Fix for bug #18014: data loss caused by altering decimal fields.

mysql-test/r/type_newdecimal.result

@@ -1397,3 +1397,13 @@ c1
 9999999999999999999999999999999999999999999999999999999999999999
 9999999999999999999999999999999999999999999999999999999999999999
 drop table t1;
+create table t1(a decimal(7,2));
+insert into t1 values(123.12);
+select * from t1;
+a
+123.12
+alter table t1 modify a decimal(10,2);
+select * from t1;
+a
+123.12
+drop table t1;

mysql-test/t/type_newdecimal.test

@@ -1095,3 +1095,14 @@ insert into t1 values(
 insert into t1 values(1e100);
 select * from t1;
 drop table t1;
+
+#
+# Bug #18014: problem with 'alter table'
+#
+
+create table t1(a decimal(7,2));
+insert into t1 values(123.12);
+select * from t1;
+alter table t1 modify a decimal(10,2);
+select * from t1;
+drop table t1;

sql/field.cc

@@ -2604,6 +2604,18 @@ void Field_new_decimal::sql_type(String &str) const
 }
 
 
+uint Field_new_decimal::is_equal(create_field *new_field)
+{
+  return ((new_field->sql_type == real_type()) &&
+          ((new_field->flags & UNSIGNED_FLAG) == 
+           (uint) (flags & UNSIGNED_FLAG)) &&
+          ((new_field->flags & AUTO_INCREMENT_FLAG) ==
+           (uint) (flags & AUTO_INCREMENT_FLAG)) &&
+          (new_field->length == max_length()) &&
+          (new_field->decimals == dec));
+}
+
+
 /****************************************************************************
 ** tiny int
 ****************************************************************************/

sql/field.h

@@ -514,6 +514,7 @@ public:
   uint32 max_length() { return field_length; }
   uint size_of() const { return sizeof(*this); } 
   uint32 pack_length() const { return (uint32) bin_size; }
+  uint is_equal(create_field *new_field);
 };