Bug#28427: Columns were renamed instead of moving by ALTER TABLE.

To avoid unnecessary work the mysql_alter_table function takes the
list of table fields and applies all changes to it (drops/moves/renames/etc).
Then this function compares the new list and the old one. If the changes
require only .frm to be modified then the actual data isn't copied. To detect
changes all columns attributes but names are compared. When a column has been
moved and has replaced another column with the same attributes except name
the mysql_alter_table function wrongly decides that two fields has been just
renamed. As a result the data from the moved column and from all columns
after it is not copied.

Now the mysql_alter_table function forces table data copying by setting
the need_copy_table flag when it finds a moved column. The flag is set at
the stage when the modified fields are created.

mysql-test/r/alter_table.result

@@ -1103,3 +1103,17 @@ Field	Type	Null	Key	Default	Extra
 unsigned_int_field	bigint(20) unsigned	NO	MUL		
 char_field	char(10)	YES		NULL	
 DROP TABLE t2;
+CREATE TABLE t1 (f1 INT, f2 INT, f3 INT);
+INSERT INTO t1 VALUES (1, 2, NULL);
+SELECT * FROM t1;
+f1	f2	f3
+1	2	NULL
+ALTER TABLE t1 MODIFY COLUMN f3 INT AFTER f1;
+SELECT * FROM t1;
+f1	f3	f2
+1	NULL	2
+ALTER TABLE t1 MODIFY COLUMN f3 INT AFTER f2;
+SELECT * FROM t1;
+f1	f2	f3
+1	2	NULL
+DROP TABLE t1;

mysql-test/t/alter_table.test

@@ -839,3 +839,15 @@ ALTER TABLE t2 MODIFY unsigned_int_field BIGINT UNSIGNED NOT NULL;
 DESCRIBE t2;
 
 DROP TABLE t2;
+
+#
+# Bug#28427: Columns were renamed instead of moving by ALTER TABLE.
+#
+CREATE TABLE t1 (f1 INT, f2 INT, f3 INT);
+INSERT INTO t1 VALUES (1, 2, NULL);
+SELECT * FROM t1;
+ALTER TABLE t1 MODIFY COLUMN f3 INT AFTER f1;
+SELECT * FROM t1;
+ALTER TABLE t1 MODIFY COLUMN f3 INT AFTER f2;
+SELECT * FROM t1;
+DROP TABLE t1;

sql/sql_table.cc

@@ -5932,6 +5932,7 @@ view_err:
         goto err;
       }
       find_it.after(def);			// Put element after this
+      need_copy_table= ALTER_TABLE_DATA_CHANGED;
     }
   }
   if (alter_info->alter_list.elements)
@@ -6170,12 +6171,14 @@ view_err:
            (uint*) thd->alloc(sizeof(uint) * prepared_key_list.elements)))
       goto err;
     /* Check how much the tables differ. */
-    need_copy_table= compare_tables(table, &prepared_create_list,
+    bool res= compare_tables(table, &prepared_create_list,
                                     key_info_buffer, key_count,
                                     create_info, alter_info, order_num,
                                     index_drop_buffer, &index_drop_count,
                                     index_add_buffer, &index_add_count,
                                     varchar);
+    if (!need_copy_table)
+      need_copy_table= res;
   }
 
   /*