Estimating disk-space usage using LENGTH(BLOB)

[Mike Moran]

I'm currently trying to write a short SQL expression that will give me a 
rough estimate of the disk-space usage of a particular bunch of rows in 
a table.

For table 'example':
CREATE TABLE `example` (
  `id` int(11) NOT NULL auto_increment,
  `blah` varchar(255) NOT NULL default '',
  `blah2` longblob NOT NULL
  PRIMARY KEY  (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;
... I can write a query as follows:
SELECT length(blah) + length(blah2) FROM example
This, I hope, gives me a rough idea how much space is used by the row 
(to get a better approximation, I could include what's listed in 
http://dev.mysql.com/doc/mysql/en/Storage_requirements.html, but the 
above would do).

Now, my questions are:
- Does length() always return a sensible value for things like 
length(LONGBLOB) (e.g. length(blah2))?
- Does it return the number of bytes in the blob?
- In general, will the approach shown above give a reasonable estimate 
of disk-space usage?

Ta,
--
Mike
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