notes.txt

 db.countries.find({"name": {$regex: "Korea"}}).explain("executionStats")

 db.countries.createIndex({"name": 1})


 db.countries.find({"name": {$regex: "Korea"}})

  db.countries.find({"region":"Americas"}).
count()


db.countries.find({"region": "Asia", "name": {$regex: "Kor"}})


db.countries.countDocuments({"region": "Asia", "name": {$regex: ""}})


 db.countries.createIndex({"alpha3Code": 1})

 db.countries.getIndexes()

 db.countries.find({_id: new ObjectId("651853149f2f79142a681852")})

 db.items.aggregate([
{ $match: { tag: 'blue' } },
{ $lookup: your_lookup_command }
])

db.countries.aggregate([
  { $match: { _id: new ObjectId("651853149f2f79142a681852")}},
  { $lookup: {
    from: "countries",
    localField: "borders",
    foreignField: "alpha3Code",
    as: "border_country"
  }}
])