~drizzle-trunk/drizzle/development

/* -*- mode: c++; c-basic-offset: 2; indent-tabs-mode: nil; -*-

 *  vim:expandtab:shiftwidth=2:tabstop=2:smarttab:

 *

 *  Copyright (C) 2008 Sun Microsystems

 *

 *  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  02110-1301  USA

 */

class Protocol;

struct TableList;

void item_init(void);			/* Init item functions */

class Item_field;

/*

   "Declared Type Collation"

   A combination of collation and its derivation.

  Flags for collation aggregation modes:

  MY_COLL_ALLOW_SUPERSET_CONV  - allow conversion to a superset

  MY_COLL_ALLOW_COERCIBLE_CONV - allow conversion of a coercible value

                                 (i.e. constant).

  MY_COLL_ALLOW_CONV           - allow any kind of conversion

                                 (combination of the above two)

  MY_COLL_DISALLOW_NONE        - don't allow return DERIVATION_NONE

                                 (e.g. when aggregating for comparison)

  MY_COLL_CMP_CONV             - combination of MY_COLL_ALLOW_CONV

                                 and MY_COLL_DISALLOW_NONE

*/

#define MY_COLL_ALLOW_SUPERSET_CONV   1

#define MY_COLL_ALLOW_COERCIBLE_CONV  2

#define MY_COLL_ALLOW_CONV            3

#define MY_COLL_DISALLOW_NONE         4

#define MY_COLL_CMP_CONV              7

class DTCollation {

public:

  const CHARSET_INFO *collation;

  enum Derivation derivation;

  uint repertoire;

  void set_repertoire_from_charset(const CHARSET_INFO * const cs)

  {

    repertoire= cs->state & MY_CS_PUREASCII ?

                MY_REPERTOIRE_ASCII : MY_REPERTOIRE_UNICODE30;

  }

  DTCollation()

  {

    collation= &my_charset_bin;

    derivation= DERIVATION_NONE;

    repertoire= MY_REPERTOIRE_UNICODE30;

  }

  DTCollation(const CHARSET_INFO * const collation_arg, Derivation derivation_arg)

  {

    collation= collation_arg;

    derivation= derivation_arg;

    set_repertoire_from_charset(collation_arg);

  }

  void set(DTCollation &dt)

  { 

    collation= dt.collation;

    derivation= dt.derivation;

    repertoire= dt.repertoire;

  }

  void set(const CHARSET_INFO * const collation_arg, Derivation derivation_arg)

  {

    collation= collation_arg;

    derivation= derivation_arg;

    set_repertoire_from_charset(collation_arg);

  }

  void set(const CHARSET_INFO * const collation_arg,

           Derivation derivation_arg,

           uint repertoire_arg)

  {

    collation= collation_arg;

    derivation= derivation_arg;

    repertoire= repertoire_arg;

  }

  void set(const CHARSET_INFO * const collation_arg)

  {

    collation= collation_arg;

    set_repertoire_from_charset(collation_arg);

  }

  void set(Derivation derivation_arg)

  { derivation= derivation_arg; }

  bool aggregate(DTCollation &dt, uint flags= 0);

  bool set(DTCollation &dt1, DTCollation &dt2, uint flags= 0)

  { set(dt1); return aggregate(dt2, flags); }

  const char *derivation_name() const

  {

    switch(derivation)

    {

      case DERIVATION_IGNORABLE: return "IGNORABLE";

      case DERIVATION_COERCIBLE: return "COERCIBLE";

      case DERIVATION_IMPLICIT:  return "IMPLICIT";

      case DERIVATION_SYSCONST:  return "SYSCONST";

      case DERIVATION_EXPLICIT:  return "EXPLICIT";

      case DERIVATION_NONE:      return "NONE";

      default: return "UNKNOWN";

    }

  }

};

/*************************************************************************/

/*

  A framework to easily handle different return types for hybrid items

  (hybrid item is an item whose operand can be of any type, e.g. integer,

  real, decimal).

*/

struct Hybrid_type_traits;

struct Hybrid_type

{

  int64_t integer;

  double real;

  /*

    Use two decimal buffers interchangeably to speed up += operation

    which has no native support in decimal library.

    Hybrid_type+= arg is implemented as dec_buf[1]= dec_buf[0] + arg.

    The third decimal is used as a handy temporary storage.

  */

  my_decimal dec_buf[3];

  int used_dec_buf_no;

  /*

    Traits moved to a separate class to

      a) be able to easily change object traits in runtime

      b) they work as a differentiator for the union above

  */

  const Hybrid_type_traits *traits;

  Hybrid_type() {}

  /* XXX: add traits->copy() when needed */

  Hybrid_type(const Hybrid_type &rhs) :traits(rhs.traits) {}

};

/* Hybryd_type_traits interface + default implementation for REAL_RESULT */

struct Hybrid_type_traits

{

  virtual Item_result type() const { return REAL_RESULT; }

  virtual void

  fix_length_and_dec(Item *item, Item *arg) const;

  /* Hybrid_type operations. */

  virtual void set_zero(Hybrid_type *val) const { val->real= 0.0; }

  virtual void add(Hybrid_type *val, Field *f) const

  { val->real+= f->val_real(); }

  virtual void div(Hybrid_type *val, uint64_t u) const

  { val->real/= uint64_t2double(u); }

  virtual int64_t val_int(Hybrid_type *val,

                           bool unsigned_flag __attribute__((unused))) const

  { return (int64_t) rint(val->real); }

  virtual double val_real(Hybrid_type *val) const { return val->real; }

  virtual my_decimal *val_decimal(Hybrid_type *val, my_decimal *buf) const;

  virtual String *val_str(Hybrid_type *val, String *buf, uint8_t decimals) const;

  static const Hybrid_type_traits *instance();

  Hybrid_type_traits() {}

  virtual ~Hybrid_type_traits() {}

};

struct Hybrid_type_traits_decimal: public Hybrid_type_traits

{

  virtual Item_result type() const { return DECIMAL_RESULT; }

  virtual void

  fix_length_and_dec(Item *arg, Item *item) const;

  /* Hybrid_type operations. */

  virtual void set_zero(Hybrid_type *val) const;

  virtual void add(Hybrid_type *val, Field *f) const;

  virtual void div(Hybrid_type *val, uint64_t u) const;

  virtual int64_t val_int(Hybrid_type *val, bool unsigned_flag) const;

  virtual double val_real(Hybrid_type *val) const;

  virtual my_decimal *val_decimal(Hybrid_type *val,

                                  my_decimal *buf __attribute__((unused))) const

  { return &val->dec_buf[val->used_dec_buf_no]; }

  virtual String *val_str(Hybrid_type *val, String *buf, uint8_t decimals) const;

  static const Hybrid_type_traits_decimal *instance();

  Hybrid_type_traits_decimal() {};

};

struct Hybrid_type_traits_integer: public Hybrid_type_traits

{

  virtual Item_result type() const { return INT_RESULT; }

  virtual void

  fix_length_and_dec(Item *arg, Item *item) const;

  /* Hybrid_type operations. */

  virtual void set_zero(Hybrid_type *val) const

  { val->integer= 0; }

  virtual void add(Hybrid_type *val, Field *f) const

  { val->integer+= f->val_int(); }

  virtual void div(Hybrid_type *val, uint64_t u) const

  { val->integer/= (int64_t) u; }

  virtual int64_t val_int(Hybrid_type *val,

                           bool unsigned_flag __attribute__((unused))) const

  { return val->integer; }

  virtual double val_real(Hybrid_type *val) const

  { return (double) val->integer; }

  virtual my_decimal *val_decimal(Hybrid_type *val,

                                  my_decimal *buf __attribute__((unused))) const

  {

    int2my_decimal(E_DEC_FATAL_ERROR, val->integer, 0, &val->dec_buf[2]);

    return &val->dec_buf[2];

  }

  virtual String *val_str(Hybrid_type *val, String *buf,

                          uint8_t decimals __attribute__((unused))) const

  { buf->set(val->integer, &my_charset_bin); return buf;}

  static const Hybrid_type_traits_integer *instance();

  Hybrid_type_traits_integer() {};

};

void dummy_error_processor(THD *thd, void *data);

void view_error_processor(THD *thd, void *data);

/*

  Instances of Name_resolution_context store the information necesary for

  name resolution of Items and other context analysis of a query made in

  fix_fields().

  This structure is a part of SELECT_LEX, a pointer to this structure is

  assigned when an item is created (which happens mostly during  parsing

  (sql_yacc.yy)), but the structure itself will be initialized after parsing

  is complete

  TODO: move subquery of INSERT ... SELECT and CREATE ... SELECT to

  separate SELECT_LEX which allow to remove tricks of changing this

  structure before and after INSERT/CREATE and its SELECT to make correct

  field name resolution.

*/

struct Name_resolution_context: Sql_alloc

{

  /*

    The name resolution context to search in when an Item cannot be

    resolved in this context (the context of an outer select)

  */

  Name_resolution_context *outer_context;

  /*

    List of tables used to resolve the items of this context.  Usually these

    are tables from the FROM clause of SELECT statement.  The exceptions are

    INSERT ... SELECT and CREATE ... SELECT statements, where SELECT

    subquery is not moved to a separate SELECT_LEX.  For these types of

    statements we have to change this member dynamically to ensure correct

    name resolution of different parts of the statement.

  */

  TableList *table_list;

  /*

    In most cases the two table references below replace 'table_list' above

    for the purpose of name resolution. The first and last name resolution

    table references allow us to search only in a sub-tree of the nested

    join tree in a FROM clause. This is needed for NATURAL JOIN, JOIN ... USING

    and JOIN ... ON. 

  */

  TableList *first_name_resolution_table;

  /*

    Last table to search in the list of leaf table references that begins

    with first_name_resolution_table.

  */

  TableList *last_name_resolution_table;

  /*

    SELECT_LEX item belong to, in case of merged VIEW it can differ from

    SELECT_LEX where item was created, so we can't use table_list/field_list

    from there

  */

  st_select_lex *select_lex;

  /*

    Processor of errors caused during Item name resolving, now used only to

    hide underlying tables in errors about views (i.e. it substitute some

    errors for views)

  */

  void (*error_processor)(THD *, void *);

  void *error_processor_data;

  /*

    When true items are resolved in this context both against the

    SELECT list and this->table_list. If false, items are resolved

    only against this->table_list.

  */

  bool resolve_in_select_list;

  /*

    Security context of this name resolution context. It's used for views

    and is non-zero only if the view is defined with SQL SECURITY DEFINER.

  */

  Security_context *security_ctx;

  Name_resolution_context()

    :outer_context(0), table_list(0), select_lex(0),

    error_processor_data(0),

    security_ctx(0)

    {}

  void init()

  {

    resolve_in_select_list= false;

    error_processor= &dummy_error_processor;

    first_name_resolution_table= NULL;

    last_name_resolution_table= NULL;

  }

  void resolve_in_table_list_only(TableList *tables)

  {

    table_list= first_name_resolution_table= tables;

    resolve_in_select_list= false;

  }

  void process_error(THD *thd)

  {

    (*error_processor)(thd, error_processor_data);

  }

};

/*

  Store and restore the current state of a name resolution context.

*/

class Name_resolution_context_state

{

private:

  TableList *save_table_list;

  TableList *save_first_name_resolution_table;

  TableList *save_next_name_resolution_table;

  bool        save_resolve_in_select_list;

  TableList *save_next_local;

public:

  Name_resolution_context_state() {}          /* Remove gcc warning */

public:

  /* Save the state of a name resolution context. */

  void save_state(Name_resolution_context *context, TableList *table_list)

  {

    save_table_list=                  context->table_list;

    save_first_name_resolution_table= context->first_name_resolution_table;

    save_resolve_in_select_list=      context->resolve_in_select_list;

    save_next_local=                  table_list->next_local;

    save_next_name_resolution_table=  table_list->next_name_resolution_table;

  }

  /* Restore a name resolution context from saved state. */

  void restore_state(Name_resolution_context *context, TableList *table_list)

  {

    table_list->next_local=                save_next_local;

    table_list->next_name_resolution_table= save_next_name_resolution_table;

    context->table_list=                   save_table_list;

    context->first_name_resolution_table=  save_first_name_resolution_table;

    context->resolve_in_select_list=       save_resolve_in_select_list;

  }

  TableList *get_first_name_resolution_table()

  {

    return save_first_name_resolution_table;

  }

};

/*

  This enum is used to report information about monotonicity of function

  represented by Item* tree.

  Monotonicity is defined only for Item* trees that represent table

  partitioning expressions (i.e. have no subselects/user vars/PS parameters

  etc etc). An Item* tree is assumed to have the same monotonicity properties

  as its correspoinding function F:

  [signed] int64_t F(field1, field2, ...) {

    put values of field_i into table record buffer;

    return item->val_int(); 

  }

  NOTE

  At the moment function monotonicity is not well defined (and so may be

  incorrect) for Item trees with parameters/return types that are different

  from INT_RESULT, may be NULL, or are unsigned.

  It will be possible to address this issue once the related partitioning bugs

  (BUG#16002, BUG#15447, BUG#13436) are fixed.

*/

typedef enum monotonicity_info 

{

   NON_MONOTONIC,              /* none of the below holds */

   MONOTONIC_INCREASING,       /* F() is unary and (x < y) => (F(x) <= F(y)) */

   MONOTONIC_STRICT_INCREASING /* F() is unary and (x < y) => (F(x) <  F(y)) */

} enum_monotonicity_info;

/*************************************************************************/

typedef bool (Item::*Item_processor) (uchar *arg);

/*

  Analyzer function

    SYNOPSIS

      argp   in/out IN:  Analysis parameter

                    OUT: Parameter to be passed to the transformer

    RETURN 

      true   Invoke the transformer

      false  Don't do it

*/

typedef bool (Item::*Item_analyzer) (uchar **argp);

typedef Item* (Item::*Item_transformer) (uchar *arg);

typedef void (*Cond_traverser) (const Item *item, void *arg);

class Item

{

  Item(const Item &);			/* Prevent use of these */

  void operator=(Item &);

  /* Cache of the result of is_expensive(). */

  int8_t is_expensive_cache;

  virtual bool is_expensive_processor(uchar *arg __attribute__((unused)))

  { return 0; }

public:

  static void *operator new(size_t size)

  { return sql_alloc(size); }

  static void *operator new(size_t size, MEM_ROOT *mem_root)

  { return alloc_root(mem_root, size); }

  static void operator delete(void *ptr __attribute__((unused)),

                              size_t size __attribute__((unused)))

  { TRASH(ptr, size); }

  static void operator delete(void *ptr __attribute__((unused)),

                              MEM_ROOT *mem_root __attribute__((unused)))

  {}

  enum Type {FIELD_ITEM= 0, FUNC_ITEM, SUM_FUNC_ITEM, STRING_ITEM,

	     INT_ITEM, REAL_ITEM, NULL_ITEM, VARBIN_ITEM,

	     COPY_STR_ITEM, FIELD_AVG_ITEM, DEFAULT_VALUE_ITEM,

	     PROC_ITEM,COND_ITEM, REF_ITEM, FIELD_STD_ITEM,

	     FIELD_VARIANCE_ITEM, INSERT_VALUE_ITEM,

             SUBSELECT_ITEM, ROW_ITEM, CACHE_ITEM, TYPE_HOLDER,

             PARAM_ITEM, DECIMAL_ITEM,

             VIEW_FIXER_ITEM};

  enum cond_result { COND_UNDEF,COND_OK,COND_TRUE,COND_FALSE };

  enum traverse_order { POSTFIX, PREFIX };

  /* Reuse size, only used by SP local variable assignment, otherwize 0 */

  uint rsize;

  /*

    str_values's main purpose is to be used to cache the value in

    save_in_field

  */

  String str_value;

  char * name;			/* Name from select */

  /* Original item name (if it was renamed)*/

  char * orig_name;

  Item *next;

  uint32_t max_length;

  uint name_length;                     /* Length of name */

  int8_t marker;

  uint8_t decimals;

  bool maybe_null;			/* If item may be null */

  bool null_value;			/* if item is null */

  bool unsigned_flag;

  bool with_sum_func;

  bool fixed;                        /* If item fixed with fix_fields */

  bool is_autogenerated_name;        /* indicate was name of this Item

                                           autogenerated or set by user */

  DTCollation collation;

  bool with_subselect;               /* If this item is a subselect or some

                                           of its arguments is or contains a

                                           subselect. Computed by fix_fields. */

  Item_result cmp_context;              /* Comparison context */

  // alloc & destruct is done as start of select using sql_alloc

  Item();

  /*

     Constructor used by Item_field, Item_ref & aggregate (sum) functions.

     Used for duplicating lists in processing queries with temporary

     tables

     Also it used for Item_cond_and/Item_cond_or for creating

     top AND/OR structure of WHERE clause to protect it of

     optimisation changes in prepared statements

  */

  Item(THD *thd, Item *item);

  virtual ~Item()

  {

#ifdef EXTRA_DEBUG

    name=0;

#endif

  }		/*lint -e1509 */

  void set_name(const char *str, uint length, const CHARSET_INFO * const cs);

  void rename(char *new_name);

  void init_make_field(Send_field *tmp_field,enum enum_field_types type);

  virtual void cleanup();

  virtual void make_field(Send_field *field);

  Field *make_string_field(Table *table);

  virtual bool fix_fields(THD *, Item **);

  /*

    Fix after some tables has been pulled out. Basically re-calculate all

    attributes that are dependent on the tables.

  */

  virtual void fix_after_pullout(st_select_lex *new_parent __attribute__((unused)),

                                 Item **ref __attribute__((unused))) {};

  /*

    should be used in case where we are sure that we do not need

    complete fix_fields() procedure.

  */

  inline void quick_fix_field() { fixed= 1; }

  /* Function returns 1 on overflow and -1 on fatal errors */

  int save_in_field_no_warnings(Field *field, bool no_conversions);

  virtual int save_in_field(Field *field, bool no_conversions);

  virtual void save_org_in_field(Field *field)

  { (void) save_in_field(field, 1); }

  virtual int save_safe_in_field(Field *field)

  { return save_in_field(field, 1); }

  virtual bool send(Protocol *protocol, String *str);

  virtual bool eq(const Item *, bool binary_cmp) const;

  virtual Item_result result_type() const { return REAL_RESULT; }

  virtual Item_result cast_to_int_type() const { return result_type(); }

  virtual enum_field_types string_field_type() const;

  virtual enum_field_types field_type() const;

  virtual enum Type type() const =0;

  /*

    Return information about function monotonicity. See comment for

    enum_monotonicity_info for details. This function can only be called

    after fix_fields() call.

  */

  virtual enum_monotonicity_info get_monotonicity_info() const

  { return NON_MONOTONIC; }

  /*

    Convert "func_arg $CMP$ const" half-interval into "FUNC(func_arg) $CMP2$ const2"

    SYNOPSIS

      val_int_endpoint()

        left_endp  false  <=> The interval is "x < const" or "x <= const"

                   true   <=> The interval is "x > const" or "x >= const"

        incl_endp  IN   true <=> the comparison is '<' or '>'

                        false <=> the comparison is '<=' or '>='

                   OUT  The same but for the "F(x) $CMP$ F(const)" comparison

    DESCRIPTION

      This function is defined only for unary monotonic functions. The caller

      supplies the source half-interval

         x $CMP$ const

      The value of const is supplied implicitly as the value this item's

      argument, the form of $CMP$ comparison is specified through the

      function's arguments. The calle returns the result interval

         F(x) $CMP2$ F(const)

      passing back F(const) as the return value, and the form of $CMP2$ 

      through the out parameter. NULL values are assumed to be comparable and

      be less than any non-NULL values.

    RETURN

      The output range bound, which equal to the value of val_int()

        - If the value of the function is NULL then the bound is the

          smallest possible value of INT64_MIN

  */

  virtual int64_t val_int_endpoint(bool left_endp __attribute__((unused)),

                                    bool *incl_endp __attribute__((unused)))

  { assert(0); return 0; }

  /* valXXX methods must return NULL or 0 or 0.0 if null_value is set. */

  /*

    Return double precision floating point representation of item.

    SYNOPSIS

      val_real()

    RETURN

      In case of NULL value return 0.0 and set null_value flag to true.

      If value is not null null_value flag will be reset to false.

  */

  virtual double val_real()=0;

  /*

    Return integer representation of item.

    SYNOPSIS

      val_int()

    RETURN

      In case of NULL value return 0 and set null_value flag to true.

      If value is not null null_value flag will be reset to false.

  */

  virtual int64_t val_int()=0;

  /*

    This is just a shortcut to avoid the cast. You should still use

    unsigned_flag to check the sign of the item.

  */

  inline uint64_t val_uint() { return (uint64_t) val_int(); }

  /*

    Return string representation of this item object.

    SYNOPSIS

      val_str()

      str   an allocated buffer this or any nested Item object can use to

            store return value of this method.

    NOTE

      Buffer passed via argument  should only be used if the item itself

      doesn't have an own String buffer. In case when the item maintains

      it's own string buffer, it's preferable to return it instead to

      minimize number of mallocs/memcpys.

      The caller of this method can modify returned string, but only in case

      when it was allocated on heap, (is_alloced() is true).  This allows

      the caller to efficiently use a buffer allocated by a child without

      having to allocate a buffer of it's own. The buffer, given to

      val_str() as argument, belongs to the caller and is later used by the

      caller at it's own choosing.

      A few implications from the above:

      - unless you return a string object which only points to your buffer

        but doesn't manages it you should be ready that it will be

        modified.

      - even for not allocated strings (is_alloced() == false) the caller

        can change charset (see Item_func_{typecast/binary}. XXX: is this

        a bug?

      - still you should try to minimize data copying and return internal

        object whenever possible.

    RETURN

      In case of NULL value return 0 (NULL pointer) and set null_value flag

      to true.

      If value is not null null_value flag will be reset to false.

  */

  virtual String *val_str(String *str)=0;

  /*

    Return decimal representation of item with fixed point.

    SYNOPSIS

      val_decimal()

      decimal_buffer  buffer which can be used by Item for returning value

                      (but can be not)

    NOTE

      Returned value should not be changed if it is not the same which was

      passed via argument.

    RETURN

      Return pointer on my_decimal (it can be other then passed via argument)

        if value is not NULL (null_value flag will be reset to false).

      In case of NULL value it return 0 pointer and set null_value flag

        to true.

  */

  virtual my_decimal *val_decimal(my_decimal *decimal_buffer)= 0;

  /*

    Return boolean value of item.

    RETURN

      false value is false or NULL

      true value is true (not equal to 0)

  */

  virtual bool val_bool();

  virtual String *val_nodeset(String*) { return 0; }

  /* Helper functions, see item_sum.cc */

  String *val_string_from_real(String *str);

  String *val_string_from_int(String *str);

  String *val_string_from_decimal(String *str);

  my_decimal *val_decimal_from_real(my_decimal *decimal_value);

  my_decimal *val_decimal_from_int(my_decimal *decimal_value);

  my_decimal *val_decimal_from_string(my_decimal *decimal_value);

  my_decimal *val_decimal_from_date(my_decimal *decimal_value);

  my_decimal *val_decimal_from_time(my_decimal *decimal_value);

  int64_t val_int_from_decimal();

  double val_real_from_decimal();

  int save_time_in_field(Field *field);

  int save_date_in_field(Field *field);

  int save_str_value_in_field(Field *field, String *result);

  virtual Field *get_tmp_table_field(void) { return 0; }

  /* This is also used to create fields in CREATE ... SELECT: */

  virtual Field *tmp_table_field(Table *t_arg __attribute__((unused)))

  { return 0; }

  virtual const char *full_name(void) const { return name ? name : "???"; }

  /*

    *result* family of methods is analog of *val* family (see above) but

    return value of result_field of item if it is present. If Item have not

    result field, it return val(). This methods set null_value flag in same

    way as *val* methods do it.

  */

  virtual double  val_result() { return val_real(); }

  virtual int64_t val_int_result() { return val_int(); }

  virtual String *str_result(String* tmp) { return val_str(tmp); }

  virtual my_decimal *val_decimal_result(my_decimal *val)

  { return val_decimal(val); }

  virtual bool val_bool_result() { return val_bool(); }

  /* bit map of tables used by item */

  virtual table_map used_tables() const { return (table_map) 0L; }

  /*

    Return table map of tables that can't be NULL tables (tables that are

    used in a context where if they would contain a NULL row generated

    by a LEFT or RIGHT join, the item would not be true).

    This expression is used on WHERE item to determinate if a LEFT JOIN can be

    converted to a normal join.

    Generally this function should return used_tables() if the function

    would return null if any of the arguments are null

    As this is only used in the beginning of optimization, the value don't

    have to be updated in update_used_tables()

  */

  virtual table_map not_null_tables() const { return used_tables(); }

  /*

    Returns true if this is a simple constant item like an integer, not

    a constant expression. Used in the optimizer to propagate basic constants.

  */

  virtual bool basic_const_item() const { return 0; }

  /* cloning of constant items (0 if it is not const) */

  virtual Item *clone_item() { return 0; }

  virtual cond_result eq_cmp_result() const { return COND_OK; }

  inline uint float_length(uint decimals_par) const

  { return decimals != NOT_FIXED_DEC ? (DBL_DIG+2+decimals_par) : DBL_DIG+8;}

  virtual uint decimal_precision() const;

  inline int decimal_int_part() const

  { return my_decimal_int_part(decimal_precision(), decimals); }

  /* 

    Returns true if this is constant (during query execution, i.e. its value

    will not change until next fix_fields) and its value is known.

  */

  virtual bool const_item() const { return used_tables() == 0; }

  /* 

    Returns true if this is constant but its value may be not known yet.

    (Can be used for parameters of prep. stmts or of stored procedures.)

  */

  virtual bool const_during_execution() const 

  { return (used_tables() & ~PARAM_TABLE_BIT) == 0; }

  /**

    This method is used for to:

      - to generate a view definition query (SELECT-statement);

      - to generate a SQL-query for EXPLAIN EXTENDED;

      - to generate a SQL-query to be shown in INFORMATION_SCHEMA;

      - debug.

    For more information about view definition query, INFORMATION_SCHEMA

    query and why they should be generated from the Item-tree, @see

    mysql_register_view().

  */

  virtual inline void print(String *str,

                            enum_query_type query_type __attribute__((unused)))

  {

    str->append(full_name());

  }

  void print_item_w_name(String *, enum_query_type query_type);

  virtual void update_used_tables() {}

  virtual void split_sum_func(THD *thd __attribute__((unused)),

                              Item **ref_pointer_array __attribute__((unused)),

                              List<Item> &fields __attribute__((unused))) {}

  /* Called for items that really have to be split */

  void split_sum_func2(THD *thd, Item **ref_pointer_array, List<Item> &fields,

                       Item **ref, bool skip_registered);