计划缓存:什么时候会失效重选

  test> planCacheStats(刚建好索引,还没查过)
  []
  test> 第 1 次:db.t.find({a: 0, b: 2})
  2
  test> planCacheStats
  [
    {
      isActive: false,
      works: Long('11'),
      plan: 'FETCH > IXSCAN(a_1)',
      createdFrom: '{"a":0,"b":2}'
    }
  ]
  test> 第 2 次:同一个形状
  2
  test> planCacheStats
  [
    {
      isActive: true,
      works: Long('11'),
      plan: 'FETCH > IXSCAN(a_1)',
      createdFrom: '{"a":0,"b":2}'
    }
  ]

  test> db.t.find({a: 0, b: 4}).toArray().length; db.t.find({a: 0, b: 3}).toArray().length
  2
  test> 缓存条目数(同形状不同常量)
  1
  test> db.t.find({a: 0, b: 2, c: 7}).toArray().length; db.t.find({a: 0, b: 2}).sort({c: 1}).toArray().length
  2
  test> 缓存条目数(加了 c 条件、加了 sort 之后)
  3

  test> db.t.find({a: 0, b: 1}) 再执行一次后 explain().queryPlanner.winningPlan.isCached
  true

  test> db.setProfilingLevel(2)
  {
    was: 0,
    slowms: 100,
    slowinprogms: Long('5000'),
    sampleRate: 1,
    ok: 1
  }
  test> db.t.find({a: 1, b: 2})    // a=1 占 99.99% 的文档
  19998
  test> db.system.profile.find({"command.filter": {a: 1, b: 2}, op: "query"}, {_id: 0, replanned: 1, replanReason: 1, planSummary: 1, keysExamined: 1, docsExamined: 1, planCacheShapeHash: 1}).toArray()
  [
    {
      keysExamined: 103,
      docsExamined: 103,
      replanned: true,
      replanReason: 'cached plan was less efficient than expected: expected trial execution to take 11 works but it took at least 110 works',
      planCacheShapeHash: 'D1F2A12D',
      planSummary: 'IXSCAN { b: 1 }'
    }
  ]
  test> db.setProfilingLevel(0)
  {
    was: 2,
    slowms: 100,
    slowinprogms: Long('5000'),
    sampleRate: 1,
    ok: 1
  }
  test> planCacheStats(重选之后条目里的计划)
  [
    {
      isActive: false,
      works: Long('22'),
      plan: 'FETCH > IXSCAN(b_1)',
      createdFrom: '{"a":1,"b":2}'
    },
    {
      isActive: false,
      works: Long('14'),
      plan: 'SORT > FETCH > IXSCAN(a_1)',
      createdFrom: '{"a":0,"b":2}'
    },
    {
      isActive: false,
      works: Long('11'),
      plan: 'FETCH > IXSCAN(a_1)',
      createdFrom: '{"a":0,"b":2,"c":7}'
    }
  ]

  test> 缓存条目数(之前)
  3
  test> db.t.createIndex({c: 1, b: 1})
  c_1_b_1
  test> 缓存条目数(创建索引 c_1_b_1 之后)
  0
  test> 重新执行两次后
  1
  test> hideIndex 之后的缓存条目数
  0

  test> 一个缓存条目的字段
  {
    keys: [
      'version',            'queryHash',
      'planCacheShapeHash', 'planCacheKey',
      'isActive',           'works',
      'reads',              'timeOfCreation',
      'createdFromQuery',   'cachedPlan',
      'creationExecStats',  'candidatePlanScores',
      'indexFilterSet',     'estimatedSizeBytes',
      'solutionHash',       'host'
    ],
    candidatePlanScores: [ 2.182018181818182, 1.1820181818181819 ],
    creationWorks: [ 11, 11 ]
  }
  test> db.t.getPlanCache().clear() 之后的条目数
  0