documentdb aggregate query not using index - mongodb

Question

I am trying to find the max of a value in a range of dates. The aggregate query I use has a match on indexed column _id. But the query takes too long and the explain plan tells me its going of a COLLSCAN and not an index scan. Can you please suggest why it wont make use of the index on _id?
Would it help if I created another index on colId?
{$match:{_id:{ $regex: 'regex'}}},
{$match:{$and:[{"colId":'DATA'}]}},
{$unwind:"$data"},
{$match:{$and:[{"data.time":{$gte:ISODate("xyz"),$lte:ISODate("zyx")}}]}},
{$match:{$and: [{ "data.col": { $exists: true}}] }},
{$group:{_id:"$data.time",maxCol:{$max:"$data.col"}}} ,
{$sort:{"maxCol":-1,_id:-1}},
{$limit:1}
])
Explain plan snippet:
"winningPlan" : {
"stage" : "LIMIT_SKIP",
"inputStage" : {
"stage" : "SORT",
"sortPattern" : {
"_id" : -1,
"maxCol" : -1
},
"inputStage" : {
"stage" : "SUBSCAN",
"inputStage" : {
"stage" : "HASH_AGGREGATE",
"inputStage" : {
"stage" : "SUBSCAN",
"inputStage" : {
"stage" : "PROJECTION",
"inputStage" : {
"stage" : "COLLSCAN"
}
}
}
}
}
}
This is on DocumentDB (mongo4)

Answer 1

I think regular expression cannot use indexes. $match works also on array, try this one:
db.collection.aggregate([
{
$match: {
"colId": 'DATA',
"data.time": { $gte: ISODate("xyz"), $lte: ISODate("zyx") },
"data.col": { $exists: true }
}
},
{ $match: { _id: { $regex: 'regex' } } },
{ $unwind: "$data" },
{ $group: { _id: "$data.time", maxCol: { $max: "$data.col" } } },
{ $sort: { "maxCol": -1, _id: -1 } },
{ $limit: 1 }
])
As consequence put an index on {colId: 1, "data.time": 1} or {colId: 1, "data.time": 1, "data.col": 1}