MongoDB Query plan not using compound index - mongodb

Question

I am trying MongoDB with a dataset about the company profile margin for learning purpose. Here is the sample document
{
"parent_comp" : 1
"child_comp" : 101
"profit" : NumberLong(70320020)
}
I have created two indexes i.e one on child_comp field and the other one is a compound index with parent_comp, child_comp, and last_outage_timestamp.
For the below query, I executed the explain command to see the query plan.
MongoDB Enterprise > db.data.find({ "$and" : [{ "parent_comp" : 951, "child_comp" : 9351, "profit" : { "$gte" : { "$numberLong" : "500000000" } } }, { "profit" : { "$lte" : { "$numberLong" : "1000000000" } } }] }).sort({"profit" : 1}).limit(3).explain();
{
"queryPlanner" : {
"plannerVersion" : 1,
"namespace" : "test.data",
"indexFilterSet" : false,
"parsedQuery" : {
"$and" : [
{
"child_comp" : {
"$eq" : 9351
}
},
{
"parent_comp" : {
"$eq" : 951
}
},
{
"profit" : {
"$lte" : {
"$numberLong" : "1000000000"
}
}
},
{
"profit" : {
"$gte" : {
"$numberLong" : "500000000"
}
}
}
]
},
"queryHash" : "B570EF0C",
"planCacheKey" : "187EF74B",
"winningPlan" : {
"stage" : "LIMIT",
"limitAmount" : 3,
"inputStage" : {
"stage" : "FETCH",
"filter" : {
"$and" : [
{
"child_comp" : {
"$eq" : 9351
}
},
{
"parent_comp" : {
"$eq" : 951
}
}
]
},
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"profit" : 1
},
"indexName" : "profit_index",
"isMultiKey" : false,
"multiKeyPaths" : {
"profit" : [ ]
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {
"profit" : [
"[{ $numberLong: \"500000000\" }, { $numberLong: \"1000000000\" }]"
]
}
}
}
},
"rejectedPlans" : [
{
"stage" : "SORT",
"sortPattern" : {
"profit" : 1
},
"limitAmount" : 3,
"inputStage" : {
"stage" : "SORT_KEY_GENERATOR",
"inputStage" : {
"stage" : "FETCH",
"filter" : {
"$and" : [
{
"parent_comp" : {
"$eq" : 951
}
},
{
"profit" : {
"$lte" : {
"$numberLong" : "1000000000"
}
}
},
{
"profit" : {
"$gte" : {
"$numberLong" : "500000000"
}
}
}
]
},
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"child_comp" : 1
},
"indexName" : "child_comp_index",
"isMultiKey" : false,
"multiKeyPaths" : {
"child_comp" : [ ]
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {
"child_comp" : [
"[9351.0, 9351.0]"
]
}
}
}
}
},
{
"stage" : "LIMIT",
"limitAmount" : 3,
"inputStage" : {
"stage" : "FETCH",
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"parent_comp" : 1,
"child_comp" : 1,
"profit" : 1
},
"indexName" : "parent_comp_1_child_comp_1_profit_1",
"isMultiKey" : false,
"multiKeyPaths" : {
"parent_comp" : [ ],
"child_comp" : [ ],
"profit" : [ ]
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {
"parent_comp" : [
"[951.0, 951.0]"
],
"child_comp" : [
"[9351.0, 9351.0]"
],
"profit" : [
"[{ $numberLong: \"500000000\" }, { $numberLong: \"1000000000\" }]"
]
}
}
}
}
]
},
"serverInfo" : {
"host" : "localhost",
"port" : 27017,
"version" : "4.2.8",
"gitVersion" : "43d25888249164d76d5e04dd6cf38f6111e21f5f"
},
"ok" : 1
}
As you can see winning plan used single index instead of compound index. So could you please let me know why compound index was not used.

Answer 1

Your query is sorting on profit, and the compound index does not include the field you are sorting on hence using the compound index would necessitate an additional sort stage.
The trade-offs and reasoning is further explained in the docs.
See also https://www.alexbevi.com/blog/2020/05/16/optimizing-mongodb-compound-indexes-the-equality-sort-range-esr-rule/.