Maximum precision for DECIMAL column in MySQL

[Matt Solnit]

It looks like the maximum precision for a DECIMAL column is 17 in MySQL.
Is this documented anywhere?  I get the following results:

mysql> CREATE TABLE `scale_test6` (`a_dec` decimal(30,23) default NULL)
TYPE=MyISAM;

mysql> INSERT INTO scale_test6 (a_dec) VALUES
(12345678.123456789012345678901234);

mysql> SELECT * FROM scale_test6;
+----------------------------------+
| a_dec                            |
+----------------------------------+
| 12345678.12345678900000000000000 |
+----------------------------------+
1 row in set (0.00 sec)

Am I doing everything correctly?  I would have expected to see the
entire decimal stored correctly in the table.

-- Matt Solnit <[EMAIL PROTECTED]>