MongoDB 游標(biāo)詳解及實(shí)例代碼

  db.collection.find()方法返回一個(gè)游標(biāo)，對(duì)于文檔的訪問(wèn)，我們需要進(jìn)行游標(biāo)迭代
  mongoDB的游標(biāo)與關(guān)系型數(shù)據(jù)庫(kù)SQL中的游標(biāo)類似，可以通過(guò)對(duì)游標(biāo)進(jìn)行(如限制查詢結(jié)果數(shù)，跳過(guò)的結(jié)果數(shù)等)設(shè)置來(lái)控制查詢結(jié)果
  游標(biāo)會(huì)消耗內(nèi)存和相關(guān)系統(tǒng)資源，游標(biāo)使用完后應(yīng)盡快釋放資源
  在mongo shell中，如果返回的游標(biāo)結(jié)果集未指定給某個(gè)var定義的變量，則，游標(biāo)自動(dòng)迭代20次，即輸出前20個(gè)文檔，超出20的情形則需要輸入it來(lái)翻頁(yè)
  本文內(nèi)容描述手動(dòng)方式來(lái)實(shí)現(xiàn)游標(biāo)迭代來(lái)訪問(wèn)文檔或者是用索引迭代

  聲明游標(biāo)
      var cursor = db.collectioName.find(query,projection);

  打開游標(biāo)
      Cursor.hasNext() 判斷游標(biāo)是否已經(jīng)取到盡頭

  讀取數(shù)據(jù)
      Cursor.Next()  取出游標(biāo)的下一個(gè)文檔

  關(guān)閉游標(biāo)
      cursor.close()  此步驟可省略，通常為自動(dòng)關(guān)閉，也可以顯示關(guān)閉

  用while循環(huán)來(lái)遍歷游標(biāo)示例
      var mycursor = db.bar.find({_id:{$lte:5}})
      while(mycursor.hasNext()) {
          printjson(mycursor.next());
          }

  游標(biāo)生命周期
      a、游標(biāo)完成匹配結(jié)果的迭代后，它會(huì)清除自身；
      b、客戶端的游標(biāo)已經(jīng)不在作用域內(nèi)，驅(qū)動(dòng)程序回向服務(wù)器發(fā)送一條特別的消息，讓其銷毀；
      c、缺省情況下，游標(biāo)在十分鐘內(nèi)沒(méi)有使用，游標(biāo)自動(dòng)關(guān)閉或者客戶端已經(jīng)迭代完整個(gè)游標(biāo)；
      d、可以通過(guò)cursor.noCursorTimeout()來(lái)定義游標(biāo)超時(shí)時(shí)間
          如：var myCursor = db.users.find().noCursorTimeout()
      e、對(duì)于自定義超時(shí)時(shí)長(zhǎng)的游標(biāo)可以使用cursor.close() 來(lái)關(guān)閉游標(biāo)
          如：db.collection.find(query>).close()

 repSetTest:PRIMARY> db.version()
  3.0.12
  //創(chuàng)建包含29個(gè)文檔的集合user
  repSetTest:PRIMARY> for (var i=1;i30;i++){
  ... db.user.insert({"id":i,"ename":"usr"+i});
  ... }
  WriteResult({ "nInserted" : 1 })
  repSetTest:PRIMARY> db.user.count()
  29

  //查詢集合user上所有文檔
  repSetTest:PRIMARY> db.user.find()
  { "_id" : ObjectId("5804d07fd974b32430ea9748"), "id" : 1, "ename" : "usr1" }
  { "_id" : ObjectId("5804d07fd974b32430ea9749"), "id" : 2, "ename" : "usr2" }
        .............................
  { "_id" : ObjectId("5804d07fd974b32430ea975b"), "id" : 20, "ename" : "usr20" }
  Type "it" for more  //上面的結(jié)果只輸出了20行，這個(gè)提示表明查看更多應(yīng)輸入it

  repSetTest:PRIMARY> it
  { "_id" : ObjectId("5804d07fd974b32430ea975c"), "id" : 21, "ename" : "usr21" }
   ..............
  { "_id" : ObjectId("5804d07fd974b32430ea9764"), "id" : 29, "ename" : "usr29" }

 repSetTest:PRIMARY> var myCursor = db.user.find()
      while (myCursor.hasNext()) {
      print(tojson(myCursor.next()))
  }

  { "_id" : ObjectId("5804d07fd974b32430ea9748"), "id" : 1, "ename" : "usr1" }
     ..........
  {
      "_id" : ObjectId("5804d07fd974b32430ea9751"),
      "id" : 10,
      "ename" : "usr10"
  }
     ................
  {
      "_id" : ObjectId("5804d07fd974b32430ea9764"),
      "id" : 29,
      "ename" : "usr29"
  }
  //上述查詢中通過(guò)var myCursor進(jìn)行變量的定義，相當(dāng)于SQL中的declare cursor cur_name is select ..
  //變量 myCursor定義僅僅是定義，并不會(huì)訪問(wèn)數(shù)據(jù)庫(kù)，而是在myCursor.hasNext()真正訪問(wèn)數(shù)據(jù)庫(kù)
  //myCursor.next()則是輸出下一條記錄，hasNext()訪問(wèn)數(shù)據(jù)庫(kù)時(shí)會(huì)根據(jù)缺省游標(biāo)設(shè)定將結(jié)果讀取到本地

  repSetTest:PRIMARY> var myCursor = db.user.find({id:{$gt:20}})
      while (myCursor.hasNext()) {
      printjson(myCursor.next());}
  {
      "_id" : ObjectId("5804d07fd974b32430ea975c"),
      "id" : 21,
      "ename" : "usr21"
  }
       .......
  {
      "_id" : ObjectId("5804d07fd974b32430ea9764"),
      "id" : 29,
      "ename" : "usr29"
  }

  repSetTest:PRIMARY> var myCursor = db.user.find({id:{$gt:20}})
  repSetTest:PRIMARY> myCursor.forEach(printjson);
  {
      "_id" : ObjectId("5804d07fd974b32430ea975c"),
      "id" : 21,
      "ename" : "usr21"
  }
      ................
  {
      "_id" : ObjectId("5804d07fd974b32430ea9764"),
      "id" : 29,
      "ename" : "usr29"
  }

 //如下示例，將游標(biāo)返回的內(nèi)容傳遞到數(shù)組，然后使用 printjson (documentArray[3])輸出其中的元素
  repSetTest:PRIMARY> var myCursor = db.user.find({id:{$gt:20}})
  repSetTest:PRIMARY> var documentArray = myCursor.toArray();
  repSetTest:PRIMARY> printjson (documentArray[3])
  {
      "_id" : ObjectId("580d775edeb57e4d05eec0f2"),
      "id" : 24,     //Author : Leshami
      "ename" : "usr24" //Blog  : http://blog.csdn.net/leshami
  }

  //也可以將數(shù)組元素輸出到某個(gè)變量，然后在用printjson(myDocument)輸出這個(gè)變量，如下
  repSetTest:PRIMARY> var myDocument = documentArray[3];
  repSetTest:PRIMARY> printjson(myDocument)
  {
      "_id" : ObjectId("580d775edeb57e4d05eec0f2"),
      "id" : 24,
      "ename" : "usr24"
  }

 //設(shè)置迭代顯示的次數(shù)，如下設(shè)置為5
  repSetTest:PRIMARY> DBQuery.shellBatchSize = 5
  5
  repSetTest:PRIMARY> db.user.find()
  { "_id" : ObjectId("5804d07fd974b32430ea9748"), "id" : 1, "ename" : "usr1" }
  { "_id" : ObjectId("5804d07fd974b32430ea9749"), "id" : 2, "ename" : "usr2" }
  { "_id" : ObjectId("5804d07fd974b32430ea974a"), "id" : 3, "ename" : "usr3" }
  { "_id" : ObjectId("5804d07fd974b32430ea974b"), "id" : 4, "ename" : "usr4" }
  { "_id" : ObjectId("5804d07fd974b32430ea974c"), "id" : 5, "ename" : "usr5" }
  Type "it" for more //從上面的查詢結(jié)果可知，當(dāng)輸出5個(gè)文檔就提示需要輸入it來(lái)查看更多
  repSetTest:PRIMARY> it
  { "_id" : ObjectId("5804d07fd974b32430ea974d"), "id" : 6, "ename" : "usr6" }
  { "_id" : ObjectId("5804d07fd974b32430ea974e"), "id" : 7, "ename" : "usr7" }
  { "_id" : ObjectId("5804d07fd974b32430ea974f"), "id" : 8, "ename" : "usr8" }
  { "_id" : ObjectId("5804d07fd974b32430ea9750"), "id" : 9, "ename" : "usr9" }
  { "_id" : ObjectId("5804d07fd974b32430ea9751"), "id" : 10, "ename" : "usr10" }
  Type "it" for more

 可以通過(guò)db.serverStatus()查看游標(biāo)狀態(tài)相關(guān)的信息，這些信息通常包括
      從服務(wù)器上次啟動(dòng)之后游標(biāo)超時(shí)的數(shù)量
      自定義游標(biāo)超時(shí)的數(shù)量
      游標(biāo)打開后已經(jīng)pinned的數(shù)量
      打開游標(biāo)的總數(shù)目
  //如下查詢本機(jī)游標(biāo)的信息    
  repSetTest:PRIMARY> db.serverStatus().metrics.cursor
  {
      "timedOut" : NumberLong(2),
      "open" : {
          "noTimeout" : NumberLong(0),
          "pinned" : NumberLong(0),
          "total" : NumberLong(2)
      }
  }