Best way to get the first inserted record in a collection of MongoDB - mongodb

Question

I need to fetch the first inserted record in a collection in MongoDB for which I am currently using the below query:
db.users.find({}).sort({"created_at":1}).limit(1);
But, this takes up a lot of memory. The collection has about 100K records.
What is the efficient way to do this?

Answer 1

MongoDB _id is unique identifier which is automatically generated upon insertion of document into MongoDB collection
_id field stores ObjectId value and is automatically indexed.
According to MongoDB documentation,
The 12-byte ObjectId value consists of:
4-byte value representing the seconds since the Unix epoch,
3-byte machine identifier,
2-byte process id, and
3-byte counter, starting with a random value.
According to description as mentioned into above question to fetch first inserted record please try executing following mongodb find operation into MongoDB shell.
db.users.find({}).sort({"_id":1}).limit(1);
In above query we have sorted result according to _id field since _id Object ID value consists of unix epoch timestamp

Answer 2

further to this you can add specific filters in query to get first record inserted for that criteria:
like suppose you collection contains data for storing employees from IT, ADMIN, FINANCE department and you want to look for the first document inserted for IT (i.e. first IT employee) then you can execute:
db.users.find({"Dept" : "IT"}).sort({"_id":1}).limit(1);
and similarly to find last employee:
db.users.find({"Dept" : "IT"}).sort({"_id":-1}).limit(1);
Note: for bigger collections/sharded collection it will take considerable time to get the result as it iterates entire _id field for ascending and descending criteria.