Same mongodb query gives different explain plan - mongodb

Question

I have two mongo queries, the only change in a query is merchantId field still both queries giving me different winning plan.
First Query
db.transactions.find({"created":{"$gte":1527465600000,"$lte":1527551999000},"merchantId":940,"additionalInformation.REQUESTOR":{"$ne":"MOTO"},"$or":[{"paymentMode":{"$ne":"UPI"}},{"bankCode":{"$ne":"GTEZ"}}]}).sort({ _id: -1 }).limit(200).explain()
Output of above query
{
"queryPlanner" : {
"plannerVersion" : 1,
"indexFilterSet" : false,
"parsedQuery" : {
"$and" : [
{
"$or" : [
{
"$not" : {
"bankCode" : {
"$eq" : "GTEZ"
}
}
},
{
"$not" : {
"paymentMode" : {
"$eq" : "UPI"
}
}
}
]
},
{
"merchantId" : {
"$eq" : 940
}
},
{
"created" : {
"$lte" : 1527551999000
}
},
{
"created" : {
"$gte" : 1527465600000
}
},
{
"$not" : {
"additionalInformation.REQUESTOR" : {
"$eq" : "MOTO"
}
}
}
]
},
"winningPlan" : {
"stage" : "SORT",
"sortPattern" : {
"_id" : -1
},
"limitAmount" : 200,
"inputStage" : {
"stage" : "SORT_KEY_GENERATOR",
"inputStage" : {
"stage" : "FETCH",
"filter" : {
"$and" : [
{
"$or" : [
{
"$not" : {
"bankCode" : {
"$eq" : "GTEZ"
}
}
},
{
"$not" : {
"paymentMode" : {
"$eq" : "UPI"
}
}
}
]
},
{
"$not" : {
"additionalInformation.REQUESTOR" : {
"$eq" : "MOTO"
}
}
}
]
},
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"created" : 1,
"merchantId" : 1
},
"indexName" : "created_1_merchantId_1",
"isMultiKey" : false,
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 1,
"direction" : "forward",
"indexBounds" : {
"created" : [
"[1527465600000.0, 1527551999000.0]"
],
"merchantId" : [
"[940.0, 940.0]"
]
}
}
}
}
}
},
"serverInfo" : {
},
"ok" : 1
}
Second Query
db.transactions.find({"created":{"$gte":1527465600000,"$lte":1527551999000},"merchantId":1429,"additionalInformation.REQUESTOR":{"$ne":"MOTO"},"$or":[{"paymentMode":{"$ne":"UPI"}},{"bankCode":{"$ne":"GTEZ"}}]}).sort({ _id: -1 }).limit(200).explain()
Output of above query
{
"queryPlanner" : {
"plannerVersion" : 1,
"indexFilterSet" : false,
"parsedQuery" : {
"$and" : [
{
"$or" : [
{
"$not" : {
"bankCode" : {
"$eq" : "GTEZ"
}
}
},
{
"$not" : {
"paymentMode" : {
"$eq" : "UPI"
}
}
}
]
},
{
"merchantId" : {
"$eq" : 1429
}
},
{
"created" : {
"$lte" : 1527551999000
}
},
{
"created" : {
"$gte" : 1527465600000
}
},
{
"$not" : {
"additionalInformation.REQUESTOR" : {
"$eq" : "MOTO"
}
}
}
]
},
"winningPlan" : {
"stage" : "LIMIT",
"limitAmount" : 200,
"inputStage" : {
"stage" : "FETCH",
"filter" : {
"$and" : [
{
"$or" : [
{
"$not" : {
"bankCode" : {
"$eq" : "GTEZ"
}
}
},
{
"$not" : {
"paymentMode" : {
"$eq" : "UPI"
}
}
}
]
},
{
"merchantId" : {
"$eq" : 1429
}
},
{
"created" : {
"$lte" : 1527551999000
}
},
{
"created" : {
"$gte" : 1527465600000
}
},
{
"$not" : {
"additionalInformation.REQUESTOR" : {
"$eq" : "MOTO"
}
}
}
]
},
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"_id" : 1
},
"indexName" : "_id_",
"isMultiKey" : false,
"isUnique" : true,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 1,
"direction" : "backward",
"indexBounds" : {
"_id" : [
"[MaxKey, MinKey]"
]
}
}
}
}
},
"serverInfo" : {
},
"ok" : 1
}
As you can see only param difference is merchantId, still explain gives different winning plan also IXSCAN also shows different indexes used. In first query created_1_merchantId_1 index is used and in second query id index is used. First query takes 40 seconds to get results while second query gives 1 sec. Quick will be highly appreciated.