mongodb aggregate $limit and $lookup sequence problems - sql

Question

db.getCollection('xxxxxxxx').aggregate(
[
{
"$match": {
"campaigns.campaign_id":ObjectId("5c6e50932fb955f81b0c9f59")
}
},
{
"$sort": {
"campaigns.updatedAt": 1,
"_id": -1
}
},
{
"$limit": 15
},
{
"$lookup": {
"from": "callresults",
"localField": "currentStat.sales_funnel_id",
"foreignField": "_id",
"as": "sale_funnels"
}
},
{
"$lookup": {
"from": "callresults",
"localField": "currentStat.callresult_id",
"foreignField": "_id",
"as": "callresults"
}
},
{
"$lookup": {
"from": "accounts",
"localField": "currentStat.qc.account_id",
"foreignField": "_id",
"as": "accounts"
}
},
{
"$match": {
"$or": [
{
"姓名": /137/
},
{
"电话号码": /137/
},
{
"电子邮件": /137/
},
{
"城市": /137/
},
{
"区域": /137/
},
{
"备注": /137/
}
]
}
}
]
)
The result of executing the above SQL query is 0 ($limlit before $lookup)
The result of executing the above SQL query is 0 ($limlit before $lookup)
The result of executing the above SQL query is 0 ($limlit before $lookup)
if $limlit follows $lookup
db.getCollection('xxxxxxxxxxx').aggregate(
[
{
"$match": {
"campaigns.campaign_id":ObjectId("5c6e50932fb955f81b0c9f59")
}
},
{
"$sort": {
"campaigns.updatedAt": 1,
"_id": -1
}
},
{
"$lookup": {
"from": "callresults",
"localField": "currentStat.sales_funnel_id",
"foreignField": "_id",
"as": "sale_funnels"
}
},
{
"$lookup": {
"from": "callresults",
"localField": "currentStat.callresult_id",
"foreignField": "_id",
"as": "callresults"
}
},
{
"$lookup": {
"from": "accounts",
"localField": "currentStat.qc.account_id",
"foreignField": "_id",
"as": "accounts"
}
},
{
"$match": {
"$or": [
{
"姓名": /137/
},
{
"电话号码": /137/
},
{
"电子邮件": /137/
},
{
"城市": /137/
},
{
"区域": /137/
},
{
"备注": /137/
}
]
}
},
{
"$limit": 15
}
]
)
Why is that?
Why is that?
Why is that?
Why is that?
Why is that?

Answer 1

In the first case (limit before lookup), lookup is done on the first 15 matched documents only. However, when limit is in the end of the pipe line lookup is done on all matched documents and then the limit is applied.
Taking a simpler example,
This query finds all documents where value of field "n" is 1 and then shows the first 15 matching documents.
db.collection.aggregate([{$match: {"n" : 1}}, {$limit: 15}])
However, the below query takes the top 15 documents and then runs a match on those 15 documents only.
db.collection.aggregate([{$limit: 15}, {$match: {"n" : 1}}])

Answer 2

1) In the first case. you are limiting results upto 15 and match condition is executed. So the match condition only works on the 15 documents.
2) In the second case, you are matching with all the documents in the collection, and then limiting the result.