ALTER TABLE increase INDEX LENGTH - mysql

Question

I have a mysql table named t_media_items. I have index on 3 cols (parent_id, type, weight). The index size is 2.52MB.
mysql> show indexes from t_media_items;
+---------------+------------+--------------+--------------+-----------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+---------------+------------+--------------+--------------+-----------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| t_media_items | 0 | PRIMARY | 1 | id | A | 113779 | NULL | NULL | | BTREE | | |
| t_media_items | 1 | idx_ptw | 1 | parent_id | A | 16254 | NULL | NULL | | BTREE | | |
| t_media_items | 1 | idx_ptw | 2 | type | A | 16254 | NULL | NULL | | BTREE | | |
| t_media_items | 1 | idx_ptw | 3 | weight | A | 113779 | NULL | NULL | | BTREE | | |
+---------------+------------+--------------+--------------+-----------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
4 rows in set (0.01 sec)
mysql> SELECT table_name AS "Tables", round(((index_length) / 1024 / 1024), 2) SIB
FROM information_schema.TABLES
WHERE table_schema = "XXXX" and table_name='t_media_items'
ORDER BY (index_length ) DESC;
+---------------+------+
| Tables | SIB |
+---------------+------+
| t_media_items | 2.52 |
+---------------+------+
1 row in set (0.00 sec)
I tried to alter length on another column, name "rand_key". The strange issue is after columned alter, the INDEX size suddenly increase to 5.52MB, even the "rand_key" is not part of the index.
mysql> ALTER TABLE `t_media_items` CHANGE `rand_key` `rand_key` VARCHAR( 255 ) CHARACTER SET utf8 COLLATE utf8_general_ci NULL DEFAULT NULL;
Query OK, 108503 rows affected (7.24 sec)
Records: 108503 Duplicates: 0 Warnings: 0
Here is INDEX_LENGTH after ALTER
mysql> SELECT table_name AS "Tables", round(((index_length) / 1024 / 1024), 2) SIB
FROM information_schema.TABLES
WHERE table_schema = "tallcat" and table_name='t_media_items'
ORDER BY (index_length ) DESC;
+---------------+------+
| Tables | SIB |
+---------------+------+
| t_media_items | 5.52 |
+---------------+------+
1 row in set (0.00 sec)
Could anyone help me explain the issue? Thank you

Answer 1

ALTER TABLE performs a table restructure, which is make an empty clone of the table, apply the alter, and copy data to it. Indexes are rebuild as a by-product of this process.
But as it builds the index incrementally while filling the table with data, it doesn't take advantage of fast index creation. The indexes are stored less compactly.
I can't tell if this explains the more than 2x increase in size, but it's possible.
I'm assuming that you're using MySQL 5.5 or later, or 5.1 with the InnoDB plugin. Earlier versions do not support fast index creation anyway.