Fix bug #30219.

This bug manifested itself for queries with grouping by columns of
the BIT type. It led to wrong comparisons of bit-field values and
wrong result sets.
Bit-field values never cannot be compared as binary values. Yet
the class Field_bit had an implementation of the cmp method that
compared bit-fields values as binary values. 
Also the get_image and set_image methods of the base class Field 
cannot be used for objects of the Field_bit class. 
Now these methods are declared as virtual and specific implementations
of the methods are provided for the class Field_bit.

mysql-test/r/type_bit.result

@@ -618,4 +618,36 @@ bit_field	int_field
 
	2
 handler t1 close;
 drop table t1;
+CREATE TABLE t1 (b BIT(2));
+INSERT INTO t1 (b) VALUES (1), (3), (0), (3);
+SELECT b+0, COUNT(DISTINCT b) FROM t1 GROUP BY b;
+b+0	COUNT(DISTINCT b)
+0	1
+1	1
+3	1
+DROP TABLE t1;
+CREATE TABLE t1 (b BIT(2), a VARCHAR(5));
+INSERT INTO t1 (b, a) VALUES (1, "x"), (3, "zz"), (0, "y"), (3, "z");
+SELECT b+0, COUNT(DISTINCT a) FROM t1 GROUP BY b;
+b+0	COUNT(DISTINCT a)
+0	1
+1	1
+3	2
+DROP TABLE t1;
+CREATE TABLE t1 (a CHAR(5), b BIT(2));
+INSERT INTO t1 (b, a) VALUES (1, "x"), (3, "zz"), (0, "y"), (3, "z");
+SELECT b+0, COUNT(DISTINCT a) FROM t1 GROUP BY b;
+b+0	COUNT(DISTINCT a)
+0	1
+1	1
+3	2
+DROP TABLE t1;
+CREATE TABLE t1 (a INT, b BIT(2));
+INSERT INTO t1 (b, a) VALUES (1, 1), (3, 2), (0, 3), (3, 4);
+SELECT b+0, COUNT(DISTINCT a) FROM t1 GROUP BY b;
+b+0	COUNT(DISTINCT a)
+0	1
+1	1
+3	2
+DROP TABLE t1;
 End of 5.0 tests

mysql-test/t/type_bit.test

@@ -272,4 +272,28 @@ handler t1 read a=(1);
 handler t1 close;
 drop table t1;
 
+#
+# Bug #30219: GROUP BY a column of the BIT type
+#
+
+CREATE TABLE t1 (b BIT(2));
+INSERT INTO t1 (b) VALUES (1), (3), (0), (3);
+SELECT b+0, COUNT(DISTINCT b) FROM t1 GROUP BY b;
+DROP TABLE t1;
+
+CREATE TABLE t1 (b BIT(2), a VARCHAR(5));
+INSERT INTO t1 (b, a) VALUES (1, "x"), (3, "zz"), (0, "y"), (3, "z");
+SELECT b+0, COUNT(DISTINCT a) FROM t1 GROUP BY b;
+DROP TABLE t1;
+
+CREATE TABLE t1 (a CHAR(5), b BIT(2));
+INSERT INTO t1 (b, a) VALUES (1, "x"), (3, "zz"), (0, "y"), (3, "z");
+SELECT b+0, COUNT(DISTINCT a) FROM t1 GROUP BY b;
+DROP TABLE t1;
+
+CREATE TABLE t1 (a INT, b BIT(2));
+INSERT INTO t1 (b, a) VALUES (1, 1), (3, 2), (0, 3), (3, 4);
+SELECT b+0, COUNT(DISTINCT a) FROM t1 GROUP BY b;
+DROP TABLE t1;
+
 --echo End of 5.0 tests

sql/field.h

@@ -231,9 +231,9 @@ public:
     if (null_ptr)
       null_ptr=ADD_TO_PTR(null_ptr,ptr_diff,uchar*);
   }
-  inline void get_image(char *buff,uint length, CHARSET_INFO *cs)
+  virtual void get_image(char *buff, uint length, CHARSET_INFO *cs)
     { memcpy(buff,ptr,length); }
-  inline void set_image(char *buff,uint length, CHARSET_INFO *cs)
+  virtual void set_image(char *buff,uint length, CHARSET_INFO *cs)
     { memcpy(ptr,buff,length); }
 
 
@@ -1430,13 +1430,20 @@ public:
   String *val_str(String*, String *);
   my_decimal *val_decimal(my_decimal *);
   int cmp(const char *a, const char *b)
-  { return cmp_binary(a, b); }
+  { 
+    DBUG_ASSERT(ptr == a);
+    return Field_bit::key_cmp(b, bytes_in_rec+test(bit_len));
+  }
   int key_cmp(const byte *a, const byte *b)
   { return cmp_binary((char *) a, (char *) b); }
   int key_cmp(const byte *str, uint length);
   int cmp_offset(uint row_offset);
   int cmp_binary_offset(uint row_offset)
   { return cmp_offset(row_offset); }
+  void get_image(char *buff, uint length, CHARSET_INFO *cs)
+  { get_key_image(buff, length, itRAW); }   
+  void set_image(char *buff,uint length, CHARSET_INFO *cs)
+  { Field_bit::store(buff, length, cs); }
   uint get_key_image(char *buff, uint length, imagetype type);
   void set_key_image(char *buff, uint length)
   { Field_bit::store(buff, length, &my_charset_bin); }