How well does full-text scale?

[Andrey Osenenko]

Hello.

I have a mysql table; It's pretty large but straightforward. It has 12767901 entries; here's output of show create (irrelevant parts omitted):

CREATE TABLE `a` (
  `num` int(10) unsigned NOT NULL,
  `subnum` int(10) unsigned NOT NULL,
  `comment` text,

  ...

  PRIMARY KEY (`num`,`subnum`),
  FULLTEXT KEY `comment` (`comment`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8

I want to use fulltext search on it, but it's ridiculously slow:

mysql> select count(*) from a where match(comment) against('pretty');
+----------+
| count(*) |
+----------+
|   176203 |
+----------+
1 row in set (13 min 23.29 sec)

mysql> select count(*) from a where match(comment) against('pretty');
+----------+
| count(*) |
+----------+
|   176203 |
+----------+
1 row in set (0.00 sec)

mysql> explain select count(*) from a where match(comment) against('pretty');
+----+-------------+-------+----------+---------------+---------+---------+------+------+-------------+
| id | select_type | table | type     | possible_keys | key     | key_len | ref  | rows | Extra       |
+----+-------------+-------+----------+---------------+---------+---------+------+------+-------------+
|  1 | SIMPLE      | a     | fulltext | comment       | comment | 0       |      |    1 | Using where |
+----+-------------+-------+----------+---------------+---------+---------+------+------+-------------+
1 row in set (0.12 sec)

Notice how the second time the same query completes immediately. Is this intended behavior or am I doing something wrong? show processlist says that query spends most time in Sending data state.