https://stackoverflow.com/questions/79899097

79899097.js

db.test.drop();
db.test.createIndex({A:1, B:1})
const letters = "abcdefghijklmnopqrstuvwxyz".split("");
const total=0; for (let i = 0; i < 1000; i++) {
 const result = db.test.insertMany(
  Array.from({ length: 10000 }, () => ({
    A: Math.floor(Math.random() * 5) + 1,
    B: letters[Math.floor(Math.random() * letters.length)]
  }))
 );
 total=total+Object.keys(result.insertedIds).length;
 print(`Total: ${total/1e6} million`);
}


db.test.find({
  A: { $gte: 1, $lte: 3 },
  B: { $in: ["x", "y", "z"] }
},{A:1, B:1, _id:0}
).explain("executionStats").executionStats.executionStages.inputStage
;

{
  stage: 'IXSCAN',
  nReturned: 692735,
  executionTimeMillisEstimate: 340,
  works: 692738,
  advanced: 692735,
  needTime: 2,
  needYield: 0,
  saveState: 29,
  restoreState: 29,
  isEOF: 1,
  keyPattern: { A: 1, B: 1 },
  indexName: 'A_1_B_1',
  isMultiKey: false,
  multiKeyPaths: { A: [], B: [] },
  isUnique: false,
  isSparse: false,
  isPartial: false,
  indexVersion: 2,
  direction: 'forward',
  indexBounds: { A: [ '[1, 3]' ], B: [ '["x", "x"]', '["y", "y"]', '["z", "z"]' ] },
  keysExamined: 692738,
  seeks: 3,
  dupsTested: 0,
  dupsDropped: 0
}


db.test.find({
  A: { $in: [1, 2, 3] },
  B: { $in: ["x", "y", "z"] }
},{A:1, B:1, _id:0}
).explain("executionStats").executionStats.executionStages.inputStage

{
  stage: 'IXSCAN',
  nReturned: 692735,
  executionTimeMillisEstimate: 371,
  works: 692738,
  advanced: 692735,
  needTime: 2,
  needYield: 0,
  saveState: 25,
  restoreState: 25,
  isEOF: 1,
  keyPattern: { A: 1, B: 1 },
  indexName: 'A_1_B_1',
  isMultiKey: false,
  multiKeyPaths: { A: [], B: [] },
  isUnique: false,
  isSparse: false,
  isPartial: false,
  indexVersion: 2,
  direction: 'forward',
  indexBounds: {
    A: [ '[1, 1]', '[2, 2]', '[3, 3]' ],
    B: [ '["x", "x"]', '["y", "y"]', '["z", "z"]' ]
  },
  keysExamined: 692738,
  seeks: 3,
  dupsTested: 0,
  dupsDropped: 0
}