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Type: Bug
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Resolution: Duplicate
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Priority: Major - P3
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None
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Affects Version/s: 2.6.1
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Component/s: Querying
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None
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ALL
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?
- duplicates
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SERVER-12923 Plan ranking is bad for plans with blocking stages
- Backlog
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SERVER-13675 Plans with differing performance can tie during plan ranking
- Closed