Indexes in MySQL

[Oyvind Fredstie]

I've noticed something strange in MySQL,
it seems that when I create indexes all the data
from the table is copied over to the index.

I discovered this when I created a btree index on
a table (called xray) that had a blob attribute (10 000 rows
where each blob were approx 250KB) and I noticed that when
I created the index the free space on the harddisk was reduced
by approx. 2.5 GB.

This is the xray table:
CREATE TABLE xray (
xray_num INT(12) NOT NULL AUTO_INCREMENT,
date_captured TIMESTAMP(14) NOT NULL,
xray_image MEDIUMBLOB NOT NULL,
comments TEXT,
consult_num INT(12) NOT NULL,
--
CONSTRAINT XRAY_PK PRIMARY KEY (xray_num),
--
CONSTRAINT XRAY_FK_TO_CONSULTATION FOREIGN KEY (consult_num)
REFERENCES consultation (consult_num)
);

and this is the index:
CREATE INDEX xray_consultnum_btree_index USING BTREE
ON xray (consult_num);

I thought that the index only stored the index key (consult_num)
and a pointer to the row in the table (xray).