INNODB and strings

mysql> CREATE TABLE innodb_string_test (h varchar(512) ) TYPE=INNODB;
Query OK, 0 rows affected, 2 warnings (0.55 sec)

mysql> show warnings;
+---------+------+--------------------------------------------------------------------------+
| Level   | Code | Message                                                                  |
+---------+------+--------------------------------------------------------------------------+
| Warning | 1246 | Converting column 'h' from CHAR to TEXT                                  |
| Warning | 1287 | 'TYPE=storage_engine' is deprecated; use 'ENGINE=storage_engine' instead |
+---------+------+--------------------------------------------------------------------------+
2 rows in set (0.00 sec)

Technically innodb supports defining varchar's greater then 255 characters (utf8 are double bytes so don't assume that 255 characters mean bytes-FYI).

MySQL the server doesn't allow this.


INNODB as of mysql-5.0.3 store stings in COMPACT format. This increases CPU usage slightly but saves nearly double the diskspace, thus given an application twice the memory. (Smaller data, more of it fits in memory).

To turn this feature off, define table ROW_FORMAT=REDUNDENT.

For more details on the Physical structure of strings in INNODB go here

In summary: use varchar over char.



[edited for correctness - thanks Ken Jacobs]

Looking at this slide from INNOBASE.com - in 5.1 they will start using a COMPRESSED format based off of zlib. Here are some numbers and details:

mysqluc2007

	Uncompressed	Compressed
File Size	2.8GB	1.4 GB
Insert/sec	1300	1000
CPU Usage	5%-50%	15%-50%