Query Resolver

Classes
class	Semijoin_decorrelation
	Context object used by semijoin equality decorrelation code. More...
struct	Lifted_fields_map

Functions
static bool	simplify_const_condition (THD *thd, Item **cond, bool remove_cond, bool *ret_cond_value)
	Try to replace a const condition with a simple constant. More...
static Item *	create_rollup_switcher (THD *thd, Query_block *query_block, Item_sum *item, int send_group_parts)
static bool	fulltext_uses_rollup_column (const Query_block *query_block)
	Checks if there are any calls to the MATCH function that take a ROLLUP column as argument in the SELECT list, GROUP BY clause, HAVING clause or ORDER BY clause. More...
static void	update_used_tables_for_join (mem_root_deque< Table_ref * > *tables)
	Update used tables information for a JOIN expression. More...
static Table_ref **	make_leaf_tables (Table_ref **list, Table_ref *tables)
	Make list of leaf tables of join table tree. More...
static void	fix_tables_after_pullout (Query_block *parent_query_block, Query_block *removed_query_block, Table_ref *tr, uint table_adjust, table_map lateral_deps)
	Update table reference information for conditions and expressions due to query blocks having been merged in from derived tables/views and due to semi-join transformation. More...
static bool	decorrelate_equality (Semijoin_decorrelation &sj_decor, Item_func *func, bool *was_correlated)
	Try to decorrelate an (in)equality node. More...
static bool	can_decorrelate_operator (Item_func *func, bool only_eq)
bool	walk_join_list (mem_root_deque< Table_ref * > &list, std::function< bool(Table_ref *)> action)
static bool	build_sj_exprs (THD *thd, mem_root_deque< Item * > *sj_outer_exprs, mem_root_deque< Item * > *sj_inner_exprs, Item_exists_subselect *subq_pred, Query_block *subq_query_block)
	Builds the list of SJ outer/inner expressions. More...
static bool	replace_subcondition (THD *thd, Item **tree, Item *old_cond, Item *new_cond, bool do_fix_fields, bool *found_ptr=nullptr)
	Destructively replaces a sub-condition inside a condition tree. More...
void	propagate_nullability (mem_root_deque< Table_ref * > *tables, bool nullable)
	Propagate nullability into inner tables of outer join operation. More...
bool	find_order_in_list (THD *thd, Ref_item_array ref_item_array, Table_ref *tables, ORDER *order, mem_root_deque< Item * > *fields, bool is_group_field, bool is_window_order)
	Resolve an ORDER BY or GROUP BY column reference. More...
bool	setup_order (THD *thd, Ref_item_array ref_item_array, Table_ref *tables, mem_root_deque< Item * > *fields, ORDER *order)
	Resolve and setup list of expressions in ORDER BY clause. More...
static ReplaceResult	wrap_grouped_expressions_for_rollup (Query_block *select, Item *item, Item *parent, unsigned argument_idx)
	Checks whether an item matches a grouped expression, creates an Item_rollup_group_item around it and replaces the reference to it with that item. More...
static bool	WalkAndReplaceInner (THD *thd, Item *parent, unsigned argument_idx, const function< ReplaceResult(Item *item, Item *parent, unsigned argument_idx)> &get_new_item, Item **child_ref)
	Helper function for WalkAndReplace() which replaces the Item referenced by "child_ref" if "get_new_item" returns a replacement, or visits the children of "child_ref" otherwise. More...
bool	WalkAndReplace (THD *thd, Item *item, const function< ReplaceResult(Item *item, Item *parent, unsigned argument_idx)> &get_new_item)
static bool	refresh_comparators_after_rollup (Item *item)
	Refreshes the comparators after ROLLUP resolving. More...
bool	validate_gc_assignment (const mem_root_deque< Item * > &fields, const mem_root_deque< Item * > &values, TABLE *table)
	validate_gc_assignment Check whether the other values except DEFAULT are assigned for generated columns. More...
static bool	baptize_item (THD *thd, Item *item, int *field_no)
	Helper function to make names for columns of a derived table replacing a scalar or table subquery. More...
static bool	update_context_to_derived (Item *expr, Query_block *new_derived)
	A minion of transform_grouped_to_derived. More...
static bool	replace_aggregate_in_list (Item::Aggregate_replacement &info, bool was_hidden, mem_root_deque< Item * > *list, Ref_item_array *ref_item_array)
	A minion of transform_grouped_to_derived. More...
static bool	collect_aggregates (Query_block *select, Item_sum::Collect_grouped_aggregate_info *aggregates)
	A minion of transform_grouped_to_derived. More...
static bool	query_block_contains_subquery (Query_block *select, Query_expression *slu)
	A minion of transform_scalar_subqueries_to_join_with_derived. More...
static bool	walk_join_conditions (mem_root_deque< Table_ref * > &list, std::function< bool(Item **expr_p)> action, Item::Collect_scalar_subquery_info *info)
static void	remember_transform (THD *thd, Query_block *select)
	Remember if this transform was performed. More...
static bool	add_partition_by_expr (THD *thd, PT_order_list *partition, Query_block *qb, Item *expr)
	Given an expression, create an ORDER expression for that expression and add it to a window's ORDER BY list in preparation for synthesizing a window function, cf. More...
bool	is_correlated_predicate_eligible (Item *cor_pred)
	Called to check if the provided correlated predicate is eligible for transformation. More...
static bool	extract_correlated_condition (THD *thd, Item **cond, Item **correlated_cond)
	Extracts the top level correlated condition in an OR condition. More...
bool	Query_block::prepare (THD *thd, mem_root_deque< Item * > *insert_field_list)
	Prepare query block for optimization. More...
bool	Query_block::push_conditions_to_derived_tables (THD *thd)
	Pushes parts of the WHERE condition of this query block to materialized derived tables. More...
bool	Query_block::prepare_values (THD *thd)
	Prepare a table value constructor query block for optimization. More...
bool	Query_block::apply_local_transforms (THD *thd, bool prune)
	Does permanent transformations which are local to a query block (which do not merge it to another block). More...
void	Query_block::update_used_tables ()
	Update used tables information for all local expressions. More...
bool	Query_block::resolve_limits (THD *thd)
	Resolve OFFSET and LIMIT clauses. More...
bool	Item_in_subselect::subquery_allows_materialization (THD *thd, Query_block *query_block, const Query_block *outer)
	Check if the subquery predicate can be executed via materialization. More...
bool	Query_block::check_view_privileges (THD *thd, ulong want_privilege_first, ulong want_privilege_next)
	Check privileges for views that are merged into query block. More...
bool	Query_block::setup_tables (THD *thd, Table_ref *tables, bool select_insert)
	Resolve and prepare information about tables for one query block. More...
void	Query_block::remap_tables (THD *thd)
	Re-map table numbers for all tables in a query block. More...
bool	Query_block::resolve_placeholder_tables (THD *thd, bool apply_semijoin)
	Resolve derived table, view, table function information for a query block. More...
bool	Query_block::is_row_count_valid_for_semi_join ()
	Check if the offset and limit are valid for a semijoin. More...
bool	Query_block::setup_wild (THD *thd)
	Expand all '*' in list of expressions with the matching column references. More...
bool	Query_block::setup_conds (THD *thd)
	Resolve WHERE condition and join conditions. More...
bool	Query_block::setup_join_cond (THD *thd, mem_root_deque< Table_ref * > *tables, bool in_update)
	Resolve join conditions for a join nest. More...
void	Query_block::reset_nj_counters (mem_root_deque< Table_ref * > *join_list=nullptr)
	Set NESTED_JOIN::counter=0 in all nested joins in passed list. More...
bool	Query_block::simplify_joins (THD *thd, mem_root_deque< Table_ref * > *join_list, bool top, bool in_sj, Item **new_conds, uint *changelog=nullptr)
	Simplify joins replacing outer joins by inner joins whenever it's possible. More...
bool	Query_block::record_join_nest_info (mem_root_deque< Table_ref * > *tables)
	Record join nest info in the select block. More...
void	Query_expression::fix_after_pullout (Query_block *parent_query_block, Query_block *removed_query_block)
	Fix used tables information for a subquery after query transformations. More...
void	Query_block::fix_after_pullout (Query_block *parent_query_block, Query_block *removed_query_block)
void	Query_block::clear_sj_expressions (NESTED_JOIN *nested_join)
	Remove semijoin condition for this query block. More...
bool	Query_block::build_sj_cond (THD *thd, NESTED_JOIN *nested_join, Query_block *subq_query_block, table_map outer_tables_map, Item **sj_cond, bool *simple_const)
	Build semijoin condition for th query block. More...
bool	Query_block::decorrelate_condition (Semijoin_decorrelation &sj_decor, Table_ref *join_nest)
	Decorrelate the WHERE clause or a join condition of a subquery used in an IN or EXISTS predicate. More...
bool	Query_block::allocate_grouping_sets (THD *thd)
	Initializes the grouping set if the query block includes GROUP BY modifiers. More...
bool	Query_block::populate_grouping_sets (THD *thd)
	Populates the grouping sets if the query block includes non-primitive grouping. More...
bool	Query_block::convert_subquery_to_semijoin (THD *thd, Item_exists_subselect *subq_pred)
	Convert a subquery predicate of this query block into a Table_ref semi-join nest. More...
bool	Query_block::merge_derived (THD *thd, Table_ref *derived_table)
	Merge derived table into query block. More...
bool	Query_block::flatten_subqueries (THD *thd)
	Convert semi-join subquery predicates into semi-join join nests. More...
void	Query_block::propagate_unique_test_exclusion ()
	Propagate exclusion from table uniqueness test into subqueries. More...
bool	Query_block::add_ftfunc_list (List< Item_func_match > *ftfuncs)
	Add full-text function elements from a list into this query block. More...
void	Query_block::repoint_contexts_of_join_nests (mem_root_deque< Table_ref * > join_list)
	Go through a list of tables and join nests, recursively, and repoint its query_block pointer. More...
void	Query_block::merge_contexts (Query_block *inner)
	Merge name resolution context objects of a subquery into its parent. More...
bool	Query_block::remove_redundant_subquery_clauses (THD *thd)
	For a table subquery predicate (IN/ANY/ALL/EXISTS/etc): since it does not support LIMIT the following clauses are redundant: More...
bool	Query_block::empty_order_list (Query_block *sl)
	Empty the ORDER list. More...
bool	Query_block::check_only_full_group_by (THD *thd)
	Runs checks mandated by ONLY_FULL_GROUP_BY. More...
bool	Query_block::setup_order_final (THD *thd)
	Do final setup of ORDER BY clause, after the query block is fully resolved. More...
bool	Query_block::setup_group (THD *thd)
	Resolve and set up the GROUP BY list. More...
ORDER *	Query_block::find_in_group_list (Item *item, int *rollup_level) const
	Finds a group expression matching the given item, or nullptr if none. More...
int	Query_block::group_list_size () const
void	Query_block::mark_item_as_maybe_null_if_non_primitive_grouped (Item *item) const
	Marks occurrences of group by fields in a function's arguments as nullable, so that we do not optimize them away before we get to add the rollup wrappers. More...
Item *	Query_block::single_visible_field () const
size_t	Query_block::num_visible_fields () const
bool	Query_block::field_list_is_empty () const
Item *	Query_block::resolve_rollup_item (THD *thd, Item *item)
	Resolve an item (and its tree) for rollup processing by replacing items matching grouped expressions with Item_rollup_group_items and updating properties (m_nullable, PROP_ROLLUP_FIELD). More...
bool	Query_block::resolve_rollup (THD *thd)
	Resolve items in SELECT list and ORDER BY list for rollup processing. More...
bool	Query_block::resolve_rollup_wfs (THD *thd)
	Replace group by field references inside window functions with references in the presence of ROLLUP. More...
void	Query_block::delete_unused_merged_columns (mem_root_deque< Table_ref * > *tables)
	Delete unused columns from merged tables. More...
Item **	Query_block::add_hidden_item (Item *item)
	Add item to the hidden part of select list. More...
void	Query_block::remove_hidden_items ()
	Remove hidden items from select list. More...
bool	Query_block::resolve_table_value_constructor_values (THD *thd)
	Resolve the rows of a table value constructor and aggregate the type of each column across rows. More...
bool	Query_block::transform_table_subquery_to_join_with_derived (THD *thd, Item_exists_subselect *subq_pred)
	Replace a table subquery ([NOT] {IN, EXISTS}) with a join to a derived table. More...
Table_ref *	Query_block::synthesize_derived (THD *thd, Query_expression *unit, Item *join_cond, bool left_outer, bool use_inner_join)
	Create a new Table_ref object for this query block, for either: 1) a derived table which will replace the subquery, or 2) an extra derived table for handling grouping, if necessary, cf. More...
bool	Query_block::remove_aggregates (THD *thd, Query_block *select)
	A minion of transform_grouped_to_derived. More...
bool	Query_block::replace_item_in_expression (Item **expr, bool was_hidden, Item::Item_replacement *info, Item_transformer transformer)
	Minion of transform_grouped_to_derived. More...
bool	Query_block::transform_grouped_to_derived (THD *thd, bool *break_off)
	Minion of transform_scalar_subqueries_to_join_with_derived. More...
bool	Query_block::replace_subquery_in_expr (THD *thd, Item::Css_info *subquery, Table_ref *tr, Item **expr)
	A minion of transform_scalar_subqueries_to_join_with_derived. More...
bool	Query_block::nest_derived (THD *thd, Item *join_cond, mem_root_deque< Table_ref * > *join_list, Table_ref *new_derived_table)
	Push the generated derived table to the correct location inside a join nest. More...
bool	Query_block::setup_counts_over_partitions (THD *thd, Table_ref *derived, Lifted_fields_map *lifted_fields, std::deque< Item_field * > &added_to_group_by, uint hidden_fields)
	Add all COUNT(0) to SELECT list of the derived table to be used for cardinality checking of the transformed subquery. More...
bool	Query_block::decorrelate_derived_scalar_subquery_pre (THD *thd, Table_ref *derived, Item *lifted_where, Lifted_fields_map *lifted_where_fields, bool *added_card_check, size_t *added_window_card_checks)
	We have a correlated scalar subquery, so we must do several things: More...
bool	Query_block::decorrelate_derived_scalar_subquery_post (THD *thd, Table_ref *derived, Lifted_fields_map *lifted_where_fields, bool added_card_check, size_t added_window_card_checks)
	See explanation in companion method decorrelate_derived_scalar_subquery_pre. More...
void	Query_block::replace_referenced_item (Item *const old_item, Item *const new_item)
	Replace item in select list and preserve its reference count. More...
bool	Query_block::transform_subquery_to_derived (THD *thd, Table_ref **out_tl, Query_expression *subs_query_expression, Item_subselect *subq, bool use_inner_join, bool reject_multiple_rows, Item *join_condition, Item *lifted_where_cond)
	Converts a subquery to a derived table and inserts it into the FROM clause of the owning query block. More...
bool	Query_block::supported_correlated_scalar_subquery (THD *thd, Item::Css_info *subquery, Item **lifted_where)
	Called when the scalar subquery is correlated. More...
bool	Query_block::transform_scalar_subqueries_to_join_with_derived (THD *thd)
	Transform eligible scalar subqueries in the SELECT list, WHERE condition, HAVING condition or JOIN conditions of a query block[*] to an equivalent derived table of a LEFT OUTER join, e.g. More...
bool	Query_block::lift_fulltext_from_having_to_select_list (THD *thd)
	Copies all non-aggregated calls to the full-text search MATCH function from the HAVING clause to the SELECT list (as hidden items), so that we can materialize their result and not only their input. More...

Variables
static const enum_walk	walk_options

Detailed Description

Function Documentation

add_ftfunc_list()

bool Query_block::add_ftfunc_list

(

List< Item_func_match > *

ftfuncs

)

Add full-text function elements from a list into this query block.

Add a list of full-text function elements into a query block.

Parameters

ftfuncs

List of full-text function elements to add.

Returns

false if success, true if error

add_hidden_item()

Item ** Query_block::add_hidden_item

(

Item *

item

)

Add item to the hidden part of select list.

Parameters

item	item to add

Returns

Pointer to reference of the added item

Parameters

item	the item to add

Returns

Pointer to reference to the added item

add_partition_by_expr()

static bool add_partition_by_expr ( THD *  thd, PT_order_list *  partition, Query_block *  qb, Item *  expr  )

static

Given an expression, create an ORDER expression for that expression and add it to a window's ORDER BY list in preparation for synthesizing a window function, cf.

setup_counts_over_partitions

allocate_grouping_sets()

bool Query_block::allocate_grouping_sets

(

THD *

thd

)

Initializes the grouping set if the query block includes GROUP BY modifiers.

apply_local_transforms()

bool Query_block::apply_local_transforms	(	THD *	thd,
		bool	prune
	)

Does permanent transformations which are local to a query block (which do not merge it to another block).

Apply local transformations, such as join nest simplification.

'Local' means that each transformation happens on one single query block. Also perform partition pruning, which is most effective after transformations have been done. This function also does condition pushdown to derived tables after all the local transformations are applied although condition pushdown is strictly not a local transform.

Parameters

thd	thread handler
prune	if true, then prune partitions based on const conditions

Returns

false if success, true if error

Since this is called after flattening of query blocks, call this function while traversing the query block hierarchy top-down.

baptize_item()

static bool baptize_item ( THD *  thd, Item *  item, int *  field_no  )

static

Helper function to make names for columns of a derived table replacing a scalar or table subquery.

Fields from the query block containing the scalar subquery are moved to the new derived table. We give them synthetic unique names here.

Parameters

thd	current session context
item	the item we want to name
field_no	the field number

Returns

true on error

build_sj_cond()

bool Query_block::build_sj_cond ( THD *  thd, NESTED_JOIN *  nested_join, Query_block *  subq_query_block, table_map  outer_tables_map, Item **  sj_cond, bool *  simple_const  )

private

Build semijoin condition for th query block.

Build equality conditions using outer expressions and inner expressions.

If the equality condition is not constant, add it to the semi-join condition. Otherwise, evaluate it and remove the constant expressions from the outer/inner expressions list if the result is true. If the result is false, remove all the expressions in outer/inner expression list and attach an always false condition to semijoin condition.

Parameters

	thd	Thread context
	nested_join	Join nest
	subq_query_block	Query block for the subquery
	outer_tables_map	Map of tables from original outer query block
[in,out]	sj_cond	Semi-join condition to be constructed Contains non-equalities on input.
[out]	simple_const	true if the returned semi-join condition is a simple true or false predicate, false otherwise.

Returns

false if success, true if error

build_sj_exprs()

static bool build_sj_exprs ( THD *  thd, mem_root_deque< Item * > *  sj_outer_exprs, mem_root_deque< Item * > *  sj_inner_exprs, Item_exists_subselect *  subq_pred, Query_block *  subq_query_block  )

static

Builds the list of SJ outer/inner expressions.

Parameters

	thd	Connection handle
[out]	sj_outer_exprs	Will add outer expressions here
[out]	sj_inner_exprs	Will add inner expressions here
	subq_pred	Item for the subquery
	subq_query_block	Single query block for the subquery

Returns

true if error

can_decorrelate_operator()

static bool can_decorrelate_operator ( Item_func *  func, bool  only_eq  )

inlinestatic

check_only_full_group_by()

bool Query_block::check_only_full_group_by ( THD *  thd )

private

Runs checks mandated by ONLY_FULL_GROUP_BY.

Parameters

thd	THD pointer

Returns

true if ONLY_FULL_GROUP_BY is violated.

check_view_privileges()

bool Query_block::check_view_privileges	(	THD *	thd,
		ulong	want_privilege_first,
		ulong	want_privilege_next
	)

Check privileges for views that are merged into query block.

Check privileges for the view tables merged into a query block.

Parameters

thd	Thread context.
want_privilege_first	Privileges requested for the first leaf.
want_privilege_next	Privileges requested for the remaining leaves.

Note

Beware that it can't properly check privileges in cases when table being changed is not the first table in the list of leaf tables (for example, for multi-UPDATE).

The inner loop is slightly inefficient. A view will have its privileges checked once for every base table that it refers to.

Returns

false if success, true if error.

clear_sj_expressions()

void Query_block::clear_sj_expressions ( NESTED_JOIN *  nested_join )

private

Remove semijoin condition for this query block.

Remove SJ outer/inner expressions.

Parameters

nested_join

join nest

collect_aggregates()

static bool collect_aggregates ( Query_block *  select, Item_sum::Collect_grouped_aggregate_info *  aggregates  )

static

A minion of transform_grouped_to_derived.

Collect a unique list of aggregate functions used in the transformed query block, which will need to be replaced with fields from the derived table containing the grouping during transform_grouped_to_derived.

Parameters

[in]	select	the query block
[in,out]	aggregates	the accumulator which will contain the aggregates

Returns

true on error

convert_subquery_to_semijoin()

bool Query_block::convert_subquery_to_semijoin ( THD *  thd, Item_exists_subselect *  subq_pred  )

private

Convert a subquery predicate of this query block into a Table_ref semi-join nest.

Parameters

thd	Thread handle
subq_pred	Subquery predicate to be converted. This is either an IN, =ANY or EXISTS predicate, possibly negated.

Returns

false if success, true if error

The following transformations are performed:

1. IN/=ANY predicates on the form:

SELECT ...
FROM ot1 ... otN
WHERE (oe1, ... oeM) IN (SELECT ie1, ..., ieM
                         FROM it1 ... itK
                        [WHERE inner-cond])
 [AND outer-cond]
[GROUP BY ...] [HAVING ...] [ORDER BY ...]

are transformed into:

SELECT ...
FROM (ot1 ... otN) SJ (it1 ... itK)
                   ON (oe1, ... oeM) = (ie1, ..., ieM)
                      [AND inner-cond]
[WHERE outer-cond]
[GROUP BY ...] [HAVING ...] [ORDER BY ...]

Notice that the inner-cond may contain correlated and non-correlated expressions. Further transformations will analyze and break up such expressions.

1. EXISTS predicates on the form:

SELECT ...
FROM ot1 ... otN
WHERE EXISTS (SELECT expressions
              FROM it1 ... itK
              [WHERE inner-cond])
 [AND outer-cond]
[GROUP BY ...] [HAVING ...] [ORDER BY ...]

are transformed into:

SELECT ...
FROM (ot1 ... otN) SJ (it1 ... itK)
                   [ON inner-cond]
[WHERE outer-cond]
[GROUP BY ...] [HAVING ...] [ORDER BY ...]

1. Negated EXISTS predicates on the form:

SELECT ...
FROM ot1 ... otN
WHERE NOT EXISTS (SELECT expressions
              FROM it1 ... itK
              [WHERE inner-cond])
 [AND outer-cond]
[GROUP BY ...] [HAVING ...] [ORDER BY ...]

are transformed into:

SELECT ...
FROM (ot1 ... otN) AJ (it1 ... itK)
                   [ON inner-cond]
[WHERE outer-cond AND is-null-cond(it1)]
[GROUP BY ...] [HAVING ...] [ORDER BY ...]

where AJ means "antijoin" and is like a LEFT JOIN; and is-null-cond is false if the row of it1 is "found" and "not_null_compl" (i.e. matches inner-cond).

1. Negated IN predicates on the form:

SELECT ...
FROM ot1 ... otN
WHERE (oe1, ... oeM) NOT IN (SELECT ie1, ..., ieM
                             FROM it1 ... itK
                             [WHERE inner-cond])
 [AND outer-cond]
[GROUP BY ...] [HAVING ...] [ORDER BY ...]

are transformed into:

SELECT ...
FROM (ot1 ... otN) AJ (it1 ... itK)
                   ON (oe1, ... oeM) = (ie1, ..., ieM)
                      [AND inner-cond]
[WHERE outer-cond]
[GROUP BY ...] [HAVING ...] [ORDER BY ...]

1. The cases 1/2 (respectively 3/4) above also apply when the predicate is decorated with IS TRUE or IS NOT FALSE (respectively IS NOT TRUE or IS FALSE).

create_rollup_switcher()

Item * create_rollup_switcher ( THD *  thd, Query_block *  query_block, Item_sum *  item, int  send_group_parts  )

static

decorrelate_condition()

bool Query_block::decorrelate_condition ( Semijoin_decorrelation &  sj_decor, Table_ref *  join_nest  )

private

Decorrelate the WHERE clause or a join condition of a subquery used in an IN or EXISTS predicate.

Correlated predicates are removed from the condition and added to the supplied semi-join nest. The predicate must be either a simple (in)equality, or an AND condition that contains one or more simple equalities, in order for decorrelation to be possible.

Parameters

sj_decor	Object for recording the decorrelated expressions
join_nest	Nest containing join condition to be decorrelated =NULL: decorrelate the WHERE condition

Returns

false if success, true if error

Decorrelation for subqueries containing non-deterministic components:

There are two types of IN and EXISTS queries with non-deterministic functions that may be meaningful (the EXISTS queries below are correlated equivalents of the respective IN queries):

1. Non-deterministic function as substitute for expression from outer query block:

A SELECT * FROM t1 WHERE RAND() IN (SELECT t2.x FROM t2)

B SELECT * FROM t1 WHERE EXISTS (SELECT * FROM t2 WHERE RAND() = t2.x);

Pick a set of random rows that matches against a fixed set (the subquery).

The intuitive interpretation of the IN subquery is that the random function is evaluated per row of the outer query block, whereas in the EXISTS subquery, it should be evaluated per row of the inner query block, and the subquery is evaluated once per row of the outer query block.

1. Non-deterministic function as substitute for expression from inner query block:

A SELECT * FROM t1 WHERE t1.x IN (SELECT RAND() FROM t2)

B SELECT * FROM t1 WHERE EXISTS (SELECT * FROM t2 WHERE RAND() = t1.x);

This is another way of picking a random row, but now the non-determinism occurs in the inner query block.

The user will expect that only query 1A has the evaluation of non-deterministic functions being performed in the outer query block. Using decorrelation for query 1B would change the apparent semantics of the query.

The purpose of decorrelation is to be able to use more execution strategies. Without decorrelation, EXISTS is limited to FirstMatch and DupsWeedout strategies. Decorrelation enables LooseScan and Materialization. We can rule out LooseScan for case 2B, since it requires an indexed column from the subquery, and for case 1B, since it requires that the outer table is partitioned according to the distinct values of the index, and random values do not fulfill that partitioning requirement.

The only strategy left is Materialization. With decorrelation, 1B would be evaluated like 1A, which is not the intuitive way. 2B would also be implemented like 2A, meaning that evaluation of non-deterministic functions would move to the materialization function.

Thus, the intuitive interpretation is to avoid materialization for subqueries with non-deterministic components in the inner query block, and hence such predicates will not be decorrelated.

decorrelate_derived_scalar_subquery_post()

bool Query_block::decorrelate_derived_scalar_subquery_post ( THD *  thd, Table_ref *  derived, Lifted_fields_map *  lifted_where_fields, bool  added_card_check, size_t  added_window_card_checks  )

private

See explanation in companion method decorrelate_derived_scalar_subquery_pre.

decorrelate_derived_scalar_subquery_pre()

bool Query_block::decorrelate_derived_scalar_subquery_pre ( THD *  thd, Table_ref *  derived, Item *  lifted_where, Lifted_fields_map *  lifted_fields, bool *  added_card_check, size_t *  added_window_card_checks  )

private

We have a correlated scalar subquery, so we must do several things:

1. Add the relevant non-correlated fields "NCF"(*) to the select list so they can be referenced in the JOIN condition which now holds the earlier WHERE AND predicates that were correlated. (*) i.e. the inner fields involved in a predicate that contains an outer field reference. These were identified in supported_correlated_scalar_subquery, and passed in as 'lifted_where'.
2. Add a COUNT(*) to select list so it can be referenced from the transformed query's WHERE clause for cardinality check, if needed, i.e. when there is no aggregate function in the subquery's single(*) select expression. (*) single because we have a scalar subquery. Add this to NCF. If it does contain an aggregate function, there will be only one row per group iff the NCF are part of any GROUP BY list, and we add them to it, so that property holds.
3. Add grouping on NCF to the subquery. If already grouped, add the NCF at end of grouping list. Note that this might result in a grouped query that might fail the functional dependency checks. So we wrap any non-grouped field in the select list in Item_func_any_value. We can safely add the Item_func_any_value because subqueries with cardinalities greater than one will be rejected anyway.
4. Remember the set of NCF so we can create derived.field and derived.count(field) (NCF'), after setting up the materialized derived table, cf. 'lifted_fields'.
5. Update the correlated fields in the JOIN condition to no longer be outer references, and the NCF' to refer to the derived table's fields, NCF.

This logic is partially done before setting up the materialized derived table, in the present method ("_pre"), and partly after setting up the materialized derived table, cf. the companion method ("_post").

Parameters

	thd	session context
	derived	the derived table being created in the transform
	lifted_where	the WHERE condition we move out to the JOIN cond
[out]	lifted_fields	mapping of where inner fields end up in the derived table's fields.
[out]	added_card_check	set to true if we are adding a cardinality check
[out]	added_window_card_checks	set to # of window functions added to SELECT if the subquery is initially grouped and we need COUNT(*) OVER (...) to be checked

decorrelate_equality()

static bool decorrelate_equality ( Semijoin_decorrelation &  sj_decor, Item_func *  func, bool *  was_correlated  )

static

Try to decorrelate an (in)equality node.

The node can be decorrelated if one argument contains only outer references and the other argument contains references only to local tables. Both arguments should be deterministic. const-for-execution values are accepted in both arguments.

Note

that a predicate like '(a,b) IN ((c,d))' is changed to two equalities only during optimization, so at the present stage it isn't decorrelate-able.

Parameters

	sj_decor	Object for recording the decorrelated expressions
	func	The query function node
[out]	was_correlated	= true if comparison is correlated and the the expressions are added to sj_nest.

Returns

false if success, true if error

delete_unused_merged_columns()

void Query_block::delete_unused_merged_columns ( mem_root_deque< Table_ref * > *  tables )

private

Delete unused columns from merged tables.

This function is called recursively for each join nest and/or table in the query block. For each merged table that it finds, each column that contains a subquery and is not marked as used is removed and the translation item is set to NULL.

Parameters

tables

List of tables and join nests

empty_order_list()

bool Query_block::empty_order_list ( Query_block *  sl )

private

Empty the ORDER list.

Delete corresponding elements from fields and base_ref_items too. If ORDER list contain any subqueries, delete them from the query block list.

Parameters

sl	Query block that possible subquery blocks in the ORDER BY clause are attached to (may be different from "this" when query block has been merged into an outer query block).

Returns

true on error

extract_correlated_condition()

static bool extract_correlated_condition ( THD *  thd, Item **  cond, Item **  correlated_cond  )

static

Extracts the top level correlated condition in an OR condition.

For ex: (((t1.a = t2.b ) and (t1.c =10)) OR ((t1.a = t2.b) and (t1.d =10))) is the same as (t1.a = t2.b) and ((t1.c = 10) or (t1.d = 10))

So we extract the (t1.a = t2.b) as the correlated condition and leave ((t1.c = 10) or (t1.d = 10)) in the original condition that is passed as the argument.

The caller of the function has to send an OR condition. Only the top level correlated condition is extracted. Caller could repeatedly call this function to extract the inner level correlated conditions as well.

Parameters

	thd	session context
[in,out]	cond	Original condition that is looked into, to extract the correlated condition.
[out]	correlated_cond	correlated condition that is extracted

Returns

false when a correlated condition is successfully extracted. true when no correlated condition could be extracted.

field_list_is_empty()

bool Query_block::field_list_is_empty

(

)

const

find_in_group_list()

ORDER * Query_block::find_in_group_list	(	Item *	item,
		int *	rollup_level
	)		const

Finds a group expression matching the given item, or nullptr if none.

When there are multiple candidates, ones that match in name are given priority (such that “a AS c GROUP BY a,b,c” resolves to c, not a); if there is still a tie, the leftmost is given priority.

Parameters

	item	The item to search for.
[out]	rollup_level	If not nullptr, will be set to the group expression's index (0-based).

find_order_in_list()

bool find_order_in_list	(	THD *	thd,
		Ref_item_array	ref_item_array,
		Table_ref *	tables,
		ORDER *	order,
		mem_root_deque< Item * > *	fields,
		bool	is_group_field,
		bool	is_window_order
	)

Resolve an ORDER BY or GROUP BY column reference.

Given a column reference (represented by 'order') from a GROUP BY or ORDER BY clause, find the actual column it represents. If the column being resolved is from the GROUP BY clause, the procedure searches the SELECT list 'fields' and the columns in the FROM list 'tables'. If 'order' is from the ORDER BY clause, only the SELECT list is being searched.

If 'order' is resolved to an Item, then order->item is set to the found Item. If there is no item for the found column (that is, it was resolved into a table field), order->item is 'fixed' and is added to fields and ref_item_array.

ref_item_array and fields are updated.

Parameters

[in]	thd	Pointer to current thread structure
[in,out]	ref_item_array	All select, group and order by fields
[in]	tables	List of tables to search in (usually FROM clause)
[in]	order	Column reference to be resolved
[in,out]	fields	List of fields to search in (usually SELECT list; hidden items are ignored)
[in]	is_group_field	True if order is a GROUP field, false if ORDER by field
[in]	is_window_order	True if order is a Window function's PARTITION BY or ORDER BY field

Return values

false	if OK
true	if error occurred

fix_after_pullout() [1/2]

void Query_expression::fix_after_pullout	(	Query_block *	parent_query_block,
		Query_block *	removed_query_block
	)

Fix used tables information for a subquery after query transformations.

This is for transformations where the subquery remains a subquery - it is not merged, it merely moves up by effect of a transformation on a containing query block. Most actions here involve re-resolving information for conditions and items belonging to the subquery. If the subquery contains an outer reference into removed_query_block or parent_query_block, the relevant information is updated by Item_ident::fix_after_pullout().

fix_after_pullout() [2/2]

void Query_block::fix_after_pullout ( Query_block *  parent_query_block, Query_block *  removed_query_block  )

private

See also

Query_expression::fix_after_pullout

fix_tables_after_pullout()

static void fix_tables_after_pullout ( Query_block *  parent_query_block, Query_block *  removed_query_block, Table_ref *  tr, uint  table_adjust, table_map  lateral_deps  )

static

Update table reference information for conditions and expressions due to query blocks having been merged in from derived tables/views and due to semi-join transformation.

This is needed for two reasons:

1. Since table numbers are changed, we need to update used_tables information for all conditions and expressions that are possibly touched.
2. For semi-join, some column references are changed from outer references to local references.

The function needs to recursively walk down into join nests, in order to cover all conditions and expressions.

For a semi-join, tables from the subquery are added last in the query block. This means that conditions and expressions from the outer query block are unaffected. But all conditions inside the semi-join nest, including join conditions, must have their table numbers changed.

For a derived table/view, tables from the subquery are merged into the outer query, and this function is called for every derived table that is merged in. This algorithm only works when derived tables are merged in the order of their original table numbers.

A hypothetical example with a triple self-join over a mergeable view:

CREATE VIEW v AS SELECT t1.a, t2.b FROM t1 JOIN t2 USING (a); SELECT v1.a, v1.b, v2.b, v3.b FROM v AS v1 JOIN v AS v2 ON ... JOIN v AS v3 ON ...;

The analysis starts with three tables v1, v2 and v3 having numbers 0, 1, 2. First we merge in v1, so we get (t1, t2, v2, v3). v2 and v3 are shifted up. Tables from v1 need to have their table numbers altered (actually they do not since both old and new numbers are 0 and 1, but this is a special case). v2 and v3 are not merged in yet, so we delay pullout on them until they are merged. Conditions and expressions from the outer query are not resolved yet, so regular resolving will take of them later. Then we merge in v2, so we get (t1, t2, t1, t2, v3). The tables from this view gets numbers 2 and 3, and v3 gets number 4. Because v2 had a higher number than the tables from v1, the join nest representing v1 is unaffected. And v3 is still not merged, so the only join nest we need to consider is v2. Finally we merge in v3, and then we have tables (t1, t2, t1, t2, t1, t2), with numbers 0 through 5. Again, since v3 has higher number than any of the already merged in views, only this join nest needs the pullout.

Parameters

parent_query_block	Query block being merged into
removed_query_block	Query block that is removed (subquery)
tr	Table object this pullout is applied to
table_adjust	Number of positions that a derived table nest is adjusted, used to fix up semi-join related fields. Tables are adjusted from position N to N+table_adjust
lateral_deps	Lateral dependencies of the unit owning removed_query_block

flatten_subqueries()

bool Query_block::flatten_subqueries

(

THD *

thd

)

Convert semi-join subquery predicates into semi-join join nests.

Convert candidate subquery predicates into semi-join join nests. This transformation is performed once in query lifetime and is irreversible.

Conversion of one subquery predicate

We start with a query block that has a semi-join subquery predicate:

SELECT ...
FROM ot, ...
WHERE oe IN (SELECT ie FROM it1 ... itN WHERE subq_where) AND outer_where

and convert the predicate and subquery into a semi-join nest:

SELECT ...
FROM ot SEMI JOIN (it1 ... itN), ...
WHERE outer_where AND subq_where AND oe=ie

that is, in order to do the conversion, we need to

Create the "SEMI JOIN (it1 .. itN)" part and add it into the parent query block's FROM structure. Add "AND subq_where AND oe=ie" into parent query block's WHERE (or ON if the subquery predicate was in an ON condition) Remove the subquery predicate from the parent query block's WHERE

Considerations when converting many predicates

A join may have at most MAX_TABLES tables. This may prevent us from flattening all subqueries when the total number of tables in parent and child selects exceeds MAX_TABLES. In addition, one slot is reserved per semi-join nest, in case the subquery needs to be materialized in a temporary table. We deal with this problem by flattening children's subqueries first and then using a heuristic rule to determine each subquery predicate's priority, which is calculated in this order:

1. Prefer dependent subqueries over non-dependent ones
2. Prefer subqueries with many tables over those with fewer tables
3. Prefer early subqueries over later ones (to make sort deterministic)

Returns

false if success, true if error

fulltext_uses_rollup_column()

static bool fulltext_uses_rollup_column ( const Query_block *  query_block )

static

Checks if there are any calls to the MATCH function that take a ROLLUP column as argument in the SELECT list, GROUP BY clause, HAVING clause or ORDER BY clause.

Such calls should be rejected, since MATCH only works on base columns.

group_list_size()

int Query_block::group_list_size

(

)

const

is_correlated_predicate_eligible()

bool is_correlated_predicate_eligible

(

Item *

cor_pred

)

Called to check if the provided correlated predicate is eligible for transformation.

To be eligible, it must have one non-correlated operand and one correlated operand, and the non-correlated operand must be a simple column reference (Else we need to group on expressions in the derived table - not supported currently).

Parameters

cor_pred

correlated predicate that needs to be examined

Returns

true if predicate is eligible for transformation.

is_row_count_valid_for_semi_join()

bool Query_block::is_row_count_valid_for_semi_join

(

)

Check if the offset and limit are valid for a semijoin.

A semijoin can be used only if OFFSET is 0 and select LIMIT is not 0.

Return values

false	if OFFSET and LIMIT does not permit a semijoin,
true	otherwise.

lift_fulltext_from_having_to_select_list()

bool Query_block::lift_fulltext_from_having_to_select_list ( THD *  thd )

private

Copies all non-aggregated calls to the full-text search MATCH function from the HAVING clause to the SELECT list (as hidden items), so that we can materialize their result and not only their input.

This is needed when the result will be accessed after aggregation, as the result from MATCH cannot be recalculated from its input alone. It also needs the underlying scan to be positioned on the correct row. Storing the value before aggregation removes the need for evaluating MATCH again after materialization.

make_leaf_tables()

static Table_ref ** make_leaf_tables ( Table_ref **  list, Table_ref *  tables  )

static

Make list of leaf tables of join table tree.

Parameters

list	pointer to pointer on list first element Must be set to NULL before first (recursive) call
tables	table list

Returns

pointer on pointer to next_leaf of last element

mark_item_as_maybe_null_if_non_primitive_grouped()

void Query_block::mark_item_as_maybe_null_if_non_primitive_grouped ( Item *  item ) const

private

Marks occurrences of group by fields in a function's arguments as nullable, so that we do not optimize them away before we get to add the rollup wrappers.

merge_contexts()

void Query_block::merge_contexts

(

Query_block *

inner

)

Merge name resolution context objects of a subquery into its parent.

Merge name resolution context objects belonging to an inner subquery to parent query block.

Update all context objects to have this base query block. Used when a subquery's query block is merged into its parent.

Parameters

inner

Subquery for which context objects are to be merged.

merge_derived()

bool Query_block::merge_derived	(	THD *	thd,
		Table_ref *	derived_table
	)

Merge derived table into query block.

Merge a derived table or view into a query block.

If some constraint prevents the derived table from being merged then do nothing, which means the table will be prepared for materialization later.

After this call, check is_merged() to see if the table was really merged.

Parameters

thd	Thread handler
derived_table	Derived table which is to be merged.

Returns

false if successful, true if error

A view is updatable if any underlying table is updatable. A view is insertable-into if all underlying tables are insertable. A view is not updatable nor insertable if it contains an outer join

See also

mysql_register_view()

nest_derived()

bool Query_block::nest_derived ( THD *  thd, Item *  join_cond, mem_root_deque< Table_ref * > *  nested_join_list, Table_ref *  derived_table  )

private

Push the generated derived table to the correct location inside a join nest.

It will be nested in a new nest along with the outer table to the join which owns the search condition in which we found the scalar subquery. For example:

select t1.i,
       t2.i
from t1
     left outer join
     t2 on
     (t1.i < (select max(t2.i) from t2));

in transformed to

select t1.i,
       t2.i
from t1
     left join
     (select max(t2.i) AS `max(t2.i)` from t2) derived_1_0   [*]
     on(true)
     left join
     t2
     on((t1.i < derived_1_0.`max(t2.i)`))

[*]: the derived table is nested in here, just ahead of the inner table t2 to which the join condition is attached.

In the original join nest before transformation may look like this (the join order list is reversed relative to the logical order):

(nest_join) t2 LEFT OUTER ON .. = .. (inner table) t1 (outer table)

After the transformation we have this nest structure:

(nest_join) t2 LEFT OUTER ON .. = .. (nest_last_join) derived_1_0 LEFT OUTER ON true t1

The method will recursively inspect and rebuild join nests as needed since the join with the condition may be deeply nested.

Parameters

thd	the session context
join_cond	the join condition which identifies the join we want to nest into
nested_join_list	the join list at the current nesting level
derived_table	the table we want to nest

Returns

true on error

num_visible_fields()

size_t Query_block::num_visible_fields

(

)

const

populate_grouping_sets()

bool Query_block::populate_grouping_sets

(

THD *

thd

)

Populates the grouping sets if the query block includes non-primitive grouping.

Populate the grouping set bitvector if the query block has non-primitive grouping.

If the non-primitive grouping is ROLLUP or CUBE, the grouping sets have to be computed. The representation of the grouping set is done using a bitfield in the ORDER object. case ROLLUP : Say the query has GROUP BY ROLLUP (a,b) then the grouping sets will be (a,b) (a) () where () represents single group aggregate without any grouping. Here there are 3 grouping sets ranging from 0 to 2 and 0 is the single group aggregate. The bitfield associated with GROUP BY element 'a' will be 3 (i,e. 2+1) The bitfield associated with Group by element 'b' will be 2 as it is part of only set number 2. case CUBE: Say the query has GROUP BY CUBE (a,b) then the grouping sets will be (a,b) (a) (b) (). The number of grouping sets will be (2^n) where n is the number of elements in the GROUP BY list. The bitfield associated with Group by element 'a' will be 6 (i.e. 4+2). The bitfield associated with Group by element 'b' will be 1.

prepare()

bool Query_block::prepare	(	THD *	thd,
		mem_root_deque< Item * > *	insert_field_list
	)

Prepare query block for optimization.

Resolve table and column information. Resolve all expressions (item trees), ie WHERE clause, join conditions, GROUP BY clause, HAVING clause, ORDER BY clause, LIMIT clause. Prepare all subqueries recursively as part of resolving the expressions. Apply permanent transformations to the abstract syntax tree, such as semi-join transformation, derived table transformation, elimination of constant values and redundant clauses (e.g ORDER BY, GROUP BY).

Parameters

thd	thread handler
insert_field_list	List of fields when used in INSERT, otherwise NULL

Returns

false if success, true if error

Note

on privilege checking for SELECT query that possibly contains view or derived table references:

• When this function is called, it is assumed that the precheck() function has been called. precheck() ensures that the user has some SELECT privileges to the tables involved in the query. When resolving views it has also been established that the user has some privileges for them. To prepare a view for privilege checking, it is also needed to call check_view_privileges() after views have been merged into the query. This is not necessary for unnamed derived tables since it has already been established that we have SELECT privileges for the underlying tables by the precheck functions. (precheck() checks a query without resolved views, ie. before tables are opened, so underlying tables of views are not yet available).
• When a query block is resolved, always ensure that the user has SELECT privileges to the columns referenced in the WHERE clause, the join conditions, the GROUP BY clause, the HAVING clause and the ORDER BY clause.
• When resolving the outer-most query block, ensure that the user also has SELECT privileges to the columns in the selected expressions.
• When setting up a derived table or view for materialization, ensure that the user has SELECT privileges to the columns in the selected expressions
• Column privileges are normally checked by Item_field::fix_fields(). Exceptions are select list of derived tables/views which are checked in Table_ref::setup_materialized_derived(), and natural/using join conditions that are checked in mark_common_columns().
• As far as INSERT, UPDATE and DELETE statements have the same expressions as a SELECT statement, this note applies to those statements as well.

prepare_values()

bool Query_block::prepare_values ( THD *  thd )

private

Prepare a table value constructor query block for optimization.

In the case of a table value constructor Query_block, we return the result of this function from Query_block::prepare, instead of doing the standard prepare routine.

For a table value constructor block, most preparation of a standard Query_block becomes irrelevant (in particular INTO, FROM, WHERE, GROUP, HAVING and WINDOW). We therefore substitute the standard resolving routine with this one, which is simply responsible for resolving the expressions contained in VALUES, as well as the query result.

Parameters

thd	thread handler

Returns

false if success, true if error

propagate_nullability()

void propagate_nullability	(	mem_root_deque< Table_ref * > *	tables,
		bool	nullable
	)

Propagate nullability into inner tables of outer join operation.

Parameters

tables	List of tables and join nests, start at m_table_nest
nullable	true: Set all underlying tables as nullable

propagate_unique_test_exclusion()

void Query_block::propagate_unique_test_exclusion

(

)

Propagate exclusion from table uniqueness test into subqueries.

Propagate exclusion from unique table check into all subqueries belonging to this query block.

This function can be applied to all subqueries of a materialized derived table or view.

push_conditions_to_derived_tables()

bool Query_block::push_conditions_to_derived_tables

(

THD *

thd

)

Pushes parts of the WHERE condition of this query block to materialized derived tables.

query_block_contains_subquery()

static bool query_block_contains_subquery ( Query_block *  select, Query_expression *  slu  )

static

A minion of transform_scalar_subqueries_to_join_with_derived.

Determine if the query expression is directly contained in the query block, i.e. it is a subquery.

Parameters

select	the query block
slu	the query expression

Returns

true if slu is directly contained in select, else false

record_join_nest_info()

bool Query_block::record_join_nest_info ( mem_root_deque< Table_ref * > *  tables )

private

Record join nest info in the select block.

After simplification of inner join, outer join and semi-join structures:

• record the remaining semi-join structures in the enclosing query block.
• record transformed join conditions in Table_ref objects.

This function is called recursively for each join nest and/or table in the query block.

Parameters

tables

List of tables and join nests

Returns

False if successful, True if failure

refresh_comparators_after_rollup()

static bool refresh_comparators_after_rollup ( Item *  item )

static

Refreshes the comparators after ROLLUP resolving.

This is needed because ROLLUP resolving happens after the comparators have been set up. In ROLLUP resolving, it may turn out that something initially believed to be constant, is not constant after all (e.g., group items that may be NULL in some cases). So we call set_cmp_func() to make Arg_comparator adjust/remove its caches accordingly.

remap_tables()

void Query_block::remap_tables ( THD *  thd )

private

Re-map table numbers for all tables in a query block.

Parameters

thd	Thread handler

Note

This function needs to be called after setup_tables() has been called, and after a query block for a subquery has been merged into a parent quary block.

remember_transform()

static void remember_transform ( THD *  thd, Query_block *  select  )

static

Remember if this transform was performed.

It it was done by a secondary engine, it may need to be rolled back before falling back on primary engine execution.

remove_aggregates()

bool Query_block::remove_aggregates	(	THD *	thd,
		Query_block *	select
	)

A minion of transform_grouped_to_derived.

"Remove" any non-window aggregate functions from fields unconditionally. If such an aggregate is found, the query block should have a HAVING clause. This is asserted in debug mode. We "remove" them by replacing them with an Item_int, which should have no adverse effects. This avoids creating trouble for Query_block::add_hidden_item which would otherwise need to keep track of removed items.

Parameters

thd	session context
select	the query block whose aggregates are being moved into a derived table

Returns

true on error, else false

remove_hidden_items()

void Query_block::remove_hidden_items

(

)

Remove hidden items from select list.

remove_redundant_subquery_clauses()

bool Query_block::remove_redundant_subquery_clauses ( THD *  thd )

private

For a table subquery predicate (IN/ANY/ALL/EXISTS/etc): since it does not support LIMIT the following clauses are redundant:

ORDER BY DISTINCT GROUP BY if there are no aggregate functions and no HAVING clause

For a scalar subquery without LIMIT: ORDER BY is redundant, as the number of rows to order must be 1.

This removal is permanent. Thus, it only makes sense to call this function for regular queries and on first execution of SP/PS

Parameters

thd	thread handler

Returns

true on error

replace_aggregate_in_list()

static bool replace_aggregate_in_list ( Item::Aggregate_replacement &  info, bool  was_hidden, mem_root_deque< Item * > *  list, Ref_item_array *  ref_item_array  )

static

A minion of transform_grouped_to_derived.

Replace occurrences of the aggregate function identified in info.m_target with the the field info.m_replacement in the expressions contained in list. Note that since this is part of a permanent transformation, we use the extra m_permanent_transform flag in the THD

Parameters

info	a tuple containing {aggregate, replacement field}
was_hidden	true if the aggregate was originally hidden
list	the list of expressions
ref_item_array	to be kept in sync with any changes in 'list'

Returns

true on error (can not happen currently unless replacement field is empty)

replace_item_in_expression()

bool Query_block::replace_item_in_expression ( Item **  expr, bool  was_hidden, Item::Item_replacement *  info, Item_transformer  transformer  )

private

Minion of transform_grouped_to_derived.

Do a replacement in expr using Item::transform as specified in info using transformer.

replace_referenced_item()

void Query_block::replace_referenced_item ( Item *const  old_item, Item *const  new_item  )

private

Replace item in select list and preserve its reference count.

Parameters

old_item	Item to be replaced.
new_item	Item to replace the old item.

If old item is present in base_ref_items, make sure it is replaced there.

Also make sure that reference count for old item is preserved in new item.

replace_subcondition()

static bool replace_subcondition ( THD *  thd, Item **  tree, Item *  old_cond, Item *  new_cond, bool  do_fix_fields, bool *  found_ptr = nullptr  )

static

Destructively replaces a sub-condition inside a condition tree.

The parse tree is also altered.

Parameters

	thd	thread handler
	tree	Must be the handle to the top level condition. This is needed when the top-level condition changes.
	old_cond	The condition to be replaced.
	new_cond	The condition to be substituted.
	do_fix_fields	If true, Item::fix_fields(THD*, Item**) is called for the new condition.
[out]	found_ptr	Pointer to boolean; used only in recursive sub-calls; top call must not specify this argument. Function deposits there if it found the searched Item or not.

Returns

error status

Return values

true	If there was an error.
false	If successful.

replace_subquery_in_expr()

bool Query_block::replace_subquery_in_expr ( THD *  thd, Item::Css_info *  subquery, Table_ref *  tr, Item **  expr  )

private

A minion of transform_scalar_subqueries_to_join_with_derived.

A transform creates a field representing the value of the derived table and adds it as a hidden field to the select list. Next, it replaces the subquery in the item tree with this field. If we replace in a HAVING condition, we build an Item_ref, cf. PTI_simple_ident_ident::itemize which also creates a Item_ref for a field reference in HAVING, because we may need to access the field in a tmp table.

Parameters

thd	The session context
subquery	The scalar subquery
tr	The table reference for the derived table
expr	The expression we are replacing (in)

repoint_contexts_of_join_nests()

void Query_block::repoint_contexts_of_join_nests ( mem_root_deque< Table_ref * >  join_list )

private

Go through a list of tables and join nests, recursively, and repoint its query_block pointer.

Parameters

join_list

List of tables and join nests

reset_nj_counters()

void Query_block::reset_nj_counters

(

mem_root_deque< Table_ref * > *

join_list = nullptr

)

Set NESTED_JOIN::counter=0 in all nested joins in passed list.

Parameters

join_list

Pass NULL. Non-NULL is reserved for recursive inner calls, then it is a list of nested joins to process, and may also contain base tables which will be ignored.

resolve_limits()

bool Query_block::resolve_limits

(

THD *

thd

)

Resolve OFFSET and LIMIT clauses.

Resolve OFFSET and LIMIT clauses for a query block.

Parameters

thd	Thread handler

Returns

false if success, true if error

OFFSET and LIMIT clauses may be attached to query blocks that make up a query expression. OFFSET and LIMIT clauses that apply to a whole query expression are attached to the fake_query_block, hence we can use this interface to resolve them as well.

OFFSET and LIMIT may be unsigned integer literal values or parameters. If parameters, ensure that the type is unsigned integer.

resolve_placeholder_tables()

bool Query_block::resolve_placeholder_tables	(	THD *	thd,
		bool	apply_semijoin
	)

Resolve derived table, view, table function information for a query block.

Resolve derived table, view or table function references in query block.

Parameters

thd	Pointer to THD.
apply_semijoin	if true, apply semi-join transform when possible

Returns

false if success, true if error

resolve_rollup()

bool Query_block::resolve_rollup ( THD *  thd )

private

Resolve items in SELECT list and ORDER BY list for rollup processing.

Parameters

thd	session context

Returns

false if success, true if error

resolve_rollup_item()

Item * Query_block::resolve_rollup_item ( THD *  thd, Item *  item  )

private

Resolve an item (and its tree) for rollup processing by replacing items matching grouped expressions with Item_rollup_group_items and updating properties (m_nullable, PROP_ROLLUP_FIELD).

Also check any GROUPING function for incorrect column.

Parameters

thd	session context
item	the item to be processed

Returns

the new item, or nullptr on error

resolve_rollup_wfs()

bool Query_block::resolve_rollup_wfs

(

THD *

thd

)

Replace group by field references inside window functions with references in the presence of ROLLUP.

Parameters

thd	session context

Returns

false if success, true if error

resolve_table_value_constructor_values()

bool Query_block::resolve_table_value_constructor_values ( THD *  thd )

private

Resolve the rows of a table value constructor and aggregate the type of each column across rows.

Parameters

thd	thread handler

Returns

false if success, true if error

setup_conds()

bool Query_block::setup_conds

(

THD *

thd

)

Resolve WHERE condition and join conditions.

Parameters

thd	thread handler

Returns

false if success, true if error

setup_counts_over_partitions()

bool Query_block::setup_counts_over_partitions ( THD *  thd, Table_ref *  derived, Lifted_fields_map *  lifted_fields, std::deque< Item_field * > &  added_to_group_by, uint  hidden_fields  )

private

Add all COUNT(0) to SELECT list of the derived table to be used for cardinality checking of the transformed subquery.

Minion of decorrelate_derived_scalar_subquery_pre

a. Add COUNT(0) OVER (PARTITION BY group-by-list) b. Add COUNT(0) OVER (PARTITION BY inner-expr) for each inner-expression not already grouped on.

setup_group()

bool Query_block::setup_group ( THD *  thd )

private

Resolve and set up the GROUP BY list.

Parameters

thd	Thread handler

Returns

false if success, true if error

setup_join_cond()

bool Query_block::setup_join_cond ( THD *  thd, mem_root_deque< Table_ref * > *  tables, bool  in_update  )

private

Resolve join conditions for a join nest.

Parameters

thd	thread handler
tables	List of tables with join conditions
in_update	True if used in update command that may have CHECK OPTION

Returns

false if success, true if error

setup_order()

bool setup_order	(	THD *	thd,
		Ref_item_array	ref_item_array,
		Table_ref *	tables,
		mem_root_deque< Item * > *	fields,
		ORDER *	order
	)

Resolve and setup list of expressions in ORDER BY clause.

Change order to point at item in select list. If item isn't a number and doesn't exists in the select list, add it to the the field list.

Parameters

thd	Current session.
ref_item_array	The Ref_item_array for this query block.
tables	From clause of the query.
fields	All columns, including hidden ones.
order	The query block's order clause.

Returns

false if success, true if error.

setup_order_final()

bool Query_block::setup_order_final ( THD *  thd )

private

Do final setup of ORDER BY clause, after the query block is fully resolved.

Check that ORDER BY clause is not redundant. Split any aggregate functions.

Parameters

thd	Thread handler

Returns

false if success, true if error

setup_tables()

bool Query_block::setup_tables	(	THD *	thd,
		Table_ref *	tables,
		bool	select_insert
	)

Resolve and prepare information about tables for one query block.

Set up table leaves in the query block based on list of tables.

Parameters

thd	Thread handler
tables	List of tables to handle
select_insert	It is SELECT ... INSERT command

Note

Check also that the 'used keys' and 'ignored keys' exists and set up the table structure accordingly. Create a list of leaf tables.

This function has to be called for all tables that are used by items, as otherwise table->map is not set and all Item_field will be regarded as const items.

Returns

False on success, true on error

setup_wild()

bool Query_block::setup_wild ( THD *  thd )

private

Expand all '*' in list of expressions with the matching column references.

Function should not be called with no wild cards in select list

Parameters

thd	thread handler

Returns

false if OK, true if error

simplify_const_condition()

static bool simplify_const_condition ( THD *  thd, Item **  cond, bool  remove_cond, bool *  ret_cond_value  )

static

Try to replace a const condition with a simple constant.

A true condition is replaced with an empty item pointer if remove_cond is true. Else it is replaced with the constant TRUE. A false condition is replaced with the constant FALSE.

Parameters

	thd	Thread handler
[in,out]	cond	Address of condition, may be substituted with a literal
	remove_cond	If true removes a "true" condition. Else replaces it with a constant TRUE.
	ret_cond_value	Store the result of the evaluated const condition

Returns

false if success, true if error

simplify_joins()

bool Query_block::simplify_joins ( THD *  thd, mem_root_deque< Table_ref * > *  join_list, bool  top, bool  in_sj, Item **  cond, uint *  changelog = nullptr  )

private

Simplify joins replacing outer joins by inner joins whenever it's possible.

The function, during a retrieval of join_list, eliminates those outer joins that can be converted into inner join, possibly nested. It also moves the join conditions for the converted outer joins and from inner joins to conds. The function also calculates some attributes for nested joins:

1. used_tables
2. not_null_tables
3. dep_tables.
4. join_cond_dep_tables

The first two attributes are used to test whether an outer join can be substituted by an inner join. The third attribute represents the relation 'to be dependent on' for tables. If table t2 is dependent on table t1, then in any evaluated execution plan table access to table t2 must precede access to table t2. This relation is used also to check whether the query contains invalid cross-references. The fourth attribute is an auxiliary one and is used to calculate dep_tables. As the attribute dep_tables qualifies possibles orders of tables in the execution plan, the dependencies required by the straight join modifiers are reflected in this attribute as well. The function also removes all parentheses that can be removed from the join expression without changing its meaning.

Note

An outer join can be replaced by an inner join if the where condition or the join condition for an embedding nested join contains a conjunctive predicate rejecting null values for some attribute of the inner tables.

E.g. in the query:

SELECT * FROM t1 LEFT JOIN t2 ON t2.a=t1.a WHERE t2.b < 5

the predicate t2.b < 5 rejects nulls. The query is converted first to:

SELECT * FROM t1 INNER JOIN t2 ON t2.a=t1.a WHERE t2.b < 5

then to the equivalent form:

SELECT * FROM t1, t2 ON t2.a=t1.a WHERE t2.b < 5 AND t2.a=t1.a

Similarly the following query:

SELECT * from t1 LEFT JOIN (t2, t3) ON t2.a=t1.a t3.b=t1.b
  WHERE t2.c < 5

is converted to:

SELECT * FROM t1, (t2, t3) WHERE t2.c < 5 AND t2.a=t1.a t3.b=t1.b

One conversion might trigger another:

SELECT * FROM t1 LEFT JOIN t2 ON t2.a=t1.a
                 LEFT JOIN t3 ON t3.b=t2.b
  WHERE t3 IS NOT NULL =>
SELECT * FROM t1 LEFT JOIN t2 ON t2.a=t1.a, t3
  WHERE t3 IS NOT NULL AND t3.b=t2.b =>
SELECT * FROM t1, t2, t3
  WHERE t3 IS NOT NULL AND t3.b=t2.b AND t2.a=t1.a

The function removes all unnecessary parentheses from the expression produced by the conversions. E.g.

SELECT * FROM t1, (t2, t3) WHERE t2.c < 5 AND t2.a=t1.a AND t3.b=t1.b

finally is converted to:

SELECT * FROM t1, t2, t3 WHERE t2.c < 5 AND t2.a=t1.a AND t3.b=t1.b

It also will remove parentheses from the following queries:

SELECT * from (t1 LEFT JOIN t2 ON t2.a=t1.a) LEFT JOIN t3 ON t3.b=t2.b
SELECT * from (t1, (t2,t3)) WHERE t1.a=t2.a AND t2.b=t3.b.

The benefit of this simplification procedure is that it might return a query for which the optimizer can evaluate execution plans with more join orders. With a left join operation the optimizer does not consider any plan where one of the inner tables is before some of outer tables.

IMPLEMENTATION The function is implemented by a recursive procedure. On the recursive ascent all attributes are calculated, all outer joins that can be converted are replaced and then all unnecessary parentheses are removed. As join list contains join tables in the reverse order sequential elimination of outer joins does not require extra recursive calls.

SEMI-JOIN NOTES Remove all semi-joins that have are within another semi-join (i.e. have an "ancestor" semi-join nest)

EXAMPLES Here is an example of a join query with invalid cross references:

SELECT * FROM t1 LEFT JOIN t2 ON t2.a=t3.a LEFT JOIN t3 ON t3.b=t1.b

Parameters

	thd	thread handler
	join_list	list representation of the join to be converted
	top	true <=> cond is the where condition
	in_sj	true <=> processing semi-join nest's children
[in,out]	cond	In: condition to which the join condition for converted outer joins is to be added; Out: new condition
	changelog	Don't specify this parameter, it is reserved for recursive calls inside this function

Returns

true for error, false for success

single_visible_field()

Item * Query_block::single_visible_field

(

)

const

subquery_allows_materialization()

bool Item_in_subselect::subquery_allows_materialization	(	THD *	thd,
		Query_block *	query_block,
		const Query_block *	outer
	)

Check if the subquery predicate can be executed via materialization.

Parameters

thd	THD
query_block	Query_block of the subquery
outer	Parent Query_block (outer to subquery)

Returns

true if subquery allows materialization, false otherwise.

supported_correlated_scalar_subquery()

bool Query_block::supported_correlated_scalar_subquery ( THD *  thd, Item::Css_info *  subquery, Item **  lifted_where  )

private

Called when the scalar subquery is correlated.

If the type of correlation is not supported, return false and leave *lifted_where unassigned. If it is supported, *lifted_where contains a set of correlated predicates. Currently, we can only de-correlate the WHERE clause: if the clause is not a top level AND, we lift out the entire predicate to the JOIN clause. If it is a top level AND, we lift out only those AND operand predicates which are correlated, leaving un-correlated operand predicates in the subquery's WHERE clause, as lifting all out would be too ineffective, potentially creating large cartesian products in the subquery.

Parameters

	thd	session context
	subquery	the subquery under consideration
[out]	lifted_where	set of predicates lifted out of WHERE

Returns

true for error else false

synthesize_derived()

Table_ref * Query_block::synthesize_derived ( THD *  thd, Query_expression *  unit, Item *  join_cond, bool  left_outer, bool  use_inner_join  )

private

Create a new Table_ref object for this query block, for either: 1) a derived table which will replace the subquery, or 2) an extra derived table for handling grouping, if necessary, cf.

transform_grouped_to_derived.

The derived table is added to the list of used tables for the query block ("outer").

Parameters

thd	the session context
unit	the query expression for subquery (case 1), or a new query expression for (case 2)
join_cond	!= nullptr: we are synthesizing a derived table for a subquery within this join condition = nullptr: synthesizing a derived table for a subquery where the subquery is not contained in a join condition
left_outer	true for case (1), false for (2)
use_inner_join	for case (1): if true/false use INNER/LEFT JOIN

Returns

the derived table object, or nullptr on error.

transform_grouped_to_derived()

bool Query_block::transform_grouped_to_derived ( THD *  thd, bool *  break_off  )

private

Minion of transform_scalar_subqueries_to_join_with_derived.

Moves implicit grouping down into a derived table to prepare for transform_scalar_subqueries_to_join_with_derived.

Example:

  SELECT ( SELECT COUNT(*)
           FROM t1 ) AS tot,
         IFNULL(MAX(t2.b), 0) + 6 AS mx  # MAX(t2.b), FROM t2 and
  FROM t2                                # WHERE go to derived_1_3
  WHERE ANY_VALUE(t2.b) = 10;

is transformed to

  SELECT ( SELECT COUNT(*)
           FROM t1 ) AS tot,
         IFNULL(derived_1_3.`MAX(t2.b)`, 0) + 6 AS mx
  FROM ( SELECT MAX(t2.b) AS `MAX(t2.b)`
         FROM t2
         WHERE (ANY_VALUE(t2.b) = 10)) derived_1_3

Create a new query expression object and query block object to represent the contents of a derived table ("new_derived" in the code below, "derived_1_3" in the example above), with a select list which only contains the aggregate functions lifted out of the transformed query block ("MAX(t2.b) AS `MAX(t2.b)`" above) and any fields referenced.

The transformed query block retains the original select list except aggregates and fields are replaced by fields ("derived_1_3.`MAX(t2.b)`") from the new subquery, but it loses its FROM list, replaced by the new derived table ("derived_1_3" above) and its WHERE and HAVING clauses which all go to the derived table's query block.

Any DISTINCT, WINDOW clauses and LIMITs stay in place at the transformed query block.

Parameters

	thd	session context
[out]	break_off	set to true of transformation could not be performed

Returns

true on error

transform_scalar_subqueries_to_join_with_derived()

bool Query_block::transform_scalar_subqueries_to_join_with_derived ( THD *  thd )

private

Transform eligible scalar subqueries in the SELECT list, WHERE condition, HAVING condition or JOIN conditions of a query block[*] to an equivalent derived table of a LEFT OUTER join, e.g.

as shown in this uncorrelated subquery:

[*] a.k.a "transformed query block" throughout this method and its minions.

  SELECT * FROM t1
    WHERE t1.a > (SELECT COUNT(a) AS cnt FROM t2);  ->

  SELECT t1.* FROM t1 LEFT OUTER JOIN
                   (SELECT COUNT(a) AS cnt FROM t2) AS derived
    ON TRUE WHERE t1.a > derived.cnt;

Grouping in the transformed query block may necessitate the grouping to be moved down to another derived table, cf. transform_grouped_to_derived.

Limitations:

• only implicitly grouped subqueries (guaranteed to have cardinality one) are identified as scalar subqueries. _ Correlated subqueries are not handled

Parameters

[in,out]

thd

the session context

Returns

true on error

transform_subquery_to_derived()

bool Query_block::transform_subquery_to_derived ( THD *  thd, Table_ref **  out_tl, Query_expression *  subs_query_expression, Item_subselect *  subq, bool  use_inner_join, bool  reject_multiple_rows, Item *  join_condition, Item *  lifted_where_cond  )

private

Converts a subquery to a derived table and inserts it into the FROM clause of the owning query block.

Parameters

	thd	Connection handle
[out]	out_tl	The created derived table will be stored in this.
	subs_query_expression	Unit for the subquery
	subq	Item for the subquery
	use_inner_join	Insert with INNER JOIN, or with LEFT JOIN
	reject_multiple_rows	For scalar subqueries where we need run-time cardinality check: true, else false
	join_condition	See join_cond in synthesize_derived()
	lifted_where_cond	The subquery's where condition, moving to JOIN cond of JOIN with the derived table

transform_table_subquery_to_join_with_derived()

bool Query_block::transform_table_subquery_to_join_with_derived ( THD *  thd, Item_exists_subselect *  subq  )

private

Replace a table subquery ([NOT] {IN, EXISTS}) with a join to a derived table.

The principle of this transformation is: FROM [tables] WHERE ... AND/OR oe IN (SELECT ie FROM it) ... becomes FROM (tables) LEFT JOIN (SELECT DISTINCT ie FROM it) AS derived ON oe = derived.ie WHERE ... AND/OR derived.ie IS NOT NULL ... If the subquery predicate is top-level in WHERE, and not negated, we use JOIN instead of LEFT JOIN, and use TRUE instead of IS NOT NULL. If the subquery predicate is negated, we use IS NULL instead of IS NOT NULL. If the subquery predicate is without aggregation(etc), we decorrelate any equality from it, and, if negated, we also decorrelate '<>,<,<=,>,>='; thus we handle EXISTS too. If the subquery cannot be decorrelated, the derived table could be made LATERAL, but as a certain secondary engine doesn't support that we just return an error.

Parameters

thd	Connection handle
subq	Item for subquery

Returns

true if error

update_context_to_derived()

static bool update_context_to_derived ( Item *  expr, Query_block *  new_derived  )

static

A minion of transform_grouped_to_derived.

This updates the name resolution contexts in expr to that of new_derived permanently.

Parameters

expr	the expression to be updated
new_derived	the query block of the new derived table which now holds the expression after it has been moved down.

Returns

true on error

update_used_tables()

void Query_block::update_used_tables

(

)

Update used tables information for all local expressions.

update_used_tables_for_join()

static void update_used_tables_for_join ( mem_root_deque< Table_ref * > *  tables )

static

Update used tables information for a JOIN expression.

validate_gc_assignment()

bool validate_gc_assignment	(	const mem_root_deque< Item * > &	fields,
		const mem_root_deque< Item * > &	values,
		TABLE *	table
	)

validate_gc_assignment Check whether the other values except DEFAULT are assigned for generated columns.

Parameters

fields	Item_fields list to be filled
values	values to fill with
table	table to be checked

Returns

Operation status

Return values

false	OK
true	Error occurred

Note

This function must be called after table->write_set has been filled.

walk_join_conditions()

static bool walk_join_conditions ( mem_root_deque< Table_ref * > &  list, std::function< bool(Item **expr_p)>  action, Item::Collect_scalar_subquery_info *  info  )

static

walk_join_list()

bool walk_join_list	(	mem_root_deque< Table_ref * > &	list,
		std::function< bool(Table_ref *)>	action
	)

WalkAndReplace()

bool WalkAndReplace	(	THD *	thd,
		Item *	item,
		const function< ReplaceResult(Item *item, Item *parent, unsigned argument_idx)> &	get_new_item
	)

WalkAndReplaceInner()

static bool WalkAndReplaceInner ( THD *  thd, Item *  parent, unsigned  argument_idx, const function< ReplaceResult(Item *item, Item *parent, unsigned argument_idx)> &  get_new_item, Item **  child_ref  )

static

Helper function for WalkAndReplace() which replaces the Item referenced by "child_ref" if "get_new_item" returns a replacement, or visits the children of "child_ref" otherwise.

wrap_grouped_expressions_for_rollup()

static ReplaceResult wrap_grouped_expressions_for_rollup ( Query_block *  select, Item *  item, Item *  parent, unsigned  argument_idx  )

static

Checks whether an item matches a grouped expression, creates an Item_rollup_group_item around it and replaces the reference to it with that item.

Variable Documentation

walk_options

const enum_walk walk_options

static

Initial value:

=
    enum_walk::PREFIX | enum_walk::POSTFIX | enum_walk::SUBQUERY