In load  MongoDB log file, we can see a lot of slow queries. Most of them are not using the best index and consequently the best execution plan is not used. However, when I run the same queries by myself in mongo shell, the correct index is used. So why for same query, we don't have same execution plan ?

Kindly find the log below using wrong index :

2020-05-25T04:55:51.624+0000 I COMMAND [conn301319] command mydb.mycoll command: aggregate { aggregate: "mycoll", pipeline: [ { $match: { networkId.$id:

{ $in: [ ObjectId('5e0ed9eb60b2533bda7a0fa8') ] }

, status: "0", alarmType: "1" } }, { $group: { _id:

{ networkId: "$networkId" }

, alarmCount: { $sum: 1 } } } ] } planSummary: IXSCAN { status: 1 } keysExamined:35350 docsExamined:35350 numYields:280 nreturned:0 reslen:135 locks:{ Global: { acquireCount:

{ r: 574 }

, acquireWaitCount: { r: 92 }, timeAcquiringMicros: { r: 1590945 } }, Database: { acquireCount:

{ r: 287 }

}, Collection: { acquireCount:

{ r: 286 }

} } protocol:op_query 1980ms

Index On collection:

db.getCollection('mycoll').getIndexes()
[
{
"v" : 1,
"key" :

{ "_id" : 1 }

,
"name" : "id",
"ns" : "mydb.mycoll"
},

{
"v" : 1,
"key" :

{ "networkId.$id" : 1, "status" : 1, "alarmType" : 1 }

,
"name" : "networkId.$id_1_status_1_alarmType_1",
"ns" : "mydb.mycoll"
},

{
"v" : 2,
"key" :

{ "status" : 1 }

,
"name" : "status_1",
"ns" : "mydb.mycoll",
"background" : true
}
]