Major - P3 Wed Mar 21 16:37:37 [conn1] warning: _id query on capped collection without an _id index, performance will be poor collection: test.c
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:
> db.createCollection( 'c', { capped:true, size:10000, autoIndexId:true } ) { "ok" : 1 } > db.c.getIndexes() [ { "v" : 1, "key" : { "_id" : 1 }, "ns" : "test.c", "name" : "_id_" } ] > db.c.find( { _id:0, a:0 } ).itcount() // Causes the warning to be logged. 0