Details

    • Type: Bug
    • Status: Closed
    • Priority: Major - P3
    • Resolution: Duplicate
    • Affects Version/s: 2.6.1
    • Fix Version/s: None
    • Component/s: Querying
    • Labels:
      None
    • Operating System:
      ALL

      Description

      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?

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                • Created:
                  Updated:
                  Resolved: