comments are searched by not indexed field pid

I was watching my mysql-slow.log and noticed the following:

# Query_time: 2  Lock_time: 0  Rows_sent: 1  Rows_examined: 364684
SELECT COUNT(cid) FROM comments WHERE pid = 367522 AND status = 0;
# Time: 061204 18:00:17
# User@Host: XXX[XXX] @ XXX []
# Query_time: 4  Lock_time: 0  Rows_sent: 1  Rows_examined: 364721
SELECT COUNT(cid) FROM comments WHERE pid = 367575 AND status = 0;
# Time: 061204 18:00:56
# User@Host: XXX[XXX] @ XXX []
# Query_time: 3  Lock_time: 0  Rows_sent: 1  Rows_examined: 364723
SELECT COUNT(cid) FROM comments WHERE pid = 367633 AND status = 0;

A quick look shows that there's no index on pid in the comments table, so I added one:

mysql> alter table comments add key pid (pid);
Query OK, 364784 rows affected (15.34 sec)

After this for 4/5 hours there were no slow queries any more. I don't remember seeing this with 4.6 but...

PP: There are three places comment.module selects by pid - lines:

/*   549 */      // validated/filtered data to perform such check.
/*   550 */      $duplicate = db_result(db_query("SELECT COUNT(cid) FROM {comments} WHERE pid = %d ..."
/*   551 */      if ($duplicate != 0) {
/*--
/*  1121 */  if (!isset($cache[$pid])) {
/*  1122 */    $cache[$pid] = db_result(db_query('SELECT COUNT(cid) FROM {comments} WHERE pid = %d ...'
/*  1123 */  }
/*--
/*  1727 */function _comment_update_node_statistics($nid) {
/*  1728 */  $count = db_result(db_query('SELECT COUNT(cid) FROM {comments} WHERE nid = %d AND...'
/*  1729 */

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