Wrong index is being picked

Hi guys,

We have a weird issue with MongoDB 2.6.0/2.6.1 picking a wrong index.

So we have a really huge collection (~250MM docs) with the following indexes defined:

[

	{

		"_id" : 1

	},

	{

		"mid" : 1,

		"r1" : -1

	},

	{

		"mid" : 1,

		"r2" : -1

	},

	{

		"key" : 1

	},

	{

		"mid" : 1,

		"key" : 1

	},

	{

		"mid" : 1

	}

]

Yet, sometimes, under a heavy load, the database starts freaking out - consumes >1000% CPU, performance degrades dramatically and I see the following entries in the logs:

2014-05-07T15:54:30.587+0000 [conn153098] query <collection>: { mid: ObjectId('4fa0d0914711596b17039e29'), key: "liuruile@163.com" } planSummary: IXSCAN { mid: 1, r1: -1 } ntoskip:0 nscanned:731072 nscannedObjects:731071 keyUpdates:0 numYields:6048 locks(micros) r:181118904 nreturned:1 reslen:214 486438ms

As you can see the wrong index is being picked. In fact, I see the same behavior for a lot of other collections as well. It looks like the very first index (after _id) is being picked all the time.

Do you have any idea on what might be causing this?