expressions must be evaluated as part of a regular execution, indicated with AGGR_REGULAR. Notice that some of the expressions may have been given values and are marked as const, but no expressions will be candidates for delayed execution.

reckey_in_range()

static bool reckey_in_range ( bool  max_fl, Index_lookup *  ref, Item_field *  item_field, Item *  cond, uint  range_fl, uint  prefix_len  )

static

Check whether found key is in range specified by conditions.

Parameters

[in]	max_fl	false for MIN(field) / true for MAX(field)
[in]	ref	Reference to the key value and info
[in]	item_field	Item representing field used in MIN/MAX expression
[in]	cond	Condition to check nullptr means that all keys are within the range.
[in]	range_fl	Says whether there is a condition to to be checked
[in]	prefix_len	Length of the constant part of the key

Returns

true if the condition is not true for the found row, false otherwise.