title: MySQL very slow for alter table query
body: <p>Why is it taking more than an hour to simply update this table to add a column? This table has 15M rows.  It has 2 indexes and a single key primary key. The ALTER TABLE query has been in "copy to tmp table" state for 1 hour 15 minutes now.</p>
  
  <pre><code>ALTER TABLE `frugg`.`item_catalog_map` 
  ADD COLUMN `conversion_url` TEXT NULL DEFAULT NULL
  </code></pre>
  
  <p>Table:</p>
  
  <pre><code>mysql&gt; describe item_catalog_map;
  +------------------------+---------------+------+-----+---------+-------+
  | Field                  | Type          | Null | Key | Default | Extra |
  +------------------------+---------------+------+-----+---------+-------+
  | catalog_unique_item_id | varchar(255)  | NO   | PRI | NULL    |       |
  | catalog_id             | int(11)       | YES  | MUL | NULL    |       |
  | item_id                | int(11)       | YES  | MUL | NULL    |       |
  | price                  | decimal(10,2) | YES  |     | 0.00    |       |
  +------------------------+---------------+------+-----+---------+-------+
  
  mysql&gt; show index from item_catalog_map;
  +------------------+------------+----------------------+--------------+------------------------+-----------+-------------+----------+--------+------+------------+---------+
  | Table            | Non_unique | Key_name             | Seq_in_index | Column_name            | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
  +------------------+------------+----------------------+--------------+------------------------+-----------+-------------+----------+--------+------+------------+---------+
  | item_catalog_map |          0 | PRIMARY              |            1 | catalog_unique_item_id | A         |    15485115 |     NULL | NULL   |      | BTREE      |         |
  | item_catalog_map |          1 | IDX_ACD6184FCC3C66FC |            1 | catalog_id             | A         |          18 |     NULL | NULL   | YES  | BTREE      |         |
  | item_catalog_map |          1 | IDX_ACD6184F126F525E |            1 | item_id                | A         |    15485115 |     NULL | NULL   | YES  | BTREE      |         |
  +------------------+------------+----------------------+--------------+------------------------+-----------+-------------+----------+--------+------+------------+---------+
  </code></pre>