MongoDB aggregate using scan for join collection? - mongodb

Question

I have two collection one collection is 'image' and other thing is 'map' for mapping info with artists
image collection has idx(single index) and map collection has multiple index(m_idx, i_idx)
i know mongodb is not RDBMS but i'm using this two table with aggregate pipline but i worried when aggregate image table, mongodb get image row by querying with index or select after scan all image collection.
i heard aggregate join the row after get all collection. but i'm not sure ..
add my query and explain result
Query
db.map.aggregate(
[
{
$match: {m_idx: 1111}
},
{
$lookup: {
from: "image",
localField: "i_idx",
foreignField: "idx",
as: "image_aggre"
}
},
{
$match: {
'image': {
$ne: []
}
}
},
{
$sort: {
seq: -1
}
},
{
$skip: (pageNum) * pageSize
},
{
$limit: pageSize
}
], {
explain: true
}
)
Explain Result
{
"queryPlanner" : {
"plannerVersion" : 1,
"namespace" : "db.map",
"winningPlan" : {
"stage" : "LIMIT_SKIP",
"inputStage" : {
"stage" : "SORT",
"sortPattern" : {
"seq" : -1
},
"inputStage" : {
"stage" : "SUBSCAN",
"inputStage" : {
"stage" : "HASH_AGGREGATE",
"inputStage" : {
"stage" : "HASH_LOOKUP",
"inputStages" : [
{
"stage" : "COLLSCAN"
},
{
"stage" : "HASH",
"inputStage" : {
"stage" : "IXSCAN",
"indexName" : "m_idx_-1_i_idx_-1",
"direction" : "forward"
}
}
]
}
}
}
}
}
},
i'm using aws-documentDB so result can be a little bit different