I'm trying to use indexing on my $lookup pipeline but it doesn't seem to be working as intended.
Here's my query:
db.map_levels.explain().aggregate([
{
$lookup:
{
from:
"map_level_revisions",
pipeline:
[
{
$match:
{
$expr:
{
$eq:
[
"$account_id",
ObjectId("5b66ca21d6b54f479bef62a4")
]
}
}
}
],
as:
"revisions"
}
},
])
and here's the explanation:
{
"stages" : [
{
"$cursor" : {
"query" : {
},
"queryPlanner" : {
"plannerVersion" : 1,
"namespace" : "test-creator.map_levels",
"indexFilterSet" : false,
"parsedQuery" : {
},
"winningPlan" : {
"stage" : "COLLSCAN",
"direction" : "forward"
},
"rejectedPlans" : [ ]
}
}
},
{
"$lookup" : {
"from" : "map_level_revisions",
"as" : "revisions",
"let" : {
},
"pipeline" : [
{
"$match" : {
"$expr" : {
"$eq" : [
"$account_id",
ObjectId("5b66ca21d6b54f479bef62a4")
]
}
}
}
]
}
}
],
"ok" : 1
}
How do I make it use an index instead?
Just a note, the query does return the document.
I'm trying to use indexing on my $lookup pipeline but it doesn't seem to be working as intended.
Here's my query:
db.map_levels.explain().aggregate([
{
$lookup:
{
from:
"map_level_revisions",
pipeline:
[
{
$match:
{
$expr:
{
$eq:
[
"$account_id",
ObjectId("5b66ca21d6b54f479bef62a4")
]
}
}
}
],
as:
"revisions"
}
},
])
and here's the explanation:
{
"stages" : [
{
"$cursor" : {
"query" : {
},
"queryPlanner" : {
"plannerVersion" : 1,
"namespace" : "test-creator.map_levels",
"indexFilterSet" : false,
"parsedQuery" : {
},
"winningPlan" : {
"stage" : "COLLSCAN",
"direction" : "forward"
},
"rejectedPlans" : [ ]
}
}
},
{
"$lookup" : {
"from" : "map_level_revisions",
"as" : "revisions",
"let" : {
},
"pipeline" : [
{
"$match" : {
"$expr" : {
"$eq" : [
"$account_id",
ObjectId("5b66ca21d6b54f479bef62a4")
]
}
}
}
]
}
}
],
"ok" : 1
}
How do I make it use an index instead?
Just a note, the query does return the document.
Share Improve this question edited Jan 26, 2019 at 22:01 A. L asked Jan 26, 2019 at 21:52 A. LA. L 12.7k29 gold badges98 silver badges179 bronze badges1 Answer
Reset to default 6The collection scan in your explain output is referring to the map_levels collection, as noted in the queryPlanner.namespace value. The $lookup stage merges data from another collection into the current pipeline. Since you haven't specified any query stages before the $lookup, the map_levels collection will be iterated using a collection scan. If an entire collection is being loaded without any filtering or sort criteria, a collection scan has less overhead than iterating an index and fetching the documents.
You can avoid the current collection scan by adding a $match stage before your $lookup (assuming you don't want to process the full map_levels collection).
How can I check the index used by
$lookup?
Unfortunately query explain output does not (as at MongoDB 4.0) indicate index usage for $lookup stages. A workaround for this would be running explain using your lookup's pipeline as a top level aggregation query.
There's a relevant issue to watch/upvote in the MongoDB Issue tracker: SERVER-22622: Improve $lookup explain to indicate query plan on the "from" collection.