Why can't a text column have a default value in MySQL?

Question

If you try to create a TEXT column on a table, and give it a default value in MySQL, you get an error (on Windows at least). I cannot see any reason why a text column should not have a default value. No explanation is given by the MySQL documentation. It seems illogical to me (and somewhat frustrating, as I want a default value!). Anybody know why this is not allowed?

Answer 1

Without any deep knowledge of the mySQL engine, I'd say this sounds like a memory saving strategy. I assume the reason is behind this paragraph from the docs:

Each BLOB or TEXT value is represented internally by a separately allocated object. This is in contrast to all other data types, for which storage is allocated once per column when the table is opened.

It seems like pre-filling these column types would lead to memory usage and performance penalties.

Answer 2

Are you sure you want a TEXT column, not a VARCHAR one? TEXT columns are for things which can become more than 255 bytes long.

Answer 3

What exactly are you doing? A quick test here showed that a text column can quite happily have a default value:

    mysql> create table test (a char(32) not null default 'this works', b char(32) not null);
    Query OK, 0 rows affected (0.03 sec)
    mysql> insert into test (b) values('hello');
    Query OK, 1 row affected (0.01 sec)
    mysql> select * from test;

+------------+-------+
| a          | b     |
+------------+-------+
| this works | hello |
+------------+-------+
1 row in set (0.00 sec)