Adding upstream version 1:10.5.12.upstream/1%10.5.12upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 1545 insertions, 0 deletions

diff --git a/sql/item_subselect.h b/sql/item_subselect.h
new file mode 100644
index 00000000..1f1c3454
--- /dev/null
+++ b/sql/item_subselect.h
@@ -0,0 +1,1545 @@
+#ifndef ITEM_SUBSELECT_INCLUDED
+#define ITEM_SUBSELECT_INCLUDED
+
+/* Copyright (c) 2002, 2011, Oracle and/or its affiliates.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; version 2 of the License.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program; if not, write to the Free Software
+   Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1335  USA */
+
+/* subselect Item */
+
+#ifdef USE_PRAGMA_INTERFACE
+#pragma interface			/* gcc class implementation */
+#endif
+
+#include <queues.h>
+
+class st_select_lex;
+class st_select_lex_unit;
+class JOIN;
+class select_result_interceptor;
+class subselect_engine;
+class subselect_hash_sj_engine;
+class Item_bool_func2;
+class Comp_creator;
+class With_element;
+class Field_pair;
+
+typedef class st_select_lex SELECT_LEX;
+
+/**
+  Convenience typedef used in this file, and further used by any files
+  including this file.
+*/
+typedef Comp_creator* (*chooser_compare_func_creator)(bool invert);
+class Cached_item;
+
+/* base class for subselects */
+
+class Item_subselect :public Item_result_field,
+                      protected Used_tables_and_const_cache,
+                      protected With_sum_func_cache
+{
+  /*
+    Set to TRUE if the value is assigned for the subselect
+    FALSE: subquery not executed or the subquery returns an empty result
+  */
+  bool value_assigned;
+  bool own_engine;  /* the engine was not taken from other Item_subselect */
+protected:
+  /* thread handler, will be assigned in fix_fields only */
+  THD *thd;
+  /* old engine if engine was changed */
+  subselect_engine *old_engine;
+  /* allowed number of columns (1 for single value subqueries) */
+  uint max_columns;
+  /* where subquery is placed */
+  enum_parsing_place parsing_place;
+  /* work with 'substitution' */
+  bool have_to_be_excluded;
+  
+  bool inside_first_fix_fields;
+  bool done_first_fix_fields;
+  Item *expr_cache;
+  /*
+    Set to TRUE if at optimization or execution time we determine that this
+    item's value is a constant. We need this member because it is not possible
+    to substitute 'this' with a constant item.
+  */
+  bool forced_const;
+  /* Set to the result of the last call of is_expensive()  */
+  bool expensive_fl;
+#ifndef DBUG_OFF
+  /* Count the number of times this subquery predicate has been executed. */
+  uint exec_counter;
+#endif
+public:
+  /* 
+    Used inside Item_subselect::fix_fields() according to this scenario:
+      > Item_subselect::fix_fields
+        > engine->prepare
+          > child_join->prepare
+            (Here we realize we need to do the rewrite and set
+             substitution= some new Item, eg. Item_in_optimizer )
+          < child_join->prepare
+        < engine->prepare
+        *ref= substitution;
+        substitution= NULL;
+      < Item_subselect::fix_fields
+  */
+  /* TODO make this protected member again. */
+  Item *substitution;
+  /* engine that perform execution of subselect (single select or union) */
+  /* TODO make this protected member again. */
+  subselect_engine *engine;
+  /* unit of subquery */
+  st_select_lex_unit *unit;
+  /* Cached buffers used when calling filesort in sub queries */
+  Filesort_buffer filesort_buffer;
+  LEX_STRING sortbuffer;
+  /* A reference from inside subquery predicate to somewhere outside of it */
+  class Ref_to_outside : public Sql_alloc
+  {
+  public:
+    st_select_lex *select; /* Select where the reference is pointing to */
+    /* 
+      What is being referred. This may be NULL when we're referring to an
+      aggregate function.
+    */ 
+    Item *item; 
+  };
+  /*
+    References from within this subquery to somewhere outside of it (i.e. to
+    parent select, grandparent select, etc)
+  */
+  List<Ref_to_outside> upper_refs;
+  st_select_lex *parent_select;
+
+  /*
+   TRUE<=>Table Elimination has made it redundant to evaluate this select
+          (and so it is not part of QEP, etc)
+  */
+  bool eliminated;
+  
+  /* subquery is transformed */
+  bool changed;
+
+  /* TRUE <=> The underlying SELECT is correlated w.r.t some ancestor select */
+  bool is_correlated;
+
+  /* 
+    TRUE <=> the subquery contains a recursive reference in the FROM list
+    of one of its selects. In this case some of subquery optimization
+    strategies cannot be applied for the subquery;
+  */
+  bool with_recursive_reference; 
+
+  /* To link Item_subselects containing references to the same recursive CTE */ 
+  Item_subselect *next_with_rec_ref;
+
+  enum subs_type {UNKNOWN_SUBS, SINGLEROW_SUBS,
+		  EXISTS_SUBS, IN_SUBS, ALL_SUBS, ANY_SUBS};
+
+  Item_subselect(THD *thd);
+
+  virtual subs_type substype() { return UNKNOWN_SUBS; }
+  bool is_exists_predicate()
+  {
+    return substype() == Item_subselect::EXISTS_SUBS;
+  }
+  bool is_in_predicate()
+  {
+    return get_IN_subquery() != NULL;
+  }
+
+  /*
+    We need this method, because some compilers do not allow 'this'
+    pointer in constructor initialization list, but we need to pass a pointer
+    to subselect Item class to select_result_interceptor's constructor.
+  */
+  virtual void init (st_select_lex *select_lex,
+		     select_result_interceptor *result);
+
+  ~Item_subselect();
+  void cleanup() override;
+  virtual void reset()
+  {
+    eliminated= FALSE;
+    null_value= 1;
+  }
+  /**
+    Set the subquery result to a default value consistent with the semantics of
+    the result row produced for queries with implicit grouping.
+  */
+  void no_rows_in_result() override= 0;
+  virtual bool select_transformer(JOIN *join);
+  bool assigned() { return value_assigned; }
+  void assigned(bool a) { value_assigned= a; }
+  enum Type type() const override;
+  bool is_null() override
+  {
+    update_null_value();
+    return null_value;
+  }
+  bool fix_fields(THD *thd, Item **ref) override;
+  bool with_subquery() const override { DBUG_ASSERT(fixed); return true; }
+  bool with_sum_func() const override { return m_with_sum_func; }
+  With_sum_func_cache* get_with_sum_func_cache() override { return this; }
+  bool mark_as_dependent(THD *thd, st_select_lex *select, Item *item);
+  void fix_after_pullout(st_select_lex *new_parent, Item **ref,
+                         bool merge) override;
+  void recalc_used_tables(st_select_lex *new_parent, bool after_pullout);
+  virtual bool exec();
+  /*
+    If subquery optimization or execution determines that the subquery has
+    an empty result, mark the subquery predicate as a constant value.
+  */
+  void make_const()
+  { 
+    used_tables_cache= 0;
+    const_item_cache= 0;
+    forced_const= TRUE; 
+  }
+  virtual bool fix_length_and_dec();
+  table_map used_tables() const override;
+  table_map not_null_tables() const override { return 0; }
+  bool const_item() const override;
+  inline table_map get_used_tables_cache() { return used_tables_cache; }
+  Item *get_tmp_table_item(THD *thd) override;
+  void update_used_tables() override;
+  void print(String *str, enum_query_type query_type) override;
+  virtual bool have_guarded_conds() { return FALSE; }
+  bool change_engine(subselect_engine *eng)
+  {
+    old_engine= engine;
+    engine= eng;
+    return eng == 0;
+  }
+  bool engine_changed(subselect_engine *eng) { return 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;
+  bool is_expensive() override;
+
+  /*
+    Used by max/min subquery to initialize value presence registration
+    mechanism. Engine call this method before rexecution query.
+  */
+  virtual void reset_value_registration() {}
+  enum_parsing_place place() { return parsing_place; }
+  bool walk(Item_processor processor, bool walk_subquery, void *arg) override;
+  bool unknown_splocal_processor(void *arg) override;
+  bool mark_as_eliminated_processor(void *arg) override;
+  bool eliminate_subselect_processor(void *arg) override;
+  bool set_fake_select_as_master_processor(void *arg) override;
+  bool enumerate_field_refs_processor(void *arg) override;
+  bool check_vcol_func_processor(void *arg) override
+  {
+    return mark_unsupported_function("select ...", arg, VCOL_IMPOSSIBLE);
+  }
+  /**
+    Callback to test if an IN predicate is expensive.
+
+    @notes
+    The return value affects the behavior of make_cond_for_table().
+
+    @retval TRUE  if the predicate is expensive
+    @retval FALSE otherwise
+  */
+  bool is_expensive_processor(void *arg) override { return is_expensive(); }
+  bool update_table_bitmaps_processor(void *arg) override;
+
+  /**
+    Get the SELECT_LEX structure associated with this Item.
+    @return the SELECT_LEX structure associated with this Item
+  */
+  st_select_lex* get_select_lex();
+  bool expr_cache_is_needed(THD *) override;
+  void get_cache_parameters(List<Item> &parameters) override;
+  bool is_subquery_processor (void *opt_arg) override { return 1; }
+  bool exists2in_processor(void *opt_arg) override { return 0; }
+  bool limit_index_condition_pushdown_processor(void *opt_arg) override
+  {
+    return TRUE;
+  }
+
+  void register_as_with_rec_ref(With_element *with_elem);
+  void init_expr_cache_tracker(THD *thd);
+
+  Item* build_clone(THD *thd) override { return 0; }
+  Item* get_copy(THD *thd) override { return 0; }
+
+  st_select_lex *wrap_tvc_into_select(THD *thd, st_select_lex *tvc_sl);
+
+  friend class select_result_interceptor;
+  friend class Item_in_optimizer;
+  friend bool Item_field::fix_fields(THD *, Item **);
+  friend int  Item_field::fix_outer_field(THD *, Field **, Item **);
+  friend bool Item_ref::fix_fields(THD *, Item **);
+  friend void mark_select_range_as_dependent(THD*,
+                                             st_select_lex*, st_select_lex*,
+                                             Field*, Item*, Item_ident*,
+                                             bool);
+  friend bool convert_join_subqueries_to_semijoins(JOIN *join);
+};
+
+/* single value subselect */
+
+class Item_cache;
+class Item_singlerow_subselect :public Item_subselect
+{
+protected:
+  Item_cache *value, **row;
+public:
+  Item_singlerow_subselect(THD *thd_arg, st_select_lex *select_lex);
+  Item_singlerow_subselect(THD *thd_arg): Item_subselect(thd_arg), value(0), row (0)
+  {}
+
+  void cleanup();
+  subs_type substype() { return SINGLEROW_SUBS; }
+
+  void reset();
+  void no_rows_in_result();
+  bool select_transformer(JOIN *join);
+  void store(uint i, Item* item);
+  double val_real();
+  longlong val_int ();
+  String *val_str (String *);
+  bool val_native(THD *thd, Native *);
+  my_decimal *val_decimal(my_decimal *);
+  bool val_bool();
+  bool get_date(THD *thd, MYSQL_TIME *ltime, date_mode_t fuzzydate);
+  const Type_handler *type_handler() const;
+  bool fix_length_and_dec();
+
+  uint cols() const;
+  Item* element_index(uint i) { return reinterpret_cast<Item*>(row[i]); }
+  Item** addr(uint i) { return (Item**)row + i; }
+  bool check_cols(uint c);
+  bool null_inside();
+  void bring_value();
+
+  /**
+    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 SELECT_LEX,
+    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 SELECT_LEX structure that was given in the constructor.
+  */
+  st_select_lex* invalidate_and_restore_select_lex();
+
+  Item* expr_cache_insert_transformer(THD *thd, uchar *unused);
+
+  friend class select_singlerow_subselect;
+};
+
+/* used in static ALL/ANY optimization */
+class select_max_min_finder_subselect;
+class Item_maxmin_subselect :public Item_singlerow_subselect
+{
+protected:
+  bool max;
+  bool was_values;  // Set if we have found at least one row
+public:
+  Item_maxmin_subselect(THD *thd, Item_subselect *parent,
+			st_select_lex *select_lex, bool max);
+  virtual void print(String *str, enum_query_type query_type);
+  void cleanup();
+  bool any_value() { return was_values; }
+  void register_value() { was_values= TRUE; }
+  void reset_value_registration() { was_values= FALSE; }
+  void no_rows_in_result();
+};
+
+/* exists subselect */
+
+class Item_exists_subselect :public Item_subselect
+{
+protected:
+  Item_func_not *upper_not;
+  bool value; /* value of this item (boolean: exists/not-exists) */
+  bool abort_on_null;
+
+  void init_length_and_dec();
+  bool select_prepare_to_be_in();
+
+public:
+  /*
+    Used by subquery optimizations to keep track about in which clause this
+    subquery predicate is located: 
+      NO_JOIN_NEST      - 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
+  */
+  TABLE_LIST *emb_on_expr_nest;
+  /**
+    Reference on the Item_in_optimizer wrapper of this subquery
+  */
+  Item_in_optimizer *optimizer;
+  /* true if we got this from EXISTS or to IN */
+  bool exists_transformed;
+
+  Item_exists_subselect(THD *thd_arg, st_select_lex *select_lex);
+  Item_exists_subselect(THD *thd_arg):
+    Item_subselect(thd_arg), upper_not(NULL), abort_on_null(0),
+    emb_on_expr_nest(NULL), optimizer(0), exists_transformed(0)
+  {}
+
+  subs_type substype() override { return EXISTS_SUBS; }
+  void reset() override
+  {
+    eliminated= FALSE;
+    value= 0;
+  }
+  void no_rows_in_result() override;
+
+  const Type_handler *type_handler() const override
+  {
+    return &type_handler_bool;
+  }
+  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 get_date(THD *thd, MYSQL_TIME *ltime, date_mode_t fuzzydate) override
+  { return get_date_from_int(thd, ltime, fuzzydate); }
+  bool fix_fields(THD *thd, Item **ref) override;
+  bool fix_length_and_dec() override;
+  void print(String *str, enum_query_type query_type) override;
+  bool select_transformer(JOIN *join) override;
+  void top_level_item() override { abort_on_null=1; }
+  bool is_top_level_item() const override { return abort_on_null; }
+  bool exists2in_processor(void *opt_arg) override;
+
+  Item* expr_cache_insert_transformer(THD *thd, uchar *unused) override;
+
+  void mark_as_condition_AND_part(TABLE_LIST *embedding) override
+  {
+    emb_on_expr_nest= embedding;
+  }
+  void under_not(Item_func_not *upper) override { upper_not= upper; };
+
+  void set_exists_transformed() { exists_transformed= TRUE; }
+
+  friend class select_exists_subselect;
+  friend class subselect_uniquesubquery_engine;
+  friend class subselect_indexsubquery_engine;
+};
+
+
+TABLE_LIST * const NO_JOIN_NEST=(TABLE_LIST*)0x1;
+
+/*
+  Possible methods to execute an IN predicate. These are set by the optimizer
+  based on user-set optimizer switches, semantic analysis and cost comparison.
+*/
+#define SUBS_NOT_TRANSFORMED 0 /* No execution method was chosen for this IN. */
+/* The Final decision about the strategy is made. */
+#define SUBS_STRATEGY_CHOSEN 1
+#define SUBS_SEMI_JOIN 2       /* IN was converted to semi-join. */
+#define SUBS_IN_TO_EXISTS 4    /* IN was converted to correlated EXISTS. */
+#define SUBS_MATERIALIZATION 8 /* Execute IN via subquery materialization. */
+/* Partial matching substrategies of MATERIALIZATION. */
+#define SUBS_PARTIAL_MATCH_ROWID_MERGE 16
+#define SUBS_PARTIAL_MATCH_TABLE_SCAN 32
+/* ALL/ANY will be transformed with max/min optimization */
+/*   The subquery has not aggregates, transform it into a MAX/MIN query. */
+#define SUBS_MAXMIN_INJECTED 64
+/*   The subquery has aggregates, use a special max/min subselect engine. */
+#define SUBS_MAXMIN_ENGINE 128
+
+
+/**
+  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 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
+{
+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> *left_expr_cache;
+  bool first_execution;
+
+  /*
+    expr & optimizer used in subselect rewriting to store Item for
+    all JOIN in UNION
+  */
+  Item *expr;
+  bool was_null;
+  /* A bitmap of possible execution strategies for an IN predicate. */
+  uchar in_strategy;
+protected:
+  /* Used to trigger on/off conditions that were pushed down to subselect */
+  bool *pushed_cond_guards;
+  Comp_creator *func;
+
+protected:
+  bool init_cond_guards();
+  bool select_in_like_transformer(JOIN *join);
+  bool single_value_transformer(JOIN *join);
+  bool row_value_transformer(JOIN * join);
+  bool fix_having(Item *having, st_select_lex *select_lex);
+  bool create_single_in_to_exists_cond(JOIN * join,
+                                       Item **where_item,
+                                       Item **having_item);
+  bool create_row_in_to_exists_cond(JOIN * join,
+                                    Item **where_item,
+                                    Item **having_item);
+  Item *left_expr;
+  /*
+    Important for PS/SP: left_expr_orig is the item that left_expr originally
+    pointed at. That item is allocated on the statement arena, while
+    left_expr could later be changed to something on the execution arena.
+  */
+  Item *left_expr_orig;
+public:
+  /* Priority of this predicate in the convert-to-semi-join-nest process. */
+  int sj_convert_priority;
+  /* May be TRUE only for the candidates to semi-join conversion */
+  bool do_not_convert_to_sj; 
+  /*
+    Types of left_expr and subquery's select list allow to perform subquery
+    materialization. Currently, we set this to FALSE when it as well could
+    be TRUE. This is to be properly addressed with fix for BUG#36752.
+  */
+  bool types_allow_materialization;
+
+  /* 
+    Same as above, but they also allow to scan the materialized table. 
+  */
+  bool sjm_scan_allowed;
+
+  /* 
+    JoinTaB Materialization (JTBM) members
+  */
+  
+  /* 
+    TRUE <=> This subselect has been converted into non-mergeable semi-join
+    table.
+  */
+  bool is_jtbm_merged;
+  
+  /* (Applicable if is_jtbm_merged==TRUE) Time required to run the materialized join */
+  double jtbm_read_time;
+
+  /* (Applicable if is_jtbm_merged==TRUE) Number of output rows in materialized join */
+  double jtbm_record_count;   
+  
+  /*
+    (Applicable if is_jtbm_merged==TRUE) TRUE <=> The materialized subselect is
+    a degenerate subselect which produces 0 or 1 rows, which we know at
+    optimization phase.
+    Examples:
+    1. subquery has "Impossible WHERE": 
+
+      SELECT * FROM ot WHERE ot.column IN (SELECT it.col FROM it WHERE 2 > 3)
+    
+    2. Subquery produces one row which opt_sum.cc is able to get with one lookup:
+
+      SELECT * FROM ot WHERE ot.column IN (SELECT MAX(it.key_col) FROM it)
+  */
+  bool is_jtbm_const_tab;
+  
+  /* 
+    (Applicable if is_jtbm_const_tab==TRUE) Whether the subquery has produced 
+     the row (or not)
+  */
+  bool jtbm_const_row_found;
+  
+  /*
+    TRUE<=>this is a flattenable semi-join, false otherwise.
+  */
+  bool is_flattenable_semijoin;
+
+  /*
+    TRUE<=>registered in the list of semijoins in outer select
+  */
+  bool is_registered_semijoin;
+  
+  List<Field_pair> corresponding_fields;
+
+  /*
+    Used to determine how this subselect item is represented in the item tree,
+    in case there is a need to locate it there and replace with something else.
+    Two options are possible:
+      1. This item is there 'as-is'.
+      1. This item is wrapped within Item_in_optimizer.
+  */
+  Item *original_item()
+  {
+    return (is_flattenable_semijoin && !exists_transformed ?
+            (Item*)this :
+            (Item*)optimizer);
+  }
+  
+  bool *get_cond_guard(int i)
+  {
+    return pushed_cond_guards ? pushed_cond_guards + i : NULL;
+  }
+  void set_cond_guard_var(int i, bool v) 
+  { 
+    if ( pushed_cond_guards)
+      pushed_cond_guards[i]= v;
+  }
+  bool have_guarded_conds() override { return MY_TEST(pushed_cond_guards); }
+
+  Item_func_not_all *upper_item; // point on NOT/NOP before ALL/SOME subquery
+
+  /*
+    SET to TRUE if IN subquery is converted from an IN predicate
+  */
+  bool converted_from_in_predicate;
+
+  Item_in_subselect(THD *thd_arg, Item * left_expr, st_select_lex *select_lex);
+  Item_in_subselect(THD *thd_arg):
+    Item_exists_subselect(thd_arg), left_expr_cache(0), first_execution(TRUE),
+    in_strategy(SUBS_NOT_TRANSFORMED),
+    pushed_cond_guards(NULL), func(NULL), do_not_convert_to_sj(FALSE),
+    is_jtbm_merged(FALSE), is_jtbm_const_tab(FALSE), upper_item(0),
+    converted_from_in_predicate(FALSE) {}
+  void cleanup() override;
+  subs_type substype() override { return IN_SUBS; }
+  void reset() override
+  {
+    eliminated= FALSE;
+    value= 0;
+    null_value= 0;
+    was_null= 0;
+  }
+  bool select_transformer(JOIN *join) override;
+  bool create_in_to_exists_cond(JOIN *join_arg);
+  bool inject_in_to_exists_cond(JOIN *join_arg);
+
+  bool exec() 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(st_select_lex_unit *unit);
+  void print(String *str, enum_query_type query_type) override;
+  enum precedence precedence() const override { return IN_PRECEDENCE; }
+  bool fix_fields(THD *thd, Item **ref) override;
+  bool fix_length_and_dec() override;
+  void fix_after_pullout(st_select_lex *new_parent, Item **ref,
+                         bool merge) override;
+  bool const_item() const override
+  {
+    return Item_subselect::const_item() && left_expr->const_item();
+  }
+  void update_used_tables() override;
+  bool setup_mat_engine();
+  bool init_left_expr_cache();
+  /* Inform 'this' that it was computed, and contains a valid result. */
+  void set_first_execution() { if (first_execution) first_execution= FALSE; }
+  bool expr_cache_is_needed(THD *thd) override;
+  inline bool left_expr_has_null();
+
+  void disable_cond_guard_for_const_null_left_expr(int i)
+  {
+    if (left_expr->const_item() && !left_expr->is_expensive())
+    {
+      if (left_expr->element_index(i)->is_null())
+        set_cond_guard_var(i,FALSE);
+    }
+  }
+
+  int optimize(double *out_rows, double *cost);
+  /*
+    Return the identifier that we could use to identify the subquery for the
+    user.
+  */
+  int get_identifier();
+
+  void block_conversion_to_sj () { do_not_convert_to_sj= TRUE; }
+
+  bool test_strategy(uchar strategy)
+  { return MY_TEST(in_strategy & strategy); }
+
+  /**
+    Test that the IN strategy was chosen for execution. This is so
+    when the CHOSEN flag is ON, and there is no other strategy.
+  */
+  bool test_set_strategy(uchar strategy)
+  {
+    DBUG_ASSERT(strategy == SUBS_SEMI_JOIN ||
+                strategy == SUBS_IN_TO_EXISTS ||
+                strategy == SUBS_MATERIALIZATION ||
+                strategy == SUBS_PARTIAL_MATCH_ROWID_MERGE ||
+                strategy == SUBS_PARTIAL_MATCH_TABLE_SCAN ||
+                strategy == SUBS_MAXMIN_INJECTED ||
+                strategy == SUBS_MAXMIN_ENGINE);
+    return ((in_strategy & SUBS_STRATEGY_CHOSEN) &&
+            (in_strategy & ~SUBS_STRATEGY_CHOSEN) == strategy);
+  }
+
+  bool is_set_strategy()
+  { return MY_TEST(in_strategy & SUBS_STRATEGY_CHOSEN); }
+
+  bool has_strategy()
+  { return in_strategy != SUBS_NOT_TRANSFORMED; }
+
+  void add_strategy (uchar strategy)
+  {
+    DBUG_ENTER("Item_in_subselect::add_strategy");
+    DBUG_PRINT("enter", ("current: %u  add: %u",
+                         (uint) in_strategy, (uint) strategy));
+    DBUG_ASSERT(strategy != SUBS_NOT_TRANSFORMED);
+    DBUG_ASSERT(!(strategy & SUBS_STRATEGY_CHOSEN));
+    /*
+      TODO: PS re-execution breaks this condition, because
+      check_and_do_in_subquery_rewrites() is called for each reexecution
+      and re-adds the same strategies.
+      DBUG_ASSERT(!(in_strategy & SUBS_STRATEGY_CHOSEN));
+    */
+    in_strategy|= strategy;
+    DBUG_VOID_RETURN;
+  }
+
+  void reset_strategy(uchar strategy)
+  {
+    DBUG_ENTER("Item_in_subselect::reset_strategy");
+    DBUG_PRINT("enter", ("current: %u  new: %u",
+                         (uint) in_strategy, (uint) strategy));
+    DBUG_ASSERT(strategy != SUBS_NOT_TRANSFORMED);
+    in_strategy= strategy;
+    DBUG_VOID_RETURN;
+  }
+
+  void set_strategy(uchar strategy)
+  {
+    DBUG_ENTER("Item_in_subselect::set_strategy");
+    DBUG_PRINT("enter", ("current: %u  set: %u",
+                         (uint) in_strategy,
+                         (uint) (SUBS_STRATEGY_CHOSEN | strategy)));
+    /* Check that only one strategy is set for execution. */
+    DBUG_ASSERT(strategy == SUBS_SEMI_JOIN ||
+                strategy == SUBS_IN_TO_EXISTS ||
+                strategy == SUBS_MATERIALIZATION ||
+                strategy == SUBS_PARTIAL_MATCH_ROWID_MERGE ||
+                strategy == SUBS_PARTIAL_MATCH_TABLE_SCAN ||
+                strategy == SUBS_MAXMIN_INJECTED ||
+                strategy == SUBS_MAXMIN_ENGINE);
+    in_strategy= (SUBS_STRATEGY_CHOSEN | strategy);
+    DBUG_VOID_RETURN;
+  }
+
+  bool walk(Item_processor processor, bool walk_subquery, void *arg) override
+  {
+    return left_expr->walk(processor, walk_subquery, arg) ||
+           Item_subselect::walk(processor, walk_subquery, arg);
+  }
+
+  bool exists2in_processor(void *opt_arg __attribute__((unused))) override
+  {
+    return 0;
+  };
+
+  bool pushdown_cond_for_in_subquery(THD *thd, Item *cond);
+
+  Item_in_subselect *get_IN_subquery() override
+  { return this; }
+  inline Item** left_exp_ptr()
+  { return &left_expr; }
+  inline Item* left_exp() const
+  { return left_expr; }
+  inline Item* left_exp_orig() const
+  { return left_expr_orig; }
+
+  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;
+  friend class subselect_partial_match_engine;
+  friend class Item_exists_subselect;
+};
+
+
+/* ALL/ANY/SOME subselect */
+class Item_allany_subselect :public Item_in_subselect
+{
+public:
+  chooser_compare_func_creator func_creator;
+  bool all;
+
+  Item_allany_subselect(THD *thd_arg, Item * left_expr,
+                        chooser_compare_func_creator fc,
+                        st_select_lex *select_lex, bool all);
+
+  void cleanup();
+  // only ALL subquery has upper not
+  subs_type substype() { return all?ALL_SUBS:ANY_SUBS; }
+  bool select_transformer(JOIN *join);
+  void create_comp_func(bool invert) { func= func_creator(invert); }
+  void print(String *str, enum_query_type query_type);
+  bool is_maxmin_applicable(JOIN *join);
+  bool transform_into_max_min(JOIN *join);
+  void no_rows_in_result();
+};
+
+
+class subselect_engine: public Sql_alloc,
+                        public Type_handler_hybrid_field_type
+{
+protected:
+  select_result_interceptor *result; /* results storage class */
+  THD *thd; /* pointer to current THD */
+  Item_subselect *item; /* item, that use this engine */
+  bool maybe_null; /* may be null (first item in select) */
+public:
+
+  enum enum_engine_type {ABSTRACT_ENGINE, SINGLE_SELECT_ENGINE,
+                         UNION_ENGINE, UNIQUESUBQUERY_ENGINE,
+                         INDEXSUBQUERY_ENGINE, HASH_SJ_ENGINE,
+                         ROWID_MERGE_ENGINE, TABLE_SCAN_ENGINE};
+
+  subselect_engine(Item_subselect *si,
+                   select_result_interceptor *res):
+    Type_handler_hybrid_field_type(&type_handler_varchar),
+    thd(NULL)
+  {
+    result= res;
+    item= si;
+    maybe_null= 0;
+  }
+  virtual ~subselect_engine() {}; // to satisfy compiler
+  virtual void cleanup()= 0;
+
+  /*
+    Also sets "thd" for subselect_engine::result.
+    Should be called before prepare().
+  */
+  void set_thd(THD *thd_arg);
+  THD * get_thd() { return thd ? thd : current_thd; }
+  virtual int prepare(THD *)= 0;
+  virtual bool fix_length_and_dec(Item_cache** row)= 0;
+  /*
+    Execute the engine
+
+    SYNOPSIS
+      exec()
+
+    DESCRIPTION
+      Execute the engine. The result of execution is subquery value that is
+      either captured by previously set up select_result-based 'sink' or
+      stored somewhere by the exec() method itself.
+
+      A required side effect: If at least one pushed-down predicate is
+      disabled, subselect_engine->no_rows() must return correct result after 
+      the exec() call.
+
+    RETURN
+      0 - OK
+      1 - Either an execution error, or the engine was "changed", and the
+          caller should call exec() again for the new engine.
+  */
+  virtual int exec()= 0;
+  virtual uint cols() const= 0; /* return number of columns in select */
+  virtual uint8 uncacheable()= 0; /* query is uncacheable */
+  virtual void exclude()= 0;
+  virtual bool may_be_null() { return maybe_null; };
+  virtual table_map upper_select_const_tables()= 0;
+  static table_map calc_const_tables(TABLE_LIST *);
+  static table_map calc_const_tables(List<TABLE_LIST> &list);
+  virtual void print(String *str, enum_query_type query_type)= 0;
+  virtual bool change_result(Item_subselect *si,
+                             select_result_interceptor *result,
+                             bool temp= FALSE)= 0;
+  virtual bool no_tables()= 0;
+  virtual bool is_executed() const { return FALSE; }
+  /* Check if subquery produced any rows during last query execution */
+  virtual bool no_rows() = 0;
+  virtual enum_engine_type engine_type() { return ABSTRACT_ENGINE; }
+  virtual int get_identifier() { DBUG_ASSERT(0); return 0; }
+  virtual void force_reexecution() {}
+protected:
+  bool set_row(List<Item> &item_list, Item_cache **row);
+};
+
+class subselect_single_select_engine: public subselect_engine
+{
+  bool prepared;       /* simple subselect is prepared */
+  bool executed;       /* simple subselect is executed */
+  st_select_lex *select_lex; /* corresponding select_lex */
+  JOIN * join; /* corresponding JOIN structure */
+public:
+  subselect_single_select_engine(st_select_lex *select,
+				 select_result_interceptor *result,
+				 Item_subselect *item);
+  void cleanup();
+  int prepare(THD *thd);
+  bool fix_length_and_dec(Item_cache** row);
+  int exec();
+  uint cols() const;
+  uint8 uncacheable();
+  void exclude();
+  table_map upper_select_const_tables();
+  void print (String *str, enum_query_type query_type);
+  bool change_result(Item_subselect *si,
+                     select_result_interceptor *result,
+                     bool temp);
+  bool no_tables();
+  bool may_be_null();
+  bool is_executed() const { return executed; }
+  bool no_rows();
+  virtual enum_engine_type engine_type() { return SINGLE_SELECT_ENGINE; }
+  int get_identifier();
+  void force_reexecution();
+  void change_select(st_select_lex *new_select) { select_lex= new_select; }
+
+  friend class subselect_hash_sj_engine;
+  friend class Item_in_subselect;
+  friend bool execute_degenerate_jtbm_semi_join(THD *thd,
+                                                TABLE_LIST *tbl,
+                                                Item_in_subselect *subq_pred,
+                                                List<Item> &eq_list);
+};
+
+
+class subselect_union_engine: public subselect_engine
+{
+  st_select_lex_unit *unit;  /* corresponding unit structure */
+public:
+  subselect_union_engine(st_select_lex_unit *u,
+			 select_result_interceptor *result,
+			 Item_subselect *item);
+  void cleanup();
+  int prepare(THD *);
+  bool fix_length_and_dec(Item_cache** row);
+  int exec();
+  uint cols() const;
+  uint8 uncacheable();
+  void exclude();
+  table_map upper_select_const_tables();
+  void print (String *str, enum_query_type query_type);
+  bool change_result(Item_subselect *si,
+                     select_result_interceptor *result,
+                     bool temp= FALSE);
+  bool no_tables();
+  bool is_executed() const;
+  void force_reexecution();
+  bool no_rows();
+  virtual enum_engine_type engine_type() { return UNION_ENGINE; }
+};
+
+
+struct st_join_table;
+
+
+/*
+  A subquery execution engine that evaluates the subquery by doing one index
+  lookup in a unique index.
+
+  This engine is used to resolve subqueries in forms
+  
+    outer_expr IN (SELECT tbl.unique_key FROM tbl WHERE subq_where) 
+    
+  or, tuple-based:
+  
+    (oe1, .. oeN) IN (SELECT uniq_key_part1, ... uniq_key_partK
+                      FROM tbl WHERE subqwhere) 
+  
+  i.e. the subquery is a single table SELECT without GROUP BY, aggregate
+  functions, etc.
+*/
+
+class subselect_uniquesubquery_engine: public subselect_engine
+{
+protected:
+  st_join_table *tab;
+  Item *cond; /* The WHERE condition of subselect */
+  /* 
+    TRUE<=> last execution produced empty set. Valid only when left
+    expression is NULL.
+  */
+  bool empty_result_set;
+public:
+
+  // constructor can assign THD because it will be called after JOIN::prepare
+  subselect_uniquesubquery_engine(THD *thd_arg, st_join_table *tab_arg,
+				  Item_in_subselect *subs, Item *where)
+    :subselect_engine(subs, 0), tab(tab_arg), cond(where)
+  { DBUG_ASSERT(subs); }
+  ~subselect_uniquesubquery_engine();
+  void cleanup();
+  int prepare(THD *);
+  bool fix_length_and_dec(Item_cache** row);
+  int exec();
+  uint cols() const { return 1; }
+  uint8 uncacheable() { return UNCACHEABLE_DEPENDENT_INJECTED; }
+  void exclude();
+  table_map upper_select_const_tables() { return 0; }
+  void print (String *str, enum_query_type query_type);
+  bool change_result(Item_subselect *si,
+                     select_result_interceptor *result,
+                     bool temp= FALSE);
+  bool no_tables();
+  int index_lookup(); /* TIMOUR: this method needs refactoring. */
+  int scan_table();
+  bool copy_ref_key(bool skip_constants);
+  bool no_rows() { return empty_result_set; }
+  virtual enum_engine_type engine_type() { return UNIQUESUBQUERY_ENGINE; }
+};
+
+
+class subselect_indexsubquery_engine: public subselect_uniquesubquery_engine
+{
+  /* FALSE for 'ref', TRUE for 'ref-or-null'. */
+  bool check_null;
+  /* 
+    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.
+    
+    NOTE: This is currently here and not in the uniquesubquery_engine. Ideally
+    it should have been in uniquesubquery_engine in order to allow execution of
+    subqueries like
+    
+      (oe1, oe2) IN (SELECT primary_key, non_key_maybe_null_field FROM tbl)
+
+    We could use uniquesubquery_engine for the first component and let
+    Item_is_not_null_test( non_key_maybe_null_field) to handle the second.
+
+    However, subqueries like the above are currently not handled by index
+    lookup-based subquery engines, the engine applicability check misses
+    them: it doesn't switch the engine for case of artificial having and
+    [eq_]ref access (only for artificial having + ref_or_null or no having).
+    The above example subquery is handled as a full-blown SELECT with eq_ref
+    access to one table.
+
+    Due to this limitation, the "artificial having" currently needs to be 
+    checked by only in indexsubquery_engine.
+  */
+  Item *having;
+public:
+
+  // constructor can assign THD because it will be called after JOIN::prepare
+  subselect_indexsubquery_engine(THD *thd_arg, st_join_table *tab_arg,
+				 Item_in_subselect *subs, Item *where,
+                                 Item *having_arg, bool chk_null)
+    :subselect_uniquesubquery_engine(thd_arg, tab_arg, subs, where),
+     check_null(chk_null),
+     having(having_arg)
+  { DBUG_ASSERT(subs); }
+  int exec();
+  void print (String *str, enum_query_type query_type);
+  virtual enum_engine_type engine_type() { return INDEXSUBQUERY_ENGINE; }
+};
+
+/*
+  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, Item *left_expr,
+                                chooser_compare_func_creator cmp,
+                                bool all,
+                                SELECT_LEX *select_lex);
+
+
+inline bool Item_subselect::is_evaluated() const
+{
+  return engine->is_executed();
+}
+
+
+inline bool Item_subselect::is_uncacheable() const
+{
+  return engine->uncacheable();
+}
+
+/**
+  Compute an IN predicate via a hash semi-join. This class is responsible for
+  the materialization of the subquery, and the selection of the correct and
+  optimal execution method (e.g. direct index lookup, or partial matching) for
+  the IN predicate.
+*/
+
+class subselect_hash_sj_engine : public subselect_engine
+{
+public:
+  /* The table into which the subquery is materialized. */
+  TABLE *tmp_table;
+  /* TRUE if the subquery was materialized into a temp table. */
+  bool is_materialized;
+  /*
+    The old engine already chosen at parse time and stored in permanent memory.
+    Through this member we can re-create and re-prepare materialize_join for
+    each execution of a prepared statement. We also reuse the functionality
+    of subselect_single_select_engine::[prepare | cols].
+  */
+  subselect_single_select_engine *materialize_engine;
+  /*
+    QEP to execute the subquery and materialize its result into a
+    temporary table. Created during the first call to exec().
+  */
+  JOIN *materialize_join;
+  /*
+    A conjunction of all the equality conditions between all pairs of expressions
+    that are arguments of an IN predicate. We need these to post-filter some
+    IN results because index lookups sometimes match values that are actually
+    not equal to the search key in SQL terms.
+  */
+  Item_cond_and *semi_join_conds;
+  Name_resolution_context *semi_join_conds_context;
+
+
+  subselect_hash_sj_engine(THD *thd_arg, Item_in_subselect *in_predicate,
+                           subselect_single_select_engine *old_engine)
+    : subselect_engine(in_predicate, NULL),
+      tmp_table(NULL), is_materialized(FALSE), materialize_engine(old_engine),
+      materialize_join(NULL),  semi_join_conds(NULL), lookup_engine(NULL),
+      count_partial_match_columns(0), count_null_only_columns(0),
+      count_columns_with_nulls(0), strategy(UNDEFINED)
+  { DBUG_ASSERT(in_predicate); }
+  ~subselect_hash_sj_engine();
+
+  bool init(List<Item> *tmp_columns, uint subquery_id);
+  void cleanup();
+  int prepare(THD *);
+  int exec();
+  void print(String *str, enum_query_type query_type);
+  uint cols() const { return materialize_engine->cols(); }
+  uint8 uncacheable() { return materialize_engine->uncacheable(); }
+  table_map upper_select_const_tables() { return 0; }
+  bool no_rows() { return !tmp_table->file->stats.records; }
+  virtual enum_engine_type engine_type() { return HASH_SJ_ENGINE; }
+  /*
+    TODO: factor out all these methods in a base subselect_index_engine class
+    because all of them have dummy implementations and should never be called.
+  */
+  bool fix_length_and_dec(Item_cache** row);//=>base class
+  void exclude(); //=>base class
+  //=>base class
+  bool change_result(Item_subselect *si,
+                     select_result_interceptor *result,
+                     bool temp= FALSE);
+  bool no_tables();//=>base class
+
+protected:
+  /* The engine used to compute the IN predicate. */
+  subselect_engine *lookup_engine;
+  /* Keyparts of the only non-NULL composite index in a rowid merge. */
+  MY_BITMAP non_null_key_parts;
+  /* Keyparts of the single column indexes with NULL, one keypart per index. */
+  MY_BITMAP partial_match_key_parts;
+  uint count_partial_match_columns;
+  uint count_null_only_columns;
+  uint count_columns_with_nulls;
+  /* Possible execution strategies that can be used to compute hash semi-join.*/
+  enum exec_strategy {
+    UNDEFINED,
+    COMPLETE_MATCH, /* Use regular index lookups. */
+    PARTIAL_MATCH,  /* Use some partial matching strategy. */
+    PARTIAL_MATCH_MERGE, /* Use partial matching through index merging. */
+    PARTIAL_MATCH_SCAN,  /* Use partial matching through table scan. */
+    IMPOSSIBLE      /* Subquery materialization is not applicable. */
+  };
+  /* The chosen execution strategy. Computed after materialization. */
+  exec_strategy strategy;
+  exec_strategy get_strategy_using_schema();
+  exec_strategy get_strategy_using_data();
+  ulonglong rowid_merge_buff_size(bool has_non_null_key,
+                                  bool has_covering_null_row,
+                                  MY_BITMAP *partial_match_key_parts);
+  void choose_partial_match_strategy(bool has_non_null_key,
+                                     bool has_covering_null_row,
+                                     MY_BITMAP *partial_match_key_parts);
+  bool make_semi_join_conds();
+  subselect_uniquesubquery_engine* make_unique_engine();
+
+};
+
+
+/*
+  Distinguish the type of (0-based) row numbers from the type of the index into
+  an array of row numbers.
+*/
+typedef ha_rows rownum_t;
+
+
+/*
+  An Ordered_key is an in-memory table index that allows O(log(N)) time
+  lookups of a multi-part key.
+
+  If the index is over a single column, then this column may contain NULLs, and
+  the NULLs are stored and tested separately for NULL in O(1) via is_null().
+  Multi-part indexes assume that the indexed columns do not contain NULLs.
+
+  TODO:
+  = Due to the unnatural assymetry between single and multi-part indexes, it
+    makes sense to somehow refactor or extend the class.
+
+  = This class can be refactored into a base abstract interface, and two
+    subclasses:
+    - one to represent single-column indexes, and
+    - another to represent multi-column indexes.
+    Such separation would allow slightly more efficient implementation of
+    the single-column indexes.
+  = The current design requires such indexes to be fully recreated for each
+    PS (re)execution, however most of the comprising objects can be reused.
+*/
+
+class Ordered_key : public Sql_alloc
+{
+protected:
+  /*
+    Index of the key in an array of keys. This index allows to
+    construct (sub)sets of keys represented by bitmaps.
+  */
+  uint keyid;
+  /* The table being indexed. */
+  TABLE *tbl;
+  /* The columns being indexed. */
+  Item_field **key_columns;
+  /* Number of elements in 'key_columns' (number of key parts). */
+  uint key_column_count;
+  /*
+    An expression, or sequence of expressions that forms the search key.
+    The search key is a sequence when it is Item_row. Each element of the
+    sequence is accessible via Item::element_index(int i).
+  */
+  Item *search_key;
+
+/* Value index related members. */
+  /*
+    The actual value index, consists of a sorted sequence of row numbers.
+  */
+  rownum_t *key_buff;
+  /* Number of elements in key_buff. */
+  ha_rows key_buff_elements;
+  /* Current element in 'key_buff'. */
+  ha_rows cur_key_idx;
+  /*
+    Mapping from row numbers to row ids. The element row_num_to_rowid[i]
+    contains a buffer with the rowid for the row numbered 'i'.
+    The memory for this member is not maintanined by this class because
+    all Ordered_key indexes of the same table share the same mapping.
+  */
+  uchar *row_num_to_rowid;
+  /*
+    A sequence of predicates to compare the search key with the corresponding
+    columns of a table row from the index.
+  */
+  Item_func_lt **compare_pred;
+
+/* Null index related members. */
+  MY_BITMAP null_key;
+  /* Count of NULLs per column. */
+  ha_rows null_count;
+  /* The row number that contains the first NULL in a column. */
+  rownum_t min_null_row;
+  /* The row number that contains the last NULL in a column. */
+  rownum_t max_null_row;
+
+protected:
+  bool alloc_keys_buffers();
+  /*
+    Quick sort comparison function that compares two rows of the same table
+    indentfied with their row numbers.
+  */
+  int cmp_keys_by_row_data(rownum_t a, rownum_t b);
+  static int cmp_keys_by_row_data_and_rownum(Ordered_key *key,
+                                             rownum_t* a, rownum_t* b);
+
+  int cmp_key_with_search_key(rownum_t row_num);
+
+public:
+  Ordered_key(uint keyid_arg, TABLE *tbl_arg,
+              Item *search_key_arg, ha_rows null_count_arg,
+              ha_rows min_null_row_arg, ha_rows max_null_row_arg,
+              uchar *row_num_to_rowid_arg);
+  ~Ordered_key();
+  void cleanup();
+  /* Initialize a multi-column index. */
+  bool init(MY_BITMAP *columns_to_index);
+  /* Initialize a single-column index. */
+  bool init(int col_idx);
+
+  uint get_column_count() { return key_column_count; }
+  uint get_keyid() { return keyid; }
+  Field *get_field(uint i)
+  {
+    DBUG_ASSERT(i < key_column_count);
+    return key_columns[i]->field;
+  }
+  rownum_t get_min_null_row() { return min_null_row; }
+  rownum_t get_max_null_row() { return max_null_row; }
+  MY_BITMAP * get_null_key() { return &null_key; }
+  ha_rows get_null_count() { return null_count; }
+  /*
+    Get the search key element that corresponds to the i-th key part of this
+    index.
+  */
+  Item *get_search_key(uint i)
+  {
+    return search_key->element_index(key_columns[i]->field->field_index);
+  }
+  void add_key(rownum_t row_num)
+  {
+    /* The caller must know how many elements to add. */
+    DBUG_ASSERT(key_buff_elements && cur_key_idx < key_buff_elements);
+    key_buff[cur_key_idx]= row_num;
+    ++cur_key_idx;
+  }
+
+  bool sort_keys();
+  double null_selectivity();
+
+  /*
+    Position the current element at the first row that matches the key.
+    The key itself is propagated by evaluating the current value(s) of
+    this->search_key.
+  */
+  bool lookup();
+  /* Move the current index cursor to the first key. */
+  void first()
+  {
+    DBUG_ASSERT(key_buff_elements);
+    cur_key_idx= 0;
+  }
+  /* TODO */
+  bool next_same();
+  /* Move the current index cursor to the next key. */
+  bool next()
+  {
+    DBUG_ASSERT(key_buff_elements);
+    if (cur_key_idx < key_buff_elements - 1)
+    {
+      ++cur_key_idx;
+      return TRUE;
+    }
+    return FALSE;
+  };
+  /* Return the current index element. */
+  rownum_t current()
+  {
+    DBUG_ASSERT(key_buff_elements && cur_key_idx < key_buff_elements);
+    return key_buff[cur_key_idx];
+  }
+
+  void set_null(rownum_t row_num)
+  {
+    bitmap_set_bit(&null_key, (uint)row_num);
+  }
+  bool is_null(rownum_t row_num)
+  {
+    /*
+      Indexes consisting of only NULLs do not have a bitmap buffer at all.
+      Their only initialized member is 'n_bits', which is equal to the number
+      of temp table rows.
+    */
+    if (null_count == tbl->file->stats.records)
+    {
+      DBUG_ASSERT(tbl->file->stats.records == null_key.n_bits);
+      return TRUE;
+    }
+    if (row_num > max_null_row || row_num < min_null_row)
+      return FALSE;
+    return bitmap_is_set(&null_key, (uint)row_num);
+  }
+  void print(String *str);
+};
+
+
+class subselect_partial_match_engine : public subselect_engine
+{
+protected:
+  /* The temporary table that contains a materialized subquery. */
+  TABLE *tmp_table;
+  /*
+    The engine used to check whether an IN predicate is TRUE or not. If not
+    TRUE, then subselect_rowid_merge_engine further distinguishes between
+    FALSE and UNKNOWN.
+  */
+  subselect_uniquesubquery_engine *lookup_engine;
+  /* A list of equalities between each pair of IN operands. */
+  List<Item> *equi_join_conds;
+  /*
+    True if there is an all NULL row in tmp_table. If so, then if there is
+    no complete match, there is a guaranteed partial match.
+  */
+  bool has_covering_null_row;
+
+  /*
+    True if all nullable columns of tmp_table consist of only NULL values.
+    If so, then if there is a match in the non-null columns, there is a
+    guaranteed partial match.
+  */
+  bool has_covering_null_columns;
+  uint count_columns_with_nulls;
+
+protected:
+  virtual bool partial_match()= 0;
+public:
+  subselect_partial_match_engine(subselect_uniquesubquery_engine *engine_arg,
+                                 TABLE *tmp_table_arg, Item_subselect *item_arg,
+                                 select_result_interceptor *result_arg,
+                                 List<Item> *equi_join_conds_arg,
+                                 bool has_covering_null_row_arg,
+                                 bool has_covering_null_columns_arg,
+                                 uint count_columns_with_nulls_arg);
+  int prepare(THD *thd_arg) { set_thd(thd_arg); return 0; }
+  int exec();
+  bool fix_length_and_dec(Item_cache**) { return FALSE; }
+  uint cols() const { /* TODO: what is the correct value? */ return 1; }
+  uint8 uncacheable() { return UNCACHEABLE_DEPENDENT; }
+  void exclude() {}
+  table_map upper_select_const_tables() { return 0; }
+  bool change_result(Item_subselect*,
+                     select_result_interceptor*,
+                     bool temp= FALSE)
+  { DBUG_ASSERT(FALSE); return false; }
+  bool no_tables() { return false; }
+  bool no_rows()
+  {
+    /*
+      TODO: It is completely unclear what is the semantics of this
+      method. The current result is computed so that the call to no_rows()
+      from Item_in_optimizer::val_int() sets Item_in_optimizer::null_value
+      correctly.
+    */
+    return !(item->get_IN_subquery()->null_value);
+  }
+  void print(String*, enum_query_type);
+
+  friend void subselect_hash_sj_engine::cleanup();
+};
+
+
+class subselect_rowid_merge_engine: public subselect_partial_match_engine
+{
+protected:
+  /*
+    Mapping from row numbers to row ids. The rowids are stored sequentially
+    in the array - rowid[i] is located in row_num_to_rowid + i * rowid_length.
+  */
+  uchar *row_num_to_rowid;
+  /*
+    A subset of all the keys for which there is a match for the same row.
+    Used during execution. Computed for each outer reference
+  */
+  MY_BITMAP matching_keys;
+  /*
+    The columns of the outer reference that are NULL. Computed for each
+    outer reference.
+  */
+  MY_BITMAP matching_outer_cols;
+  /*
+    Indexes of row numbers, sorted by <column_value, row_number>. If an
+    index may contain NULLs, the NULLs are stored efficiently in a bitmap.
+
+    The indexes are sorted by the selectivity of their NULL sub-indexes, the
+    one with the fewer NULLs is first. Thus, if there is any index on
+    non-NULL columns, it is contained in keys[0].
+  */
+  Ordered_key **merge_keys;
+  /* The number of elements in merge_keys. */
+  uint merge_keys_count;
+  /* The NULL bitmaps of merge keys.*/
+  MY_BITMAP   **null_bitmaps;
+  /*
+    An index on all non-NULL columns of 'tmp_table'. The index has the
+    logical form: <[v_i1 | ... | v_ik], rownum>. It allows to find the row
+    number where the columns c_i1,...,c1_k contain the values v_i1,...,v_ik.
+    If such an index exists, it is always the first element of 'merge_keys'.
+  */
+  Ordered_key *non_null_key;
+  /*
+    Priority queue of Ordered_key indexes, one per NULLable column.
+    This queue is used by the partial match algorithm in method exec().
+  */
+  QUEUE pq;
+protected:
+  /*
+    Comparison function to compare keys in order of decreasing bitmap
+    selectivity.
+  */
+  static int cmp_keys_by_null_selectivity(Ordered_key **k1, Ordered_key **k2);
+  /*
+    Comparison function used by the priority queue pq, the 'smaller' key
+    is the one with the smaller current row number.
+  */
+  static int cmp_keys_by_cur_rownum(void *arg, uchar *k1, uchar *k2);
+
+  bool test_null_row(rownum_t row_num);
+  bool exists_complementing_null_row(MY_BITMAP *keys_to_complement);
+  bool partial_match();
+public:
+  subselect_rowid_merge_engine(subselect_uniquesubquery_engine *engine_arg,
+                               TABLE *tmp_table_arg, uint merge_keys_count_arg,
+                               bool has_covering_null_row_arg,
+                               bool has_covering_null_columns_arg,
+                               uint count_columns_with_nulls_arg,
+                               Item_subselect *item_arg,
+                               select_result_interceptor *result_arg,
+                               List<Item> *equi_join_conds_arg)
+    :subselect_partial_match_engine(engine_arg, tmp_table_arg,
+                                    item_arg, result_arg, equi_join_conds_arg,
+                                    has_covering_null_row_arg,
+                                    has_covering_null_columns_arg,
+                                    count_columns_with_nulls_arg),
+    merge_keys_count(merge_keys_count_arg), non_null_key(NULL)
+  {}
+  ~subselect_rowid_merge_engine();
+  bool init(MY_BITMAP *non_null_key_parts, MY_BITMAP *partial_match_key_parts);
+  void cleanup();
+  virtual enum_engine_type engine_type() { return ROWID_MERGE_ENGINE; }
+};
+
+
+class subselect_table_scan_engine: public subselect_partial_match_engine
+{
+protected:
+  bool partial_match();
+public:
+  subselect_table_scan_engine(subselect_uniquesubquery_engine *engine_arg,
+                              TABLE *tmp_table_arg, Item_subselect *item_arg,
+                              select_result_interceptor *result_arg,
+                              List<Item> *equi_join_conds_arg,
+                              bool has_covering_null_row_arg,
+                              bool has_covering_null_columns_arg,
+                              uint count_columns_with_nulls_arg);
+  void cleanup();
+  virtual enum_engine_type engine_type() { return TABLE_SCAN_ENGINE; }
+};
+#endif /* ITEM_SUBSELECT_INCLUDED */