我使用MongoDB2.6.4。我的索引看起来像这样:{"v":1,"key":{"isFolder":1},"name":"isFolder_1","ns":"Tenant_51.files","background":true},{"v":1,"key":{"isForeign":1},"name":"isForeign_1","ns":"Tenant_51.files","background":true},我的查询是这样的:db.files.find({isFolder:true,isForeign:false}).explain(true)出于某种原因，它选择仅使用1个索引(