我的文档结构如下所示
{
"_id" : "311acd33a0ae8dcc3101246f90af9dc5",
"created_datetime" : ISODate("2013-04-05T10:35:31.143Z"),
"installs" : [
{
"status" : 1,
"app" : "xyz",
"reg_id" : "AVJyaIFI2Q8v93YmOHI5kEOVoCLbd4CAUyVK9zLrC1QCiBcl_bw89i5PvhEuTKmxtb4x130vjMyo78zPI7cedErcRv_Jjn0BN3Wq40hhg",
"last_action_datetime" : ISODate("2013-04-05T10:35:31.143Z"),
"version" : "2"
},
{
"status" : 1,
"app" : "abc",
"reg_id" : "AVJyaIFI2Q8v93YmOHI5kEOVoCLbd4CAUyVK9zLrC1QCiBcl_bw89i5PvhEuTKmxtb4x130vjMyo78zPI7cedErcRv_Jjn0BN3Wq40hhg",
"last_action_datetime" : ISODate("2013-04-05T10:35:31.143Z"),
"version" : "5"
},
{
"status" : 1,
"app" : "pqr",
"last_action_datetime" : ISODate("2013-04-06T10:35:31.143Z"),
"version" : "1"
},
],
"last_update" : ISODate("2013-04-12T06:26:46.333Z"),
"num_updates" : 9,
.....
}
我有一个复合索引'install.reg_id'
和'installs.status'
一个单一的索引'installs.status'
现在我想找到至少installs
包含一个元素reg_id
并且它status
是1的所有文档。所以我查询
db.users.find({'installs': {'$elemMatch': {'reg_id': {'$exists': true}, 'status': 1}}}).explain()
我明白了
{
"cursor" : "BtreeCursor installs.status_1",
"isMultiKey" : true,
"n" : 1447034,
"nscannedObjects" : 1720864,
"nscanned" : 1720864,
"nscannedObjectsAllPlans" : 1720864,
"nscannedAllPlans" : 1720864,
"scanAndOrder" : false,
"indexOnly" : false,
"nYields" : 13072,
"nChunkSkips" : 0,
"millis" : 11063,
"indexBounds" : {
"installs.status" : [
[
1,
1
]
]
},
"server" : "####:27017"
}
所以这里应该使用复合索引但没有使用。我认为这$elemMatch
是罪魁祸首所以我做了这个查询
db.users.find({'installs.reg_id': {'$exists': true}}).explain()
我明白了
{
"cursor" : "BasicCursor",
"isMultiKey" : false,
"n" : 2947446,
"nscannedObjects" : 3184871,
"nscanned" : 3184871,
"nscannedObjectsAllPlans" : 3184871,
"nscannedAllPlans" : 3184871,
"scanAndOrder" : false,
"indexOnly" : false,
"nYields" : 23865,
"nChunkSkips" : 0,
"millis" : 16172,
"indexBounds" : {
},
"server" : "####:27017"
}
这表明查询没有使用任何索引。
知道这里出了什么问题吗?
更新:添加提示确实使查询使用索引
db.users.find({'installs': {'$elemMatch': {'reg_id': {'$exists': true}, 'status': 1}}}).hint({"installs.reg_id":1,"installs.status":1}).explain()
返回
{
"cursor" : "BtreeCursor installs.reg_id_1_installs.status_1",
"isMultiKey" : true,
"n" : 1451589,
"nscannedObjects" : 2464985,
"nscanned" : 4373261,
"nscannedObjectsAllPlans" : 2464985,
"nscannedAllPlans" : 4373261,
"scanAndOrder" : false,
"indexOnly" : false,
"nYields" : 20170,
"nChunkSkips" : 0,
"millis" : 106353,
"indexBounds" : {
"installs.reg_id" : [
[
{
"$minElement" : 1
},
{
"$maxElement" : 1
}
]
],
"installs.status" : [
[
1,
1
]
]
},
"server" : "####:27017"
}
这里使用复合索引。