does $lookup use indexes in the foreignField key? - mongodb

Question

In the example below, if the collection inventory has an index on the sku field, will it be used in this $lookup operation?
db.orders.insertMany( [
{ "_id" : 1, "item" : "almonds", "price" : 12, "quantity" : 2 },
{ "_id" : 2, "item" : "pecans", "price" : 20, "quantity" : 1 },
{ "_id" : 3 }
] )
db.inventory.insertMany( [
{ "_id" : 1, "sku" : "almonds", "description": "product 1", "instock" : 120 },
{ "_id" : 2, "sku" : "bread", "description": "product 2", "instock" : 80 },
{ "_id" : 3, "sku" : "cashews", "description": "product 3", "instock" : 60 },
{ "_id" : 4, "sku" : "pecans", "description": "product 4", "instock" : 70 },
{ "_id" : 5, "sku": null, "description": "Incomplete" },
{ "_id" : 6 }
] )
db.orders.aggregate( [
{
$lookup:
{
from: "inventory",
localField: "item",
foreignField: "sku",
as: "inventory_docs"
}
}
] )
EDITED:
It does not. Why not?
{
"explainVersion" : "1",
"stages" : [
{
"$cursor" : {
"queryPlanner" : {
"namespace" : "6303c64faf8ef53d8ba2062f_y22_test2.orders",
"indexFilterSet" : false,
"parsedQuery" : {
},
"queryHash" : "8B3D4AB8",
"planCacheKey" : "D542626C",
"maxIndexedOrSolutionsReached" : false,
"maxIndexedAndSolutionsReached" : false,
"maxScansToExplodeReached" : false,
"winningPlan" : {
"stage" : "COLLSCAN",
"direction" : "forward"
},
"rejectedPlans" : [
]
}
}
},
{
"$lookup" : {
"from" : "inventory",
"as" : "inventory_docs",
"localField" : "item",
"foreignField" : "sku"
}
}
],

Answer 1

In the case of simple lookups (e.g., when specifying a localField + foreignField), an index will be used
Things are sadly more complicated when using a $lookup + pipeline, the following limitations apply:
Multikey indexes are not used.
Indexes are not used for comparisons where the operand is an array or the
operand type is undefined.
Indexes are not used for comparisons with more than one field path operand.
https://www.mongodb.com/docs/manual/reference/operator/aggregation/lookup/
It is really annoying that the explain() call doesn't provide any information on index usage of lookup stages. The best way I've found to determine whether an index was used was to (separately) use the $indexStats aggregation on the collection being looked up, in the above case:
db.inventory.aggregate([{$indexStats: {}}])
Then find the index you think is being used and watch the accesses.ops field.