Mongo selecting the wrong index - mongodb

Question

I'm using mongo db 2.6.9.
I created 2 indexes on a collection and i don`t understand the reson that the query planer allways selects the wrong index.
I would like to understand what I'm missing.
First Index:
{
"TimeStamp":1,
"A":1,
"B":1,
}
Second Index:
{
"TimeStamp":1,
"A":1,
"C":1,
}
When I'm useing the following query the planner selects the first index:
db.collection.find({"TimeStamp":{ "$gte" : ISODate("2015-04-14T00:00:00Z"), "$lt" : ISODate("2015-04-15T00:00:00Z") },"C":2137,"A":1}).explain()
explain({verbose:1}) results:
{
"cursor" : "BtreeCursor IX_First",
"isMultiKey" : false,
"n" : 0,
"nscannedObjects" : 0,
"nscanned" : 0,
"nscannedObjectsAllPlans" : 0,
"nscannedAllPlans" : 0,
"scanAndOrder" : false,
"indexOnly" : false,
"nYields" : 0,
"nChunkSkips" : 0,
"millis" : 0,
"indexBounds" : {
"TimeStamp" : [
[
ISODate("2015-04-14T00:00:00Z"),
ISODate("2015-04-15T00:00:00Z")
]
],
"A" : [
[
1,
1
]
],
"B" : [
[
{
"$minElement" : 1
},
{
"$maxElement" : 1
}
]
],
},
"allPlans" : [
{
"cursor" : "BtreeCursor IX_First",
"isMultiKey" : false,
"n" : 0,
"nscannedObjects" : 0,
"nscanned" : 0,
"scanAndOrder" : false,
"indexOnly" : false,
"nChunkSkips" : 0,
"indexBounds" : {
"TimeStamp" : [
[
ISODate("2015-04-14T00:00:00Z"),
ISODate("2015-04-15T00:00:00Z")
]
],
"A" : [
[
1,
1
]
],
"B" : [
[
{
"$minElement" : 1
},
{
"$maxElement" : 1
}
]
],
},
{
"cursor" : "BtreeCursor IIX_Second",
"isMultiKey" : false,
"n" : 0,
"nscannedObjects" : 0,
"nscanned" : 0,
"scanAndOrder" : false,
"indexOnly" : false,
"nChunkSkips" : 0,
"indexBounds" : {
"TimeStamp" : [
[
ISODate("2015-04-14T00:00:00Z"),
ISODate("2015-04-15T00:00:00Z")
]
],
"A" : [
[
1,
1
]
],
"C" : [
[
2137,
2137
]
]
}
}
],
"server" : "mongo2:27017",
"filterSet" : false,
"stats" : {
"type" : "KEEP_MUTATIONS",
"works" : 2,
"yields" : 0,
"unyields" : 0,
"invalidates" : 0,
"advanced" : 0,
"needTime" : 0,
"needFetch" : 0,
"isEOF" : 1,
"children" : [
{
"type" : "FETCH",
"works" : 1,
"yields" : 0,
"unyields" : 0,
"invalidates" : 0,
"advanced" : 0,
"needTime" : 0,
"needFetch" : 0,
"isEOF" : 1,
"alreadyHasObj" : 0,
"forcedFetches" : 0,
"matchTested" : 0,
"children" : [
{
"type" : "IXSCAN",
"works" : 1,
"yields" : 0,
"unyields" : 0,
"invalidates" : 0,
"advanced" : 0,
"needTime" : 0,
"needFetch" : 0,
"isEOF" : 1,
"keyPattern" : "{ TimeStamp: 1, A: 1, B: 1}",
"isMultiKey" : 0,
"boundsVerbose" : "field #0['TimeStamp']: [new Date(1428969600000), new Date(1429056000000)), field #1['A']: [1.0, 1.0], field #2['B']: [MinKey, MaxKey]",
"yieldMovedCursor" : 0,
"dupsTested" : 0,
"dupsDropped" : 0,
"seenInvalidated" : 0,
"matchTested" : 0,
"keysExamined" : 0,
"children" : [ ]
}
]
}
]
}
}

Answer 1

That's because the order of the index fields matters in this case. Try swapping A and C:
db.collection.find({"TimeStamp":{ "$gte" : ISODate("2015-04-14T00:00:00Z"), "$lt" : ISODate("2015-04-15T00:00:00Z") },"A":1,"C":2137}).explain()