How does mongodb aggregate and return the entire data?

  mongodb, question

As follows:

After filtering, they are grouped again, and the two fields are grouping conditions.
However, I just want to get the time within the group, that is, the data with fdate being the largest.
How can it be done? ?

db.SEC_2018_05_12.aggregate([{ $match : { fyear : { $eq : 2018},fmonth:{$eq : 5},fday:{$eq : 12},fhour:{$eq : 17},fmin:{$eq : 8} } }
 ,  { $group: { _id: {dev_id: "$dev_id", data_id: "$data_id"},maxTimeValue: { $max: "$fdate" } } }  ]).pretty()

If you can give some test conditions, it will be easier for others to understand your problem.
First of all, $eq, this operator is unnecessary to write most of the time. So your query is equivalent to:

 { $match : { fyear : 2018, fmonth: 5, fday: 12, fhour: 17, fmin: 8 } },
 { $group: { _id: {dev_id: "$dev_id", data_id: "$data_id"}, maxTimeValue: { $max: "$fdate" } } }  ]).pretty()

Then your request is to get the first data, not just the largest one.fdateValue. You can change your mind to understand this problem:

Sort by dev_id asc, data_id asc, fdate desc, and then take the first piece of data from each grouping.

According to this idea, the query should be:

 { $match : { fyear : 2018, fmonth: 5, fday: 12, fhour: 17, fmin: 8 } },
 { $sort: { dev_id: 1, data_id: 1, fdate: -1} },
 { $group: { _id: {dev_id: "$dev_id", data_id: "$data_id"}, maxDoc: { $first: "$$ROOT" } } }  ]).pretty()

Finally, I would like to talk about the problem of data model design.$matchThis is actually a time, but it has been split into many parts to store. Unless there are clear reasons to support it, individuals are not very much in favor of such an approach.

  1. Originally there was only 4 bytes at a time, so you became 5 x 4 = 20 bytes. Pay attention to the storage space.
  2. $match/$sortNeed index support, so the original index will now become a joint index of 5 fields, which is also a waste of space and efficiency.

In the case of large amount of data, the above query needs index support. In your current writing style, the index should read:

 fyear: 1,
 fmonth: 1,
 fday: 1,
 fhour: 1,
 fmin: 1,
 dev_id: 1,
 data_id: 1,
 fdate: -1

It looks scary enough, doesn’t it? But in fact, the previous pile of time itself is only one time.