Wed Mar 21 16:37:37 [conn1] warning: _id query on capped collection without an _id index, performance will be poor collection: test.c
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This message is printed if there is a constraint on _id and there is a candidate collection scan plan. This means it can be printed when an _id index is present and is the basis of an additional candidate plan. For example:
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> db.createCollection( 'c', { capped:true, size:10000, autoIndexId:true } )
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{ "ok" : 1 }
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> db.c.getIndexes()
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[
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{
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"v" : 1,
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"key" : {
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"_id" : 1
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},
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"ns" : "test.c",
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"name" : "_id_"
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}
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]
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> db.c.find( { _id:0, a:0 } ).itcount() // Causes the warning to be logged.
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0
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Major - P3