test-_id-random-lookup-insert-phase.js

// node --max-old-space-size=8192 test-_id-random-lookup.js
var fs = require('node:fs');
var numDocs = 1e6;

async function main() {
  var  { MongoClient, ObjectId } = require('mongodb');
  var client = await MongoClient.connect('mongodb://localhost:27017/test');
  console.log('Connected');
  var db = client.db('testing');
  try {
    await db.dropCollection('obj');
  } catch(e) { null; }
  var coll = await db.collection('obj');
  var docs = [];
  
  console.time('insert');
  for(let i = 0; i < numDocs; i++) {
    // docs.push({ _id: new ObjectId(), i });
    docs.push({ _id: (new ObjectId()).toString(), i });
  }
  await coll.insertMany(docs);
  console.timeEnd('insert');
  
  console.log((await coll.countDocuments({})));
  fs.writeFileSync('docs.js', JSON.stringify(docs));
}

main()
  .then(() => process.exit())
  .catch((e) => { console.error(e); process.exit(); });

test-_id-random-lookup.js

// Run test-_id-random-lookup-insert-phase.js first.
var fs = require('node:fs');
var numLookup = 1e4;

async function main() {
  var  { MongoClient, ObjectId } = require('mongodb');
  var client = await MongoClient.connect('mongodb://localhost:27017/test');
  console.log('Connected');
  var db = client.db('testing');
  var coll = await db.collection('obj');
  var docs = JSON.parse(fs.readFileSync('docs.js'));
  var numDocs = docs.length;

  console.time('lookup');
  for(let i = 0; i < numLookup; i++) {
    // var doc = await coll.findOne({ _id: ObjectId(docs[randInt(numDocs)]._id) });
    var doc = await coll.findOne({ _id: docs[randInt(numDocs)]._id });
    // console.log(doc);
  }
  console.timeEnd('lookup');
  client.close();
}

function randInt(numDocs) {
  return Math.floor(Math.random()*numDocs);
}

main()
  .then(() => process.exit())
  .catch((e) => { console.error(e); process.exit(); });