Why does MySQL create temporay tables for renaming enum values? - mysql

Question

I'm faced to rename a value of a enum value. According to the documentation enums are a
list of permitted values that are enumerated explicitly in the column specification at table creation time.
The ALTER TABLE Syntax documentation of MySQL states:
In most cases, ALTER TABLE makes a temporary copy of the original table.
However there are exceptions:
Changing the definition of an ENUM or SET column by adding new enumeration or set members to the end of the list of valid member values, as long as the storage size of the data type does not change.
or
Renaming a column.
For this exceptions no temporary table has to be created. Thus the ALTER query is faster.
From the statements above I concluded that MySQL won't have to create a temporary table for renaming just the value of an enum. After all: the corresponding (internal) number representation of the value stays the same. Therefore not even the assignment of a value within the table should be affected.
I ran some test queries on the expected behavior:
mysql> CREATE TABLE `test` (`type` enum('foo','bar') NOT NULL DEFAULT 'foo');
Query OK, 0 rows affected (0.01 sec)
mysql> show global status where Variable_name = 'Created_tmp_tables';
+--------------------+-------+
| Variable_name | Value |
+--------------------+-------+
| Created_tmp_tables | 253 |
+--------------------+-------+
1 row in set (0.00 sec)
mysql> ALTER TABLE test CHANGE `type` type enum('foo','baz') NOT NULL DEFAULT 'foo';
Query OK, 0 rows affected (0.01 sec)
mysql> show global status where Variable_name = 'Created_tmp_tables';
+--------------------+-------+
| Variable_name | Value |
+--------------------+-------+
| Created_tmp_tables | 254 |
+--------------------+-------+
1 row in set (0.00 sec)
The Created_tmp_tables shows that a temporary table was created during the altering process.
Is there a reason why MySQL behaves like this? Or is it just not optimized for such kind of operations?

Answer 1

I always forget how this works and every once in a while have the same question and am glad you asked this and posted the answer in your question :)
Changing the definition of an ENUM or SET column by adding new enumeration or set members to the end of the list of valid member values
is not the same thing as changing the ENUM value itself