两个数组字段的复合索引为什么报错

  test> db.createCollection("t"); db.t.createIndex({a: 1, b: 1})
  a_1_b_1
  test> db.t.insertOne({_id: 1, a: [1, 2, 3], b: "x"})
  { acknowledged: true, insertedId: 1 }
  test> db.t.insertOne({_id: 2, a: 1, b: ["x", "y"]})
  { acknowledged: true, insertedId: 2 }

  test> try { db.t.insertOne({_id: 3, a: [1, 2], b: ["x", "y"]}) } catch (e) { ({code: e.code, name: e.codeName, message: e.message}) }
  {
    code: 171,
    name: undefined,
    message: 'cannot index parallel arrays [b] [a]'
  }
  test> db.t.updateOne({_id: 1}, {$set: {b: ["p", "q"]}})    // 把标量改成数组,让 a 和 b 同时是数组
  {
    code: 171,
    message: 'Plan executor error during update :: caused by :: cannot index parallel arrays [b] [a]'
  }
  test> db.t.find().toArray()
  [
    { _id: 1, a: [ 1, 2, 3 ], b: 'x' },
    { _id: 2, a: 1, b: [ 'x', 'y' ] }
  ]

  test> try { db.u.createIndex({a: 1, b: 1}) } catch (e) { ({code: e.code, name: e.codeName, message: e.message.slice(0, 150)}) }
  {
    code: 171,
    name: 'CannotIndexParallelArrays',
    message: 'Index build failed: 420fa910-bf98-4c68-8826-cd95ea44c52b: Collection test.u ( e3a2d792-34d6-4983-b6e4-ada0dc9c5920 ) :: caused by :: collection scan s'
  }
  test> db.u.getIndexes().map(i => i.name)
  [ '_id_' ]

  test> db.createCollection("s"); db.s.createIndex({"x.a": 1, "x.b": 1})
  x.a_1_x.b_1
  test> db.s.insertOne({x: [{a:1,b:2},{a:3,b:4}]})    // 同一个数组 x 的两个子字段,只算一个数组
  { acknowledged: true, insertedId: 1 }
  test> db.s.insertOne({x: [{a: [1,2], b: [3,4]}]})    // x.a 与 x.b 都是数组里的数组
  cannot index parallel arrays [b] [a]

  test> db.v.find({a: 3, b: 2})    // a、b 各一个单字段 multikey 索引
  {
    plan: 'FETCH > IXSCAN(a_1)',
    nReturned: 1714,
    keysExamined: 6000,
    docsExamined: 6000,
    ms: 8
  }
  test> db.v.getIndexes().map(i => i.name)
  [ '_id_', 'a_1', 'b_1' ]
  test> db.v.createIndex({a: 1, b: 1})    // 两个都是数组的集合上建复合索引
  MongoServerError: Index build failed: 653a4a12-6145-4a4d-833d-c9bcc2f7ac83: Collection test.v ( d76cba83-4382-44ea-947b-433e6070a430 ) :: caused by :: collection scan sto

  test> db.w.find({pairs: {$elemMatch: {a: 3, b: 2}}})    // 把 a、b 的组合展开成 pairs 子文档数组
  {
    plan: 'FETCH > IXSCAN(pairs.a_1_pairs.b_1)',
    nReturned: 1714,
    keysExamined: 1714,
    docsExamined: 1714,
    ms: 4
  }
  v 集合每个文档的 a、b 各 2 个元素;w 集合把它们展开成 2 × 2 = 4 个 {a, b} 子文档:
  test> db.w.validate({full: true}).keysPerIndex
  { _id_: 30000, 'pairs.a_1_pairs.b_1': 120000 }