mongodb not follow prefix rule - mongodb

Question

mongodb often use index which include the field no in the filter , and the field will using [MaxKey , MinKey] ,this is not the same as what I saw in the index prefix documentation.
the query like this :
db.Wager.aggregate([
{"$match":
{
"CompanyId": 1341,
"UpdatedDateUtc" : {"$gte" : ISODate("2021-03-25T23:35:00Z")}
}},
{$project:
{"CompanyId":1,"UpdatedDateUtc":1,_id:0}
},
{"$group":
{"_id": 1,"n": {"$sum": 1}}
}
])
I hope this can use this index :
{ "CompanyId": -1, "UpdatedDateUtc" : -1 ,"WagerEventDateUtc" : -1 }
but it always will use index :
{ "CompanyId" : 1,"BetDateUtc" : -1, "WagerEventDateUtc" : -1, "UpdatedDateUtc" : -1}
there is the explain :
{
"nReturned": 101,
"executionTimeMillisEstimate": 0,
"totalKeysExamined": 101,
"totalDocsExamined": 0,
"executionStages": {
"stage": "PROJECTION_COVERED",
"nReturned": 101,
"executionTimeMillisEstimate": 0,
"works": 101,
"advanced": 101,
"needTime": 0,
"needYield": 0,
"saveState": 3,
"restoreState": 2,
"isEOF": 0,
"transformBy": {
"CompanyId": 1,
"UpdatedDateUtc": 1,
"_id": 0
},
"inputStage": {
"stage": "IXSCAN",
"nReturned": 101,
"executionTimeMillisEstimate": 0,
"works": 101,
"advanced": 101,
"needTime": 0,
"needYield": 0,
"saveState": 3,
"restoreState": 2,
"isEOF": 0,
"keyPattern": {
"CompanyId": 1,
"BetDateUtc": -1,
"WagerEventDateUtc": -1,
"UpdatedDateUtc": -1
},
"indexName": "CompanyId_1_BetDateUtc_-1_WagerEventDateUtc_-1_UpdatedDateUtc_-1",
"isMultiKey": false,
"multiKeyPaths": {
"CompanyId": [],
"BetDateUtc": [],
"WagerEventDateUtc": [],
"UpdatedDateUtc": []
},
"isUnique": false,
"isSparse": false,
"isPartial": false,
"indexVersion": 2,
"direction": "forward",
"indexBounds": {
"CompanyId": [
"[1341.0, 1341.0]"
],
"BetDateUtc": [
"[MaxKey, MinKey]"
],
"WagerEventDateUtc": [
"[MaxKey, MinKey]"
],
"UpdatedDateUtc": [
"[new Date(9223372036854775807), new Date(1616715300000)]"
]
},
"keysExamined": 101,
"seeks": 1,
"dupsTested": 0,
"dupsDropped": 0
}
}
},
{
"nReturned": 101,
"executionTimeMillisEstimate": 0,
"totalKeysExamined": 101,
"totalDocsExamined": 0,
"executionStages": {
"stage": "PROJECTION_COVERED",
"nReturned": 101,
"executionTimeMillisEstimate": 0,
"works": 101,
"advanced": 101,
"needTime": 0,
"needYield": 0,
"saveState": 10242,
"restoreState": 10242,
"isEOF": 0,
"transformBy": {
"CompanyId": 1,
"UpdatedDateUtc": 1,
"_id": 0
},
"inputStage": {
"stage": "IXSCAN",
"nReturned": 101,
"executionTimeMillisEstimate": 0,
"works": 101,
"advanced": 101,
"needTime": 0,
"needYield": 0,
"saveState": 10242,
"restoreState": 10242,
"isEOF": 0,
"keyPattern": {
"CompanyId": -1,
"UpdatedDateUtc": -1,
"WagerEventDateUtc": -1
},
"indexName": "CompanyId_-1_UpdatedDateUtc_-1_WagerEventDateUtc_-1",
"isMultiKey": false,
"multiKeyPaths": {
"CompanyId": [],
"UpdatedDateUtc": [],
"WagerEventDateUtc": []
},
"isUnique": false,
"isSparse": false,
"isPartial": false,
"indexVersion": 2,
"direction": "forward",
"indexBounds": {
"CompanyId": [
"[1341.0, 1341.0]"
],
"UpdatedDateUtc": [
"[new Date(9223372036854775807), new Date(1616715300000)]"
],
"WagerEventDateUtc": [
"[MaxKey, MinKey]"
]
},
"keysExamined": 101,
"seeks": 1,
"dupsTested": 0,
"dupsDropped": 0
}
}
}
I think mongodb choose wrong index were because "saveState" and "restoreState" too high , but I'm not sure because I don’t know what these two fields mean.

Answer 1

The answer to why is chooses one index over another is it runs a short test, calculates which index returns the most results per unit of "work", with bonus points for completing during the test or not using a blocking sort.
If 2 or more indexes are tied after that, it randomly picks one.