Why is this query being logged as "not using indexes"?

For some reason my slow query log is reporting the following query as "not using indexes" and for the life of me I cannot understand why.

Here is the query:

update  scheduletask
set     active = 0
where   nextrun < date_sub( now(), interval 2 minute )
and     enabled = 1
and     active = 1;

Here is the table:

CREATE TABLE `scheduletask` (
  `scheduletaskid` int(11) NOT NULL AUTO_INCREMENT,
  `schedulethreadid` int(11) NOT NULL,
  `taskname` varchar(50) NOT NULL,
  `taskpath` varchar(100) NOT NULL,
  `tasknote` text,
  `recur` int(11) NOT NULL,
  `taskinterval` int(11) NOT NULL,
  `lastrunstart` datetime NOT NULL,
  `lastruncomplete` datetime NOT NULL,
  `nextrun` datetime NOT NULL,
  `active` int(11) NOT NULL,
  `enabled` int(11) NOT NULL,
  `creatorid` int(11) NOT NULL,
  `editorid` int(11) NOT NULL,
  `created` datetime NOT NULL,
  `edited` datetime NOT NULL,
  PRIMARY KEY (`scheduletaskid`),
  UNIQUE KEY `Name` (`taskname`),
  KEY `IDX_NEXTRUN` (`nextrun`)
) ENGINE=InnoDB AUTO_INCREMENT=34 DEFAULT CHARSET=latin1;

Solution

Add another index like this

KEY `IDX_COMB` (`nextrun`, `enabled`, `active`)

I'm not sure how many rows your table have but the following might apply as well

Sometimes MySQL does not use an index, even if one is available. One circumstance under which this occurs is when the optimizer estimates that using the index would require MySQL to access a very large percentage of the rows in the table. (In this case, a table scan is likely to be much faster because it requires fewer seeks.)