item_subselect.h

#ifndef ITEM_SUBSELECT_INCLUDED
#define ITEM_SUBSELECT_INCLUDED

/* Copyright (c) 2002, 2012, Oracle and/or its affiliates. All rights reserved.

   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,
   51 Franklin Street, Suite 500, Boston, MA 02110-1335 USA */

/* subselect Item */

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 Cached_item;
class Comp_creator;

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);

/* base class for subselects */

class Item_subselect : public Item_result_field
{
private:
    bool value_assigned; /* value already assigned to subselect */
    /**
        Whether or not execution of this subselect 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 traced_before;
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;
        < Item_subselect::fix_fields
    */
    Item *substitution;
public:
    /* unit of subquery */
    st_select_lex_unit *unit;
    /**
       If !=INT_MIN: this Item is in the condition attached to the JOIN_TAB
       having this index in the parent JOIN.
    */
    int in_cond_of_tab;

    /// EXPLAIN needs read-only access to the engine
    const subselect_engine *get_engine_for_explain() const
    {
        return engine;
    }

protected:
    /* engine that perform execution of subselect (single select or union) */
    subselect_engine *engine;
    /* old engine if engine was changed */
    subselect_engine *old_engine;
    /* cache of used external tables */
    table_map used_tables_cache;
    /* 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;
    /* cache of constant state */
    bool const_item_cache;

public:
    /* changed engine indicator */
    bool engine_changed;
    /* subquery is transformed */
    bool changed;

    enum trans_res
    {RES_OK, RES_REDUCE, RES_ERROR};
    enum subs_type
    {   UNKNOWN_SUBS, SINGLEROW_SUBS,
        EXISTS_SUBS, IN_SUBS, ALL_SUBS, ANY_SUBS
    };

    Item_subselect();

    virtual subs_type substype()
    {
        return UNKNOWN_SUBS;
    }

    /*
      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();
    virtual void cleanup();
    virtual void reset()
    {
        null_value = 1;
    }
    virtual trans_res select_transformer(JOIN *join);
    bool assigned() const
    {
        return value_assigned;
    }
    void assigned(bool a)
    {
        value_assigned = a;
    }
    enum Type type() const;
    bool is_null()
    {
        update_null_value();
        return null_value;
    }
    bool fix_fields(THD *thd, Item **ref);
    void fix_after_pullout(st_select_lex *parent_select,
                           st_select_lex *removed_select);
    virtual bool exec();
    virtual void fix_length_and_dec();
    table_map used_tables() const;
    table_map not_null_tables() const
    {
        return 0;
    }
    bool const_item() const;
    inline table_map get_used_tables_cache()
    {
        return used_tables_cache;
    }
    inline bool get_const_item_cache()
    {
        return const_item_cache;
    }
    Item *get_tmp_table_item(THD *thd);
    void update_used_tables();
    virtual void print(String *str, enum_query_type query_type);
    virtual bool have_guarded_conds()
    {
        return FALSE;
    }
    bool change_engine(subselect_engine *eng)
    {
        old_engine = engine;
        engine = eng;
        engine_changed = 1;
        return eng == 0;
    }

    /*
      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 call this method before rexecution query.
    */
    virtual void reset_value_registration() {}
    enum_parsing_place place()
    {
        return parsing_place;
    }
    bool walk_body(Item_processor processor, bool walk_subquery, uchar *arg);
    bool walk(Item_processor processor, bool walk_subquery, uchar *arg);
    virtual bool explain_subquery_checker(uchar **arg);
    bool inform_item_in_cond_of_tab(uchar *join_tab_index);

    const char *func_name() const
    {
        DBUG_ASSERT(0);
        return "subselect";
    }

    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 Item_ident::fix_after_pullout(st_select_lex *parent_select,
            st_select_lex *removed_selec);
    friend void mark_select_range_as_dependent(THD *,
            st_select_lex *, st_select_lex *,
            Field *, Item *, Item_ident *);
};

/* single value subselect */

class Item_cache;
class Item_singlerow_subselect : public Item_subselect
{
protected:
    Item_cache *value, * *row;
    bool no_rows;                              ///< @c no_rows_in_result
public:
    Item_singlerow_subselect(st_select_lex *select_lex);
    Item_singlerow_subselect() :
        Item_subselect(), value(0), row (0), no_rows(false) {}

    virtual void cleanup();
    subs_type substype()
    {
        return SINGLEROW_SUBS;
    }

    virtual void reset();
    trans_res select_transformer(JOIN *join);
    void store(uint i, Item *item);
    double val_real();
    longlong val_int ();
    String *val_str (String *);
    my_decimal *val_decimal(my_decimal *);
    bool get_date(MYSQL_TIME *ltime, uint fuzzydate);
    bool get_time(MYSQL_TIME *ltime);
    bool val_bool();
    enum Item_result result_type() const;
    enum_field_types field_type() const;
    void fix_length_and_dec();

    /*
      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().
    */
    virtual void no_rows_in_result();

    uint cols();
    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();

    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, bool ignore_nulls);
    virtual void print(String *str, enum_query_type query_type);
    virtual void cleanup();
    bool any_value()
    {
        return was_values;
    }
    void register_value()
    {
        was_values = TRUE;
    }
    void reset_value_registration()
    {
        was_values = FALSE;
    }
};

/* exists subselect */

class Item_exists_subselect : public Item_subselect
{
protected:
    bool value; /* value of this item (boolean: exists/not-exists) */

public:
    /**
      The method chosen to execute the predicate, currently used for IN, =ANY
      and EXISTS predicates.
    */
    enum enum_exec_method
    {
        EXEC_UNSPECIFIED, ///< No execution method specified yet.
        EXEC_SEMI_JOIN,   ///< Predicate is converted to semi-join nest.
        /// IN was converted to correlated EXISTS, and this is a final decision.
        EXEC_EXISTS,
        /**
           Decision between EXEC_EXISTS and EXEC_MATERIALIZATION is not yet taken.
           IN was temporarily converted to correlated EXISTS.
           All descendants of Item_in_subselect must go through this method
           before they can reach EXEC_EXISTS.
        */
        EXEC_EXISTS_OR_MAT,
        /// Predicate executed via materialization, and this is a final decision.
        EXEC_MATERIALIZATION
    };
    enum_exec_method exec_method;
    /// Priority of this predicate in the convert-to-semi-join-nest process.
    int sj_convert_priority;
    /// True if this predicate is chosen for semi-join transformation
    bool sj_chosen;
    /**
      Used by subquery optimizations to keep track about where this subquery
      predicate is located, and whether it is a candidate for transformation.
        (TABLE_LIST*) 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.
    */
    TABLE_LIST *embedding_join_nest;

    Item_exists_subselect(st_select_lex *select_lex);
    Item_exists_subselect()
        : Item_subselect(), value(false), exec_method(EXEC_UNSPECIFIED),
          sj_convert_priority(0), sj_chosen(false), embedding_join_nest(NULL)
    {}
    virtual trans_res select_transformer(JOIN *join)
    {
        exec_method = EXEC_EXISTS;
        return RES_OK;
    }
    subs_type substype()
    {
        return EXISTS_SUBS;
    }
    virtual void reset()
    {
        value = 0;
    }

    enum Item_result result_type() const
    {
        return INT_RESULT;
    }
    longlong val_int();
    double val_real();
    String *val_str(String *);
    my_decimal *val_decimal(my_decimal *);
    bool val_bool();
    bool get_date(MYSQL_TIME *ltime, uint fuzzydate)
    {
        return get_date_from_int(ltime, fuzzydate);
    }
    bool get_time(MYSQL_TIME *ltime)
    {
        return get_time_from_int(ltime);
    }
    void fix_length_and_dec();
    virtual void print(String *str, enum_query_type query_type);

    friend class select_exists_subselect;
    friend class subselect_indexsubquery_engine;
};


/**
  Representation of IN subquery predicates of the form
  "left_expr IN (SELECT ...)".

  @detail
  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
{
public:
    Item *left_expr;
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 left_expr_cache_filled; ///< Whether left_expr_cache holds a value
    /** The need for expr cache may be optimized away, @sa init_left_expr_cache. */
    bool need_expr_cache;

    /*
      expr & optimizer used in subselect rewriting to store Item for
      all JOIN in UNION
    */
    Item *expr;
    Item_in_optimizer *optimizer;
    bool was_null;
    bool abort_on_null;
private:
    /**
       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: public Sql_alloc
    {
        /**
           True: if IN->EXISTS has been done and has added a condition to the
           subquery's WHERE clause.
        */
        bool added_to_where;
        /**
           True: if original subquery was dependent (correlated) before IN->EXISTS
           was done.
        */
        bool originally_dependent;
    } *in2exists_info;

    Item *remove_in2exists_conds(Item *conds);

public:
    /* Used to trigger on/off conditions that were pushed down to subselect */
    bool *pushed_cond_guards;

    Item_func_not_all *upper_item; // point on NOT/NOP before ALL/SOME subquery

    /*
      Location of the subquery predicate. It is either
       - pointer to join nest if the subquery predicate is in the ON expression
       - (TABLE_LIST*)1 if the predicate is in the WHERE.
    */
    TABLE_LIST *expr_join_nest;

    bool in2exists_added_to_where() const
    {
        return in2exists_info && in2exists_info->added_to_where;
    }

    /// Is reliable only if IN->EXISTS has been done.
    bool originally_dependent() const
    {
        return in2exists_info->originally_dependent;
    }

    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()
    {
        return test(pushed_cond_guards);
    }

    Item_in_subselect(Item *left_expr, st_select_lex *select_lex);
    Item_in_subselect()
        : Item_exists_subselect(), left_expr(NULL), left_expr_cache(NULL),
          left_expr_cache_filled(false), need_expr_cache(TRUE), expr(NULL),
          optimizer(NULL), was_null(FALSE), abort_on_null(FALSE),
          in2exists_info(NULL), pushed_cond_guards(NULL), upper_item(NULL)
    {}
    virtual void cleanup();
    subs_type substype()
    {
        return IN_SUBS;
    }
    virtual void reset()
    {
        value = 0;
        null_value = 0;
        was_null = 0;
    }
    trans_res select_transformer(JOIN *join);
    trans_res select_in_like_transformer(JOIN *join, Comp_creator *func);
    trans_res single_value_transformer(JOIN *join, Comp_creator *func);
    trans_res row_value_transformer(JOIN *join);
    trans_res single_value_in_to_exists_transformer(JOIN *join,
            Comp_creator *func);
    trans_res row_value_in_to_exists_transformer(JOIN *join);
    bool walk(Item_processor processor, bool walk_subquery, uchar *arg);
    virtual bool exec();
    longlong val_int();
    double val_real();
    String *val_str(String *);
    my_decimal *val_decimal(my_decimal *);
    void update_null_value ()
    {
        (void) val_bool();
    }
    bool val_bool();
    void top_level_item()
    {
        abort_on_null = 1;
    }
    inline bool is_top_level_item()
    {
        return abort_on_null;
    }
    bool test_limit(st_select_lex_unit *unit);
    virtual void print(String *str, enum_query_type query_type);
    bool fix_fields(THD *thd, Item **ref);
    void fix_after_pullout(st_select_lex *parent_select,
                           st_select_lex *removed_select);
    bool init_left_expr_cache();

    /**
       Once the decision to use IN->EXISTS has been taken, performs some last
       steps of this transformation.
    */
    bool finalize_exists_transform(SELECT_LEX *select_lex);
    /**
       Once the decision to use materialization has been taken, performs some
       last steps of this transformation.
    */
    bool finalize_materialization_transform(JOIN *join);

    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;
};


/* ALL/ANY/SOME subselect */
class Item_allany_subselect : public Item_in_subselect
{
public:
    chooser_compare_func_creator func_creator;
    Comp_creator *func;
    bool all;

    Item_allany_subselect(Item *left_expr, chooser_compare_func_creator fc,
                          st_select_lex *select_lex, bool all);

    // only ALL subquery has upper not
    subs_type substype()
    {
        return all ? ALL_SUBS : ANY_SUBS;
    }
    trans_res select_transformer(JOIN *join);
    virtual void print(String *str, enum_query_type query_type);
};


class subselect_engine: public Sql_alloc
{
protected:
    select_result_interceptor *result; /* results storage class */
    Item_subselect *item; /* item, that use this engine */
    enum Item_result res_type; /* type of results */
    enum_field_types res_field_type; /* column type of the results */
    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
    };

    subselect_engine(Item_subselect *si, select_result_interceptor *res)
        : result(res), item(si), res_type(STRING_RESULT),
          res_field_type(MYSQL_TYPE_VAR_STRING), maybe_null(false)
    {}
    virtual ~subselect_engine() {}; // to satisfy compiler
    /**
      Cleanup engine after complete query execution, free all resources.
    */
    virtual void cleanup() = 0;

    /// Sets "thd" for 'result'. Should be called before prepare()
    void set_thd_for_result();
    virtual bool prepare() = 0;
    virtual void 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.

      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 bool exec() = 0;
    virtual uint cols() const = 0; /* return number of columns in select */
    virtual uint8 uncacheable() const = 0; /* query is uncacheable */
    virtual enum Item_result type() const
    {
        return res_type;
    }
    virtual enum_field_types field_type() const
    {
        return res_field_type;
    }
    virtual void exclude() = 0;
    virtual bool may_be_null() const
    {
        return maybe_null;
    };
    virtual table_map upper_select_const_tables() const = 0;
    static table_map calc_const_tables(TABLE_LIST *);
    virtual void print(String *str, enum_query_type query_type) = 0;
    virtual bool change_result(Item_subselect *si,
                               select_result_interceptor *result) = 0;
    virtual bool no_tables() const = 0;
    virtual bool is_executed() const
    {
        return FALSE;
    }
    virtual enum_engine_type engine_type() const
    {
        return ABSTRACT_ENGINE;
    }
#ifndef DBUG_OFF
    /**
       @returns the internal Item. Defined only in debug builds, because should
       be used only for debug asserts.
    */
    const Item_subselect *get_item() const
    {
        return item;
    }
#endif

protected:
    void set_row(List<Item> &item_list, Item_cache **row);
};


class subselect_single_select_engine: public subselect_engine
{
private:
    bool prepared; /* simple subselect is prepared */
    bool executed; /* simple subselect is executed */
    bool optimize_error; ///< simple subselect optimization failed
    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);
    virtual void cleanup();
    virtual bool prepare();
    virtual void fix_length_and_dec(Item_cache **row);
    virtual bool exec();
    virtual uint cols() const;
    virtual uint8 uncacheable() const;
    virtual void exclude();
    virtual table_map upper_select_const_tables() const;
    virtual void print (String *str, enum_query_type query_type);
    virtual bool change_result(Item_subselect *si,
                               select_result_interceptor *result);
    virtual bool no_tables() const;
    virtual bool may_be_null() const;
    virtual bool is_executed() const
    {
        return executed;
    }
    virtual enum_engine_type engine_type() const
    {
        return SINGLE_SELECT_ENGINE;
    }
    st_select_lex  *get_st_select_lex()
    {
        return select_lex;
    }

    friend class subselect_hash_sj_engine;
    friend class Item_in_subselect;
};


class subselect_union_engine: public subselect_engine
{
public:
    subselect_union_engine(st_select_lex_unit *u,
                           select_result_interceptor *result,
                           Item_subselect *item);
    virtual void cleanup();
    virtual bool prepare();
    virtual void fix_length_and_dec(Item_cache **row);
    virtual bool exec();
    virtual uint cols() const;
    virtual uint8 uncacheable() const;
    virtual void exclude();
    virtual table_map upper_select_const_tables() const;
    virtual void print (String *str, enum_query_type query_type);
    virtual bool change_result(Item_subselect *si,
                               select_result_interceptor *result);
    virtual bool no_tables() const;
    virtual bool is_executed() const;
    virtual enum_engine_type engine_type() const
    {
        return UNION_ENGINE;
    }

private:
    st_select_lex_unit *unit;  /* corresponding unit structure */
};


struct st_join_table;


/**
  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 : public subselect_engine
{
protected:
    st_join_table *tab;
    Item *cond; /* The WHERE condition of subselect */
private:
    /* 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.
    */
    Item *having;
    /**
       Whether a lookup for key value (x0,y0) (x0 and/or y0 being NULL or not
       NULL) will find at most one row.
    */
    bool unique;
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_subselect *subs, Item *where,
                                   Item *having_arg, bool chk_null,
                                   bool unique_arg)
        : subselect_engine(subs, 0), tab(tab_arg), cond(where),
          check_null(chk_null), having(having_arg), unique(unique_arg)
    {};
    virtual bool exec();
    virtual void print (String *str, enum_query_type query_type);
    virtual enum_engine_type engine_type() const
    {
        return unique ? UNIQUESUBQUERY_ENGINE : INDEXSUBQUERY_ENGINE;
    }
    virtual void cleanup() {}
    virtual bool prepare();
    virtual void fix_length_and_dec(Item_cache **row);
    virtual uint cols() const
    {
        return 1;
    }
    virtual uint8 uncacheable() const
    {
        return UNCACHEABLE_DEPENDENT;
    }
    virtual void exclude();
    virtual table_map upper_select_const_tables() const
    {
        return 0;
    }
    virtual bool change_result(Item_subselect *si,
                               select_result_interceptor *result);
    virtual bool no_tables() const;
    bool scan_table();
    void copy_ref_key(bool *require_scan, bool *convert_error);
};

/*
  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(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. 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: public subselect_indexsubquery_engine
{
private:
    /* TRUE if the subquery was materialized into a temp table. */
    bool is_materialized;
    /**
       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;
    /*
      The old engine already chosen at parse time and stored in permanent memory.
      Through this member we can re-create and re-prepare the join object
      used to materialize the subquery 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;
    /* Temp table context of the outer select's JOIN. */
    TMP_TABLE_PARAM *tmp_param;

public:
    subselect_hash_sj_engine(THD *thd, Item_subselect *in_predicate,
                             subselect_single_select_engine *old_engine)
        : subselect_indexsubquery_engine(thd, NULL, in_predicate, NULL,
                                         NULL, false, true),
          is_materialized(false), materialize_engine(old_engine), tmp_param(NULL)
    {}
    ~subselect_hash_sj_engine();

    bool setup(List<Item> *tmp_columns);
    virtual void cleanup();
    virtual bool prepare()
    {
        return materialize_engine->prepare();
    }
    virtual bool exec();
    virtual void print (String *str, enum_query_type query_type);
    virtual uint cols() const
    {
        return materialize_engine->cols();
    }
    virtual enum_engine_type engine_type() const
    {
        return HASH_SJ_ENGINE;
    }

    const st_join_table *get_join_tab() const
    {
        return tab;
    }
    Item *get_cond_for_explain() const
    {
        return cond;
    }
};
#endif /* ITEM_SUBSELECT_INCLUDED */