Mongodb unwind and match VS match and unwind - mongodb

Question

I'm looking to optimize the MongoDB performance by minimizing the number of records to unwind.
I do like:
unwind(injectionRecords),
match("machineID" : "machine1"),
count(counter)
But because of huge data unwind operation takes a lot of time and then it matches from unwind.
It unwinds all the 4 records then matches machineID from result and give me count of it.
Instead I would like to do something like :
match("machineID": "machine1"),
unwind(injectionRecords)
count(counter)
So, it would match records having machineID and unwind only 2 instead of 4 and give me the count of it.
Is this possible? How can I do this?
Here are sample docs,
{
"_id" : ObjectId("5981c24b90a7c215e4f166dd"),
"machineID" : "machine1",
"injectionRecords" : [
{
"startTime" : ISODate("2017-08-02T17:45:04.779+05:30"),
"endTime" : ISODate("2017-08-02T17:45:07.763+05:30"),
"counter" : 1
},
{
"startTime" : ISODate("2017-08-02T17:45:24.417+05:30"),
"endTime" : ISODate("2017-08-02T17:45:27.402+05:30"),
"counter" : 2
}
]
},
{
"_id" : ObjectId("5981c24b90a7c215e4f166de"),
"machineID" : "machine2",
"injectionRecords" : [
{
"startTime" : ISODate("2017-08-02T17:46:04.779+05:30"),
"endTime" : ISODate("2017-08-02T17:46:07.763+05:30"),
"counter" : 1
},
{
"startTime" : ISODate("2017-08-02T17:46:24.417+05:30"),
"endTime" : ISODate("2017-08-02T17:46:27.402+05:30"),
"counter" : 2
}
]
}

Answer 1

The following query will return a count of injectionRecords for a given machineId. I think this is what you are asking for.
db.collection.aggregate([
{$match: {machineID: 'machine1'}},
{$unwind: '$injectionRecords'},
{$group:{_id: "$_id",count:{$sum:1}}}
])
Of course, this query (where the unwind takes place before the match) is functionally equivalent:
db.collection.aggregate([
{$unwind: '$injectionRecords'},
{$match: {machineID: 'machine1'}},
{$group:{_id: "$_id",count:{$sum:1}}}
])
However, running that query with explain ...
db.collection.aggregate([
{$unwind: '$injectionRecords'},
{$match: {machineID: 'machine1'}},
{$group:{_id: "$_id",count:{$sum:1}}}
], {explain: true})
... shows that the unwind stage applies to the entire collection whereas if you match before unwinding then only the matched documents are unwound.