Many files:

   Coercibility, initial stage
item_func.h:
  Coercibility, initial stage

sql/item.cc

@@ -39,6 +39,7 @@ Item::Item():
 {
   marker= 0;
   maybe_null=null_value=with_sum_func=unsigned_flag=0;
+  coercibility=COER_NOCOLL;
   name= 0;
   decimals= 0; max_length= 0;
   THD *thd= current_thd;
@@ -63,7 +64,8 @@ Item::Item(THD *thd, Item &item):
   null_value(item.null_value),
   unsigned_flag(item.unsigned_flag),
   with_sum_func(item.with_sum_func),
-  fixed(item.fixed)
+  fixed(item.fixed),
+  coercibility(item.coercibility)
 {
   next=thd->free_list;			// Put in free list
   thd->free_list= this;
@@ -169,6 +171,7 @@ CHARSET_INFO * Item::thd_charset() const
 Item_field::Item_field(Field *f) :Item_ident(NullS,f->table_name,f->field_name)
 {
   set_field(f);
+  coercibility= COER_IMPLICIT;
   fixed= 1; // This item is not needed in fix_fields
 }
 
@@ -177,7 +180,7 @@ Item_field::Item_field(THD *thd, Item_field &item):
   Item_ident(thd, item),
   field(item.field),
   result_field(item.result_field)
-{}
+{ coercibility= COER_IMPLICIT; }
 
 void Item_field::set_field(Field *field_par)
 {
@@ -189,6 +192,7 @@ void Item_field::set_field(Field *field_par)
   field_name=field_par->field_name;
   unsigned_flag=test(field_par->flags & UNSIGNED_FLAG);
   set_charset(field_par->charset());
+  coercibility= COER_IMPLICIT;
 }
 
 const char *Item_ident::full_name() const

sql/item.h

@@ -39,6 +39,8 @@ public:
              SUBSELECT_ITEM, ROW_ITEM, CACHE_ITEM};
 
   enum cond_result { COND_UNDEF,COND_OK,COND_TRUE,COND_FALSE };
+  enum coercion    { COER_NOCOLL=0,   COER_COERCIBLE=1, 
+  		     COER_IMPLICIT=2, COER_EXPLICIT=3  };
 
   String str_value;			/* used to store value */
   my_string name;			/* Name from select */
@@ -50,6 +52,7 @@ public:
   my_bool unsigned_flag;
   my_bool with_sum_func;
   my_bool fixed;                        /* If item fixed with fix_fields */
+  enum coercion coercibility;		/* Precedence order of collation */
 
   // alloc & destruct is done as start of select using sql_alloc
   Item();
@@ -155,7 +158,7 @@ public:
   Item_field(const char *db_par,const char *table_name_par,
 	     const char *field_name_par)
     :Item_ident(db_par,table_name_par,field_name_par),field(0),result_field(0)
-  {}
+  { coercibility= COER_IMPLICIT; }
   // Constructor need to process subselect with temporary tables (see Item)
   Item_field(THD *thd, Item_field &item);
   Item_field(Field *field);
@@ -350,17 +353,20 @@ public:
 class Item_string :public Item
 {
 public:
-  Item_string(const char *str,uint length,CHARSET_INFO *cs)
+  Item_string(const char *str,uint length,
+  	      CHARSET_INFO *cs, enum coercion coer= COER_COERCIBLE)
   {
     str_value.set(str,length,cs);
+    coercibility= coer;
     max_length=length;
     name=(char*) str_value.ptr();
     decimals=NOT_FIXED_DEC;
   }
   Item_string(const char *name_par, const char *str, uint length,
-	      CHARSET_INFO *cs)
+	      CHARSET_INFO *cs, enum coercion coer= COER_COERCIBLE)
   {
     str_value.set(str,length,cs);
+    coercibility= coer;
     max_length=length;
     name=(char*) name_par;
     decimals=NOT_FIXED_DEC;

sql/item_create.cc

@@ -230,6 +230,11 @@ Item *create_func_bit_length(Item* a)
   return new Item_func_bit_length(a);
 }
 
+Item *create_func_coercibility(Item* a)
+{
+  return new Item_func_coercibility(a);
+}
+
 Item *create_func_char_length(Item* a)
 {
   return new Item_func_char_length(a);

sql/item_create.h

@@ -25,6 +25,7 @@ Item *create_func_asin(Item* a);
 Item *create_func_bin(Item* a);
 Item *create_func_bit_count(Item* a);
 Item *create_func_bit_length(Item* a);
+Item *create_func_coercibility(Item* a);
 Item *create_func_ceiling(Item* a);
 Item *create_func_char_length(Item* a);
 Item *create_func_cast(Item *a, Item_cast cast_type);

sql/item_func.cc

@@ -107,6 +107,7 @@ Item_func::fix_fields(THD *thd, TABLE_LIST *tables, Item **ref)
     return 0;					// Fatal error if flag is set!
   if (arg_count)
   {						// Print purify happy
+    coercibility= COER_NOCOLL;
     for (arg=args, arg_end=args+arg_count; arg != arg_end ; arg++)
     {
       if ((*arg)->fix_fields(thd, tables, arg) ||
@@ -1004,6 +1005,16 @@ longlong Item_func_char_length::val_int()
   return (longlong) (!args[0]->binary()) ? res->numchars() : res->length();
 }
 
+longlong Item_func_coercibility::val_int()
+{
+  if (args[0]->null_value)
+  {
+    null_value= 1;
+    return 0;
+  }
+  null_value= 0;
+  return (longlong) args[0]->coercibility;
+}
 
 longlong Item_func_locate::val_int()
 {

sql/item_func.h

@@ -590,6 +590,15 @@ public:
   void fix_length_and_dec() { max_length=10; }
 };
 
+class Item_func_coercibility :public Item_int_func
+{
+public:
+  Item_func_coercibility(Item *a) :Item_int_func(a) {}
+  longlong val_int();
+  const char *func_name() const { return "coercibility"; }
+  void fix_length_and_dec() { max_length=10; }
+};
+
 class Item_func_locate :public Item_int_func
 {
   String value1,value2;

sql/lex.h

@@ -441,6 +441,7 @@ static SYMBOL sql_functions[] = {
   { "CHAR_LENGTH",	SYM(FUNC_ARG1),0,CREATE_FUNC(create_func_char_length)},
   { "CHARACTER_LENGTH", SYM(FUNC_ARG1),0,CREATE_FUNC(create_func_char_length)},
   { "COALESCE",		SYM(COALESCE),0,0},
+  { "COERCIBILITY",	SYM(FUNC_ARG1),0,CREATE_FUNC(create_func_coercibility)},
   { "CONCAT",		SYM(CONCAT),0,0},
   { "CONCAT_WS",	SYM(CONCAT_WS),0,0},
   { "CONNECTION_ID",	SYM(FUNC_ARG0),0,CREATE_FUNC(create_func_connection_id)},

sql/sql_yacc.yy

@@ -3778,7 +3778,7 @@ text_literal:
 	{ $$ = new Item_string($1.str,$1.length,
 			       YYTHD->variables.thd_charset); }
 	| UNDERSCORE_CHARSET TEXT_STRING
-	  { $$ = new Item_string($2.str,$2.length,Lex->charset); }
+	  { $$ = new Item_string($2.str,$2.length,Lex->charset,Item::COER_EXPLICIT); }
 	| text_literal TEXT_STRING
 	  { ((Item_string*) $1)->append($2.str,$2.length); };