item_subselect.h

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#ifndef ITEM_SUBSELECT_INCLUDED
#define ITEM_SUBSELECT_INCLUDED
 
/* Copyright (c) 2002, 2025, Oracle and/or its affiliates.
 
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/// Implements classes that represent subqueries and predicates containing
/// table subqueries.
 
#include <assert.h>
#include <sys/types.h>
 
#include <cstddef>
#include <memory>  // unique_ptr
#include <vector>
 
#include "field_types.h"  // enum_field_types
#include "my_alloc.h"     // Destroy_only
 
#include "my_inttypes.h"
#include "my_table_map.h"
#include "my_time.h"
#include "mysql/udf_registration_types.h"
#include "mysql_time.h"
#include "sql/comp_creator.h"
#include "sql/enum_query_type.h"
#include "sql/item.h"                    // Item_result_field
#include "sql/iterators/row_iterator.h"  // IWYU pragma: keep
#include "sql/parse_location.h"          // POS
#include "sql/parse_tree_node_base.h"
#include "sql/sql_const.h"
#include "sql/sql_opt_exec_shared.h"
#include "template_utils.h"
 
class Comp_creator;
class Field;
class Item_func_not_all;
class Item_in_optimizer;
class JOIN;
class Json_wrapper;
class PT_subquery;
class Query_result_interceptor;
class Query_result_subquery;
class Query_result_union;
class Query_block;
class Query_expression;
class String;
class THD;
class Temp_table_param;
class my_decimal;
class subselect_indexsubquery_engine;
struct AccessPath;
class Table_ref;
 
template <class T>
class List;
 
/// Base class that is common to all subqueries and subquery predicates
 
class Item_subselect : public Item_result_field {
  typedef Item_result_field super;
 
 public:
  Query_expression *query_expr() const { return m_query_expr; }
 
  /**
    For Item_subselect constructor with POS parameter, the contextualized
    field must be transitioned explicitly.
  */
  void set_contextualized() {
#ifndef NDEBUG
    assert(!contextualized);
    contextualized = true;
#endif  // NDEBUG
  }
  /**
     If !=NO_PLAN_IDX: this Item is in the condition attached to the JOIN_TAB
     having this index in the parent JOIN.
  */
  int in_cond_of_tab{NO_PLAN_IDX};
 
  // For EXPLAIN.
  enum enum_engine_type { OTHER_ENGINE, INDEXSUBQUERY_ENGINE, HASH_SJ_ENGINE };
  enum_engine_type engine_type() const;
 
  // For EXPLAIN. Only valid if engine_type() == HASH_SJ_ENGINE.
  const TABLE *get_table() const;
  const Index_lookup &index_lookup() const;
  join_type get_join_type() const;
 
  void create_iterators(THD *thd);
  virtual AccessPath *root_access_path() const { return nullptr; }
 
  enum Subquery_type {
    SCALAR_SUBQUERY,  ///< Scalar or row subquery
    EXISTS_SUBQUERY,  ///< [NOT] EXISTS subquery predicate
    IN_SUBQUERY,      ///< [NOT] IN subquery predicate
    ALL_SUBQUERY,     ///< comp-op ALL quantified comparison predicate
    ANY_SUBQUERY      ///< comp-op ANY quantified comparison predicate
  };
 
  /// Accumulate used tables
  void accumulate_used_tables(table_map add_tables) {
    m_used_tables_cache |= add_tables;
    m_subquery_used_tables |= add_tables;
  }
 
  /// Return whether this subquery references any tables in the directly
  /// containing query block, i.e. whether there are outer references to the
  /// containing block inside the subquery.
  bool contains_outer_references() const {
    return (subquery_used_tables() & ~PSEUDO_TABLE_BITS) != 0;
  }
 
  virtual Subquery_type subquery_type() const = 0;
 
  /**
    @returns whether this subquery is a single column scalar subquery.
             (Note that scalar and row subqueries are both represented as
              scalar subqueries, this function can be used to distinguish them)
  */
  virtual bool is_single_column_scalar_subquery() const { return false; }
 
  void cleanup() override;
  /**
    Reset state after a single execution of a subquery, useful when a
    dependent subquery must be evaluated multiple times for varying values
    of the outer references.
    This is a lighter cleanup procedure than the one provided by cleanup().
  */
  virtual void reset() {
    m_value_assigned = false;
    null_value = true;
  }
  bool is_value_assigned() const { return m_value_assigned; }
  void set_value_assigned() { m_value_assigned = true; }
  void reset_value_assigned() { m_value_assigned = false; }
  enum Type type() const override;
  bool is_null() override { return update_null_value() || null_value; }
  bool fix_fields(THD *thd, Item **ref) override;
  void fix_after_pullout(Query_block *parent_query_block,
                         Query_block *removed_query_block) override;
  virtual bool exec(THD *thd);
  table_map used_tables() const override { return m_used_tables_cache; }
  table_map not_null_tables() const override { return 0; }
  /// @returns used tables for subquery (excluding any left-hand expression)
  table_map subquery_used_tables() const { return m_subquery_used_tables; }
  Item *get_tmp_table_item(THD *thd) override;
  void update_used_tables() override;
  void print(const THD *thd, String *str,
             enum_query_type query_type) const override;
 
  void set_indexsubquery_engine(subselect_indexsubquery_engine *eng) {
    indexsubquery_engine = eng;
  }
 
  /*
    True if this subquery has been already evaluated. Implemented only for
    single select and union subqueries only.
  */
  bool is_evaluated() const;
  bool is_uncacheable() const;
 
  /**
    Used by max/min subquery to initialize value presence registration
    mechanism. Engine calls this function before re-execution of subquery.
  */
  virtual void reset_has_values() {}
  /**
    @returns the "place" where this subquery is located in the simply
             containing query block.
  */
  enum_parsing_context place() { return m_parsing_place; }
 
  bool walk(Item_processor processor, enum_walk walk, uchar *arg) override;
  bool explain_subquery_checker(uchar **arg) override;
  bool inform_item_in_cond_of_tab(uchar *arg) override;
  bool clean_up_after_removal(uchar *arg) override;
 
  bool check_function_as_value_generator(uchar *args) override {
    Check_function_as_value_generator_parameters *func_arg =
        pointer_cast<Check_function_as_value_generator_parameters *>(args);
    func_arg->err_code = func_arg->get_unnamed_function_error_code();
    return true;
  }
 
  /// argument used by walk method collect_scalar_subqueries ("css")
  struct Collect_subq_info {
    ///< accumulated all subq (or aggregates) found
    std::vector<Item_subselect *> list;
    Query_block *m_query_block{nullptr};
    Collect_subq_info(Query_block *owner) : m_query_block(owner) {}
    bool contains(Query_expression *candidate) {
      for (auto sq : list) {
        if (sq->m_query_expr == candidate) return true;
      }
      return false;
    }
  };
 
  bool collect_subqueries(uchar *) override;
  Item *replace_item_field(uchar *arg) override;
  Item *replace_item_view_ref(uchar *arg) override;
 
 protected:
  Item_subselect() : Item_result_field() { set_subquery(); }
  explicit Item_subselect(const POS &pos) : super(pos) { set_subquery(); }
  /**
    Bind this subquery object with the supplied query expression.
    As part of transformations, this function may re-bind the subquery to
    a different query expression.
 
    @param qe  supplied query expression
  */
  void bind(Query_expression *qe);
 
  /// The query expression of the subquery
  Query_expression *m_query_expr{nullptr};  // Actual value set in construction
 
  /// The query result object associated with the query expression
  Query_result_interceptor *m_query_result{nullptr};
 
  /// Only relevant for Item_in_subselect; optimized structure used for
  /// execution in place of running the entire subquery.
  subselect_indexsubquery_engine *indexsubquery_engine{nullptr};
 
  /// cache of used tables
  table_map m_used_tables_cache{0};
  /// cache of used tables from subquery only (not including LHS of IN subquery)
  table_map m_subquery_used_tables{0};
 
  /// allowed number of columns (1 for scalar subqueries)
  uint m_max_columns{0};  // Irrelevant value, actually set during construction
  /// where subquery is placed
  enum_parsing_context m_parsing_place{CTX_NONE};
 
 private:
  bool subq_opt_away_processor(uchar *arg) override;
 
  /// Accumulate properties from underlying query expression
  void accumulate_properties();
  /// Accumulate properties from underlying query block
  void accumulate_properties(Query_block *select);
  /// Accumulate properties from a selected expression within a query block.
  void accumulate_expression(Item *item);
  /// Accumulate properties from a condition or GROUP/ORDER within a query
  /// block.
  void accumulate_condition(Item *item);
 
  /// True if value has been assigned to subquery
  bool m_value_assigned{false};
  /**
    Whether or not execution of this subquery has been traced by
    optimizer tracing already. If optimizer trace option
    REPEATED_SUBSELECT is disabled, this is used to disable tracing
    after the first one.
  */
  bool m_traced_before{false};
};
 
/// Class that represents scalar subquery and row subquery
 
class Item_singlerow_subselect : public Item_subselect {
 public:
  explicit Item_singlerow_subselect(Query_block *query_block);
  Item_singlerow_subselect(const POS &pos, Query_block *query_block);
  Item_singlerow_subselect() : Item_subselect() {}
 
  bool fix_fields(THD *thd, Item **ref) override;
  void cleanup() override;
  Subquery_type subquery_type() const override { return SCALAR_SUBQUERY; }
  bool create_row(const mem_root_deque<Item *> &item_list, Item_cache **row,
                  bool possibly_empty);
 
  bool is_single_column_scalar_subquery() const override;
 
  void reset() override;
  void store(uint i, Item *item);
  double val_real() override;
  longlong val_int() override;
  String *val_str(String *) override;
  my_decimal *val_decimal(my_decimal *) override;
  bool val_json(Json_wrapper *result) override;
  bool val_date(Date_val *date, my_time_flags_t flags) override;
  bool val_time(Time_val *time) override;
  bool val_datetime(Datetime_val *dt, my_time_flags_t flags) override;
  bool val_bool() override;
  enum Item_result result_type() const override;
  bool resolve_type(THD *) override;
 
  /*
    Mark the subquery as having no rows.
    If there are aggregate functions (in the outer query),
    we need to generate a NULL row. @c return_zero_rows().
  */
  void no_rows_in_result() override;
 
  uint cols() const override;
 
  /**
    @note that this returns the i-th element of the SELECT list.
    To check for nullability, look at this->is_nullable() and not
    element_index[i]->is_nullable(), since the selected expressions are
    always NULL if the subquery is empty.
  */
  Item *element_index(uint i) override { return m_row[i]; }
  Item **addr(uint i) override { return pointer_cast<Item **>(m_row) + i; }
  bool check_cols(uint c) override;
  bool null_inside() override;
  void bring_value() override;
 
  bool collect_scalar_subqueries(uchar *) override;
  virtual bool is_maxmin() const { return false; }
 
  /**
    Argument for walk method replace_scalar_subquery
  */
  struct Scalar_subquery_replacement {
    ///< subquery to be replaced with ␓field from derived table
    Item_singlerow_subselect *m_target;
    ///< The derived table of the transform
    TABLE *m_derived;
    ///< the replacement field
    Field *m_field;
    ///< The transformed query block.
    Query_block *m_outer_query_block;
    ///< The immediately surrounding query block. This will be the transformed
    ///< block or a subquery of it
    Query_block *m_inner_query_block;
    ///< True if subquery's selected item contains a COUNT aggregate
    bool m_add_coalesce{false};
    ///< Presence of HAVING clause in subquery: Only relevant if
    ///< \c m_add_coalesce is true
    bool m_add_having_compensation{false};
    ///< Index of field holding value of having clause in derived table's list
    ///< of fields. Only relevant if \c m_add_coalesce is true
    uint m_having_idx{0};
 
    Scalar_subquery_replacement(Item_singlerow_subselect *target,
                                TABLE *derived, Field *field,
                                Query_block *select, bool add_coalesce,
                                bool add_having_compensation, uint having_idx)
        : m_target(target),
          m_derived(derived),
          m_field(field),
          m_outer_query_block(select),
          m_inner_query_block(select),
          m_add_coalesce(add_coalesce),
          m_add_having_compensation(add_having_compensation),
          m_having_idx(having_idx) {}
  };
 
  Item *replace_scalar_subquery(uchar *arge) override;
  /**
    This method is used to implement a special case of semantic tree
    rewriting, mandated by a SQL:2003 exception in the specification.
    The only caller of this method is handle_sql2003_note184_exception(),
    see the code there for more details.
    Note that this method breaks the object internal integrity, by
    removing it's association with the corresponding Query_block,
    making this object orphan from the parse tree.
    No other method, beside the destructor, should be called on this
    object, as it is now invalid.
    @return the Query_block structure that was given in the constructor.
  */
  Query_block *invalidate_and_restore_query_block();
  std::optional<ContainedSubquery> get_contained_subquery(
      const Query_block *outer_query_block) override;
  friend class Query_result_scalar_subquery;
 
 private:
  /// Value cache of a scalar subquery
  Item_cache *m_value{nullptr};
  /**
    Value cache for a row or scalar subquery. In case of a scalar subquery,
    m_row points directly at m_value. In case of a row subquery, m_row
    is an array of pointers to individual Item_cache objects.
  */
  Item_cache **m_row{nullptr};
  bool m_no_rows{false};  ///< @c no_rows_in_result
};
 
/* used in static ALL/ANY optimization */
class Item_maxmin_subselect final : public Item_singlerow_subselect {
 public:
  /**
    Create an Item for use in transformation of quantified comparison
    predicates that can be evaluated using MAX or MIN aggregation.
 
    @param parent      The quantified comparison predicate that is transformed.
    @param query_block The query block representing the inner part of the subq.
    @param max_arg     Whether the aggregation is for MAX or MIN.
  */
  Item_maxmin_subselect(Item_subselect *parent, Query_block *query_block,
                        bool max_arg);
  void print(const THD *thd, String *str,
             enum_query_type query_type) const override;
  void cleanup() override;
  bool has_values() const { return m_has_values; }
  void register_value() { m_has_values = true; }
  void reset_has_values() override { m_has_values = false; }
  bool is_maxmin() const override { return true; }
 
 private:
  bool m_max;                ///< True if performing MAX, false if MIN
  bool m_has_values{false};  ///< Set if we have found at least one row
};
 
/// Classes that represent predicates over table subqueries:
/// [NOT] EXISTS, [NOT] IN, ANY/SOME and ALL
 
/**
  Strategy which will be used to handle this subquery: flattening to a
  semi-join, conversion to a derived table, rewrite of IN to EXISTS...
  Sometimes the strategy is first only a candidate, then the real decision
  happens in a second phase. Other times the first decision is final.
 */
enum class Subquery_strategy : int {
  /// Nothing decided yet
  UNSPECIFIED,
  /// Candidate for rewriting IN(subquery) to EXISTS, or subquery
  /// materialization
  CANDIDATE_FOR_IN2EXISTS_OR_MAT,
  /// Candidate for semi-join flattening
  CANDIDATE_FOR_SEMIJOIN,
  /// Candidate for rewriting to joined derived table
  CANDIDATE_FOR_DERIVED_TABLE,
  /// Semi-join flattening
  SEMIJOIN,
  /// Rewrite to joined derived table
  DERIVED_TABLE,
  /// Evaluate as EXISTS subquery (possibly after rewriting from another type)
  SUBQ_EXISTS,
  /// Subquery materialization (HASH_SJ_ENGINE)
  SUBQ_MATERIALIZATION,
  /// Subquery has been deleted, probably because it was always false
  DELETED,
};
 
class Item_exists_subselect : public Item_subselect {
  typedef Item_subselect super;
 
 public:
  /**
    Create an Item that represents an EXISTS subquery predicate, or any
    quantified comparison predicate that uses the same base class.
 
    @param pos         String representing the parsed item
    @param query_block First query block of query expression representing
                       the contained subquery.
  */
  Item_exists_subselect(const POS &pos, Query_block *query_block);
 
  explicit Item_exists_subselect(const POS &pos) : super(pos) {}
 
  // The left-hand expression of the subquery predicate. Unused with EXISTS
  Item *left_expr{nullptr};
 
  /// Priority of this predicate in the convert-to-semi-join-nest process.
  int sj_convert_priority{0};
  /// Execution strategy chosen for this Item
  Subquery_strategy strategy{Subquery_strategy::UNSPECIFIED};
  /// Used by the transformation to derived table
  enum_condition_context outer_condition_context{enum_condition_context::ANDS};
 
  /**
    Used by subquery optimizations to keep track about where this subquery
    predicate is located, and whether it is a candidate for transformation.
      (Table_ref*) 1 - the predicate is an AND-part of the WHERE
      join nest pointer    - the predicate is an AND-part of ON expression
                             of a join nest
      NULL                 - for all other locations. It also means that the
                             predicate is not a candidate for transformation.
    See also THD::emb_on_expr_nest.
 
    As for the second case above (the join nest pointer), note that this value
    may change if scalar subqueries are transformed to derived tables,
    cf. transform_scalar_subqueries_to_join_with_derived, due to the need to
    build new join nests. The change is performed in Query_block::nest_derived.
  */
  Table_ref *embedding_join_nest{nullptr};
 
  void notify_removal() override { strategy = Subquery_strategy::DELETED; }
 
  /*
    Transform subquery predicate to a form that can be executed, e.g.
    as an EXISTS subquery, or a MAX or MIN aggregation on the subquery.
 
    This is a virtual function that has implementations for EXISTS, IN
    and quantified comparison subquery predicates.
 
    @param thd              Thread handle
    @param[out] transformed Points to transformed representation of the object
                            if unchanged: No transformation was performed
 
    @returns false if success, true if error
  */
  virtual bool transformer(THD *thd, Item **transformed);
 
  Subquery_type subquery_type() const override { return EXISTS_SUBQUERY; }
  bool is_bool_func() const override { return true; }
  void reset() override {
    reset_value_assigned();
    m_value = false;
  }
 
  enum Item_result result_type() const override { return INT_RESULT; }
  /*
    The item is
    ([NOT] IN/EXISTS) [ IS [NOT] TRUE|FALSE ]
  */
  enum Bool_test value_transform = BOOL_IDENTITY;
  bool with_is_op() const {
    switch (value_transform) {
      case BOOL_IS_TRUE:
      case BOOL_IS_FALSE:
      case BOOL_NOT_TRUE:
      case BOOL_NOT_FALSE:
        return true;
      default:
        return false;
    }
  }
  /// True if the IS TRUE/FALSE wasn't explicit in the query
  bool implicit_is_op = false;
  Item *truth_transformer(THD *, enum Bool_test test) override;
  /**
    Convert result according to value_transform, null value and supplied value
 
    @param v boolean value from the primitive subquery predicate
 
    @returns translated boolean value
 
    @note Operation will possibly take into account and possibly change null
          value of the Item.
  */
  virtual bool return_value(bool v);
  void apply_is_true() override {
    const bool had_is = with_is_op();
    truth_transformer(nullptr, BOOL_IS_TRUE);
    if (!had_is && value_transform == BOOL_IS_TRUE)
      implicit_is_op = true;  // needn't be written by EXPLAIN
  }
  bool allow_table_subquery_transform() const;
  bool use_anti_join_transform() const;
  bool fix_fields(THD *thd, Item **ref) override;
  longlong val_int() override;
  double val_real() override;
  String *val_str(String *) override;
  my_decimal *val_decimal(my_decimal *) override;
  bool val_bool() override;
  bool val_date(Date_val *date, my_time_flags_t flags) override {
    return get_date_from_int(date, flags);
  }
  bool val_time(Time_val *time) override { return get_time_from_int(time); }
  bool val_datetime(Datetime_val *dt, my_time_flags_t flags) override {
    return get_datetime_from_int(dt, flags);
  }
  bool resolve_type(THD *thd) override;
  void print(const THD *thd, String *str,
             enum_query_type query_type) const override;
 
  friend class Query_result_exists_subquery;
 
 protected:
  bool is_semijoin_candidate(THD *thd) const;
  bool is_derived_candidate(THD *thd) const;
 
  /// value of this item (boolean: exists/not-exists)
  bool m_value{false};
};
 
/**
  Representation of IN subquery predicates of the form
  "left_expr IN (SELECT ...)".
 
  @details
  This class has:
   - A "subquery execution engine" (as a subclass of Item_subselect) that allows
     it to evaluate subqueries. (and this class participates in execution by
     having m_was_null variable where part of execution result is stored.
   - Transformation methods (todo: more on this).
 
  This class is not used directly, it is "wrapped" into Item_in_optimizer
  which provides some small bits of subquery evaluation.
*/
 
class Item_in_subselect : public Item_exists_subselect {
  typedef Item_exists_subselect super;
 
 public:
  Item_in_subselect(const POS &pos, Item *left_expr, Query_block *query_block);
  Item_in_subselect(const POS &pos, Item *left_expr,
                    PT_subquery *pt_subquery_arg);
 
  Item_in_subselect(const POS &pos)
      : Item_exists_subselect(pos), pt_subselect(nullptr) {}
 
  bool do_itemize(Parse_context *pc, Item **res) override;
 
  void cleanup() override;
  Subquery_type subquery_type() const override { return IN_SUBQUERY; }
 
  void reset() override {
    m_value = false;
    null_value = false;
    m_was_null = false;
  }
  // Whether to ignore UNKNOWN result (only return TRUE or FALSE)
  bool ignore_unknown() const {
    return value_transform != BOOL_IDENTITY && value_transform != BOOL_NEGATED;
  }
  /*
    @returns true if implementation has to process NULL values.
             Always true if ignore_unknown() is false.
             Also true if quantified comparison predicate is ALL.
             Virtual since IN and ALL/ANY need different checks.
  */
  virtual bool process_nulls() const { return value_transform != BOOL_IS_TRUE; }
  bool return_value(bool v) override;
 
  bool transformer(THD *thd, Item **transformed) override;
  bool quantified_comp_transformer(THD *thd, Comp_creator *func,
                                   Item **transformed);
  bool single_value_transformer(THD *thd, Comp_creator *func,
                                Item **transformed);
  bool row_value_transformer(THD *thd, Item **transformed);
  bool single_value_in_to_exists_transformer(THD *thd, Query_block *select,
                                             Comp_creator *func);
  bool row_value_in_to_exists_transformer(THD *thd, Query_block *select,
                                          Item_in_optimizer *optimizer);
  bool subquery_allows_materialization(THD *thd, Query_block *query_block,
                                       const Query_block *outer);
  bool walk(Item_processor processor, enum_walk walk, uchar *arg) override;
  Item *transform(Item_transformer transformer, uchar *arg) override;
  Item *compile(Item_analyzer analyzer, uchar **arg_p,
                Item_transformer transformer, uchar *arg_t) override;
 
  bool exec(THD *thd) override;
  longlong val_int() override;
  double val_real() override;
  String *val_str(String *) override;
  my_decimal *val_decimal(my_decimal *) override;
  bool val_bool() override;
  bool test_limit();
  void print(const THD *thd, String *str,
             enum_query_type query_type) const override;
  void fix_after_pullout(Query_block *parent_query_block,
                         Query_block *removed_query_block) override;
  void update_used_tables() override;
  bool init_left_expr_cache(THD *thd);
 
  /**
     Once the decision to use IN->EXISTS has been taken, performs some last
     steps of this transformation.
  */
  bool finalize_exists_transform(THD *thd, Query_block *select);
  /**
     Once the decision to use materialization has been taken, performs some
     last steps of this transformation.
  */
  bool finalize_materialization_transform(THD *thd, JOIN *join);
  AccessPath *root_access_path() const override;
  std::optional<ContainedSubquery> get_contained_subquery(
      const Query_block *outer_query_block) override;
 
  bool in2exists_added_to_where() const {
    return m_in2exists_info != nullptr && m_in2exists_info->added_to_where;
  }
 
  /// Is reliable only if IN->EXISTS has been done.
  bool dependent_before_in2exists() const {
    return m_in2exists_info->dependent_before;
  }
 
  bool *get_cond_guard(int i) const {
    return m_pushed_cond_guards != nullptr ? m_pushed_cond_guards + i : nullptr;
  }
  void set_cond_guard_var(int i, bool v) const {
    if (m_pushed_cond_guards != nullptr) m_pushed_cond_guards[i] = v;
  }
 
  friend class Item_ref_null_helper;
  friend class Item_is_not_null_test;
  friend class Item_in_optimizer;
  friend class subselect_indexsubquery_engine;
  friend class subselect_hash_sj_engine;
 
  /// Used to trigger on/off conditions that were pushed down to subquery
  bool *m_pushed_cond_guards{nullptr};
 
  /// Point on NOT/NOP before ALL/SOME subquery
  Item_func_not_all *m_upper_item{nullptr};
 
 protected:
  /**
    Cache of the left operand of the subquery predicate. Allocated in the
    runtime memory root, for each execution, thus need not be freed.
  */
  List<Cached_item> *m_left_expr_cache{nullptr};
 
  /// Whether m_left_expr_cache holds a value
  bool m_left_expr_cache_filled{false};
 
  /// The need for expr cache may be optimized away, @sa init_left_expr_cache.
  bool need_expr_cache{true};
 
 private:
  bool mark_as_outer(Item *left_row, size_t col);
 
  /**
    In the case of
 
       x COMP_OP (SELECT1 UNION SELECT2 ...)
 
    - the subquery transformation is done on SELECT1; this requires wrapping
      'x' with more Item layers, and injecting that in a condition in SELECT1.
 
    - the same transformation is done on SELECT2; but the wrapped 'x' doesn't
      need to be created again, the one created for SELECT1 could be reused
 
    - to achieve this, the wrapped 'x' is stored in member
      'm_injected_left_expr' when it is created for SELECT1, and is later
      reused for SELECT2.
 
    This will refer to a cached value which is reevaluated once for each
    candidate row, cf. setup in #single_value_transformer.
  */
  Item_ref *m_injected_left_expr{nullptr};
 
  bool m_was_null{false};
 
  /**
     This bundles several pieces of information useful when doing the
     IN->EXISTS transform. If this transform has not been done, pointer is
     NULL.
  */
  struct In2exists_info {
    /**
       True: if IN->EXISTS has been done and has added a condition to the
       subquery's WHERE clause.
    */
    bool added_to_where;
    /**
       True: if subquery was dependent (correlated) before IN->EXISTS
       was done.
    */
    bool dependent_before;
    /**
       True: if subquery was dependent (correlated) after IN->EXISTS
       was done.
    */
    bool dependent_after;
  } *m_in2exists_info{nullptr};
 
  PT_subquery *pt_subselect;
 
  bool val_bool_naked();
};
 
/**
  Class that represents a quantified comparison predicate.
 
  expression comp-op ANY ( subquery )
  expression comp-op ALL ( subquery )
 
  Note that the special cases = ANY (aka IN) and <> ALL (aka NOT IN)
  are handled by the base class Item_in_subselect.
*/
class Item_allany_subselect final : public Item_in_subselect {
 public:
  Item_allany_subselect(const POS &pos, Item *left_expr,
                        chooser_compare_func_creator fc, Query_block *select,
                        bool all);
 
  Subquery_type subquery_type() const override {
    return m_all_subquery ? ALL_SUBQUERY : ANY_SUBQUERY;
  }
  Comp_creator *compare_func() const { return m_compare_func; }
 
  Item *truth_transformer(THD *, enum Bool_test test) override;
  void apply_is_true() override {
    implicit_is_op = true;
    if (value_transform == BOOL_IDENTITY) {
      value_transform = BOOL_IS_TRUE;
    } else if (value_transform == BOOL_NEGATED) {
      value_transform = BOOL_IS_FALSE;
    }
  }
  bool process_nulls() const override {
    return !ignore_unknown() || m_all_subquery;
  }
  bool eqne_op() const;
  bool return_value(bool v) override;
 
  bool transformer(THD *thd, Item **transformed) override;
  void print(const THD *thd, String *str,
             enum_query_type query_type) const override;
 
 private:
  /// The original source for the comparison function
  chooser_compare_func_creator m_func_creator;
  /// The comparison function generator, possibly inverted by NOT
  Comp_creator *m_compare_func;
  /// Whether ALL or ANY subquery
  bool m_all_subquery;
  /// Used to generate the correct comparison function
  bool m_inverted{false};
};
 
/**
  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.
*/
class subselect_indexsubquery_engine {
 protected:
  Query_result_union *result = nullptr; /* results storage class */
  /// Table which is read, using one of eq_ref, ref, ref_or_null.
  TABLE *table{nullptr};
  Table_ref *table_ref{nullptr};
  Index_lookup ref;
  join_type type{JT_UNKNOWN};
  Item *m_cond;      ///< The WHERE condition of  the subquery
  ulonglong m_hash;  ///< Hash value calculated by RefIterator, when needed.
  /*
    The "having" clause. This clause (further referred to as "artificial
    having") was inserted by subquery transformation code. It contains
    Item(s) that have a side-effect: they record whether the subquery has
    produced a row with NULL certain components. We need to use it for cases
    like
      (oe1, oe2) IN (SELECT t.key, t.no_key FROM t1)
    where we do index lookup on t.key=oe1 but need also to check if there
    was a row such that t.no_key IS NULL.
  */
  Item *m_having;
 
  Item_in_subselect *item; /* item that uses this engine */
 
 public:
  enum enum_engine_type { INDEXSUBQUERY_ENGINE, HASH_SJ_ENGINE };
 
  subselect_indexsubquery_engine(TABLE *table, Table_ref *table_ref,
                                 const Index_lookup &ref,
                                 enum join_type join_type,
                                 Item_in_subselect *subs, Item *where,
                                 Item *having)
      : table(table),
        table_ref(table_ref),
        ref(ref),
        type(join_type),
        m_cond(where),
        m_having(having),
        item(subs) {}
  virtual ~subselect_indexsubquery_engine() = default;
  virtual bool exec(THD *thd);
  virtual void print(const THD *thd, String *str, enum_query_type query_type);
  virtual enum_engine_type engine_type() const { return INDEXSUBQUERY_ENGINE; }
  virtual void cleanup() {}
  virtual void create_iterators(THD *) {}
};
 
/*
  This function is actually defined in sql_parse.cc, but it depends on
  chooser_compare_func_creator defined in this file.
 */
Item *all_any_subquery_creator(THD *thd, const POS &pos, Item *left_expr,
                               chooser_compare_func_creator cmp, bool all,
                               Query_block *select);
 
/**
  Compute an IN predicate via a hash semi-join. The subquery is materialized
  during the first evaluation of the IN predicate. The IN predicate is executed
  via the functionality inherited from subselect_indexsubquery_engine.
*/
 
class subselect_hash_sj_engine final : public subselect_indexsubquery_engine {
 private:
  /* true if the subquery was materialized into a temp table. */
  bool is_materialized;
  // true if we know for sure that there are zero rows in the table.
  // Set only after is_materialized is true.
  bool has_zero_rows = false;
  /**
     Existence of inner NULLs in materialized table:
     By design, other values than IRRELEVANT_OR_FALSE are possible only if the
     subquery has only one inner expression.
  */
  enum nulls_exist {
    /// none, or they don't matter
    NEX_IRRELEVANT_OR_FALSE = 0,
    /// they matter, and we don't know yet if they exists
    NEX_UNKNOWN = 1,
    /// they matter, and we know there exists at least one.
    NEX_TRUE = 2
  };
  enum nulls_exist mat_table_has_nulls;
  Query_expression *const m_query_expr;
  unique_ptr_destroy_only<RowIterator> m_iterator;
  AccessPath *m_root_access_path;
 
  /// Saved result object, must be restored after use
  Query_result_interceptor *saved_result{nullptr};
 
 public:
  subselect_hash_sj_engine(Item_in_subselect *in_predicate,
                           Query_expression *query_expr)
      : subselect_indexsubquery_engine(nullptr, nullptr, {}, JT_UNKNOWN,
                                       in_predicate, nullptr, nullptr),
        is_materialized(false),
        m_query_expr(query_expr) {}
  ~subselect_hash_sj_engine() override;
 
  bool setup(THD *thd, const mem_root_deque<Item *> &tmp_columns);
  void cleanup() override;
  bool exec(THD *thd) override;
  void print(const THD *thd, String *str, enum_query_type query_type) override;
  enum_engine_type engine_type() const override { return HASH_SJ_ENGINE; }
 
  TABLE *get_table() const { return table; }
  const Index_lookup &index_lookup() const { return ref; }
  enum join_type get_join_type() const { return type; }
  AccessPath *root_access_path() const { return m_root_access_path; }
  void create_iterators(THD *thd) override;
};
 
/**
  Removes every predicate injected by IN->EXISTS.
 
  This function is different from others:
  - it wants to remove all traces of IN->EXISTS (for
  materialization)
  - remove_subq_pushed_predicates() and remove_additional_cond() want to
  remove only the conditions of IN->EXISTS which index lookup already
  satisfies (they are just an optimization).
 
  If there are no in2exists conditions, it will return the exact same
  pointer. If it returns a new Item, the old Item is left alone, so it
  can be reused in other settings.
 
  @param conds  Condition; may be nullptr.
  @returns      new condition
 */
Item *remove_in2exists_conds(Item *conds);
 
/// @returns whether the item is a quantified comparison predicate
bool is_quantified_comp_predicate(Item *item);
 
#endif /* ITEM_SUBSELECT_INCLUDED */