【Mongodb】視圖 && 索引

時間 2020-04-16

原文原文鏈接

準備工做html

準備2個集合的數據，後面視圖和索引都會用到
1個訂單集合，一個收款信息集合mongodb

var orders = new Array();
var shipping = new Array();
var addresses = ["廣西省玉林市", "湖南省岳陽市", "湖北省荊州市", "甘肅省蘭州市", "吉林省松原市", "江西省景德鎮", "遼寧省瀋陽市", "福建省廈門市", "廣東省廣州市", "北京市朝陽區"];

for (var i = 10000; i < 20000; i++) {
    var orderNo = i + Math.random().toString().substr(2, 5);
    orders[i] = { orderNo: orderNo, userId: i, price: Math.round(Math.random() * 10000) / 100, qty: Math.floor(Math.random() * 10) + 1, orderTime: new Date(new Date().setSeconds(Math.floor(Math.random() * 10000))) };

    var address = addresses[Math.floor(Math.random() * 10)];
    shipping[i] = { orderNo: orderNo, address: address, recipienter: "Wilson", province: address.substr(0, 3), city: address.substr(3, 3) }
}
db.order.insert(orders);
db.shipping.insert(shipping);

視圖數據庫

概述數組

A MongoDB view is a queryable object whose contents are defined by an aggregation pipeline on other collections or views. MongoDB does not persist the view contents to disk. A view’s content is computed on-demand when a client queries the view. MongoDB can require clients to have permission to query the view. MongoDB does not support write operations against views.app

Mongodb的視圖基本上和SQL的視圖同樣dom

數據源（集合或視圖）
提供查詢
不實際存儲硬盤
客戶端發起請求查詢時計算而得

1. 建立視圖ui

有兩種方法建立視圖spa

db.createCollection(
  "<viewName>",
  {
    "viewOn" : "<source>",
    "pipeline" : [<pipeline>],
    "collation" : { <collation> }
  }
)

db.createView(
  "<viewName>",
  "<source>",
  [<pipeline>],
  {
    "collation" : { <collation> }
  }
)

通常使用db.createView設計

viewName : 必須，視圖名稱code

source : 必須，數據源，集合/視圖

[<pipeline>] : 可選，一組管道，可見管道是Mongodb比較重要的一環

1.1 單個集合建立視圖

假設如今查看當天最高的10筆訂單視圖，例如後臺某個地方須要實時顯示金額最高的訂單

db.createView(
    "orderInfo",         //視圖名稱
    "order",             //數據源   
    [
        //篩選符合條件的訂單，大於當天，這裏要注意時區
        { $match: { "orderTime": { $gte: ISODate("2020-04-13T16:00:00.000Z") } } },
        //按金額倒序
        { $sort: { "price": -1 } },
        //限制10個文檔
        { $limit: 10 },
        //選擇要顯示的字段
        //0: 排除字段，若字段上使用（_id除外），就不能有其餘包含字段
        //1: 包含字段
        { $project: { _id: 0, orderNo: 1, price: 1, orderTime: 1 } }
    ]
)

而後就能夠直接使用orderInfo這個視圖查詢數據

db.orderInfo.find({})

返回結果

{ "orderNo" : "1755149436", "price" : 100, "orderTime" : ISODate("2020-04-14T13:49:42.220Z") }
{ "orderNo" : "1951423853", "price" : 99.99, "orderTime" : ISODate("2020-04-14T15:08:07.240Z") }
{ "orderNo" : "1196303215", "price" : 99.99, "orderTime" : ISODate("2020-04-14T15:15:41.158Z") }
{ "orderNo" : "1580069456", "price" : 99.98, "orderTime" : ISODate("2020-04-14T13:41:07.199Z") }
{ "orderNo" : "1114480559", "price" : 99.98, "orderTime" : ISODate("2020-04-14T13:31:58.150Z") }
{ "orderNo" : "1229542817", "price" : 99.98, "orderTime" : ISODate("2020-04-14T15:15:35.162Z") }
{ "orderNo" : "1208031402", "price" : 99.94, "orderTime" : ISODate("2020-04-14T14:13:02.160Z") }
{ "orderNo" : "1680622670", "price" : 99.93, "orderTime" : ISODate("2020-04-14T15:17:25.210Z") }
{ "orderNo" : "1549824953", "price" : 99.92, "orderTime" : ISODate("2020-04-14T13:09:41.196Z") }
{ "orderNo" : "1449930147", "price" : 99.92, "orderTime" : ISODate("2020-04-14T15:16:15.187Z") }

1.2 多個集合建立視圖

其實跟單個是集合是同樣，只是多了$lookup鏈接操做符，視圖根據管道最終結果顯示，因此能夠關聯多個集合（若出現這種狀況就要考慮集合設計是否合理，mongodb原本就是文檔型數據庫）

db.orderDetail.drop()
db.createView(
    "orderDetail",
    "order",
    [
        { $lookup: { from: "shipping", localField: "orderNo", foreignField: "orderNo", as: "shipping" } },
        { $project: { "orderNo": 1, "price": 1, "shipping.address": 1 } }
    ]
)

查詢視圖，獲得以下結果

{ "_id" : ObjectId("5e95af8c4ef6faf974b4a6c3"), "orderNo" : "1000039782", "price" : 85.94, "shipping" : [ { "address" : "北京市朝陽區" } ] }
{ "_id" : ObjectId("5e95af8c4ef6faf974b4a6c4"), "orderNo" : "1000102128", "price" : 29.04, "shipping" : [ { "address" : "吉林省松原市" } ] }
{ "_id" : ObjectId("5e95af8c4ef6faf974b4a6c5"), "orderNo" : "1000214514", "price" : 90.69, "shipping" : [ { "address" : "湖南省岳陽市" } ] }
{ "_id" : ObjectId("5e95af8c4ef6faf974b4a6c6"), "orderNo" : "1000337987", "price" : 75.05, "shipping" : [ { "address" : "遼寧省瀋陽市" } ] }
{ "_id" : ObjectId("5e95af8c4ef6faf974b4a6c7"), "orderNo" : "1000468969", "price" : 76.84, "shipping" : [ { "address" : "江西省景德鎮" } ] }
{ "_id" : ObjectId("5e95af8c4ef6faf974b4a6c8"), "orderNo" : "1000572219", "price" : 60.25, "shipping" : [ { "address" : "江西省景德鎮" } ] }
{ "_id" : ObjectId("5e95af8c4ef6faf974b4a6c9"), "orderNo" : "1000611743", "price" : 19.14, "shipping" : [ { "address" : "廣東省廣州市" } ] }
{ "_id" : ObjectId("5e95af8c4ef6faf974b4a6ca"), "orderNo" : "1000773917", "price" : 31.5, "shipping" : [ { "address" : "北京市朝陽區" } ] }
{ "_id" : ObjectId("5e95af8c4ef6faf974b4a6cb"), "orderNo" : "1000879146", "price" : 76.16, "shipping" : [ { "address" : "吉林省松原市" } ] }
{ "_id" : ObjectId("5e95af8c4ef6faf974b4a6cc"), "orderNo" : "1000945977", "price" : 93.98, "shipping" : [ { "address" : "遼寧省瀋陽市" } ] }

能夠看到，mongodb不是像SQL那樣把鏈接的表當成列列出，而是把鏈接結果放在數組裏面，這很符合Mongodb文檔型結構。

2. 修改視圖

假設如今須要增長一個數量的字段

db.runCommand({
    collMod: "orderInfo",
    viewOn: "order",
    pipeline: [
        { $match: { "orderTime": { $gte: ISODate("2020-04-13T16:00:00.000Z") } } },
        { $sort: { "price": -1 } },
        { $limit: 10 },
        //增長qty
        { $project: { _id: 0, orderNo: 1, price: 1, qty: 1, orderTime: 1 } }
    ]
})

固然，也能夠刪除視圖，從新用db.createView()建立視圖

3. 刪除視圖

db.orderInfo.drop();

索引

概述

Indexes support the efficient execution of queries in MongoDB. Without indexes, MongoDB must perform a collection scan, i.e. scan every document in a collection, to select those documents that match the query statement. If an appropriate index exists for a query, MongoDB can use the index to limit the number of documents it must inspect.

索引能提供高效的查詢，沒有索引的查詢，mongole執行集合掃描，至關於SQL SERVER的全表掃描，掃描每個文檔。

數據存在存儲介質上，大多數狀況是爲了查詢，查詢的快慢直接影響用戶體驗，mongodb索引也是空間換時間，添加索引，CUD操做都會致使索引從新生成，影響速度。

1. 準備工做

1.1 準備200W條數據

var orderNo = 100 * 10000;
for (var i = 0; i < 100; i++) {
    //分批次插入，每次20000條
    var orders = new Array();
    for (var j = 0; j < 20000; j++) {
        var orderNo = orderNo++;
        orders[j] = { orderNo: orderNo, userId: i + j, price: Math.round(Math.random() * 10000) / 100, qty: Math.floor(Math.random() * 10) + 1, orderTime: new Date(new Date().setSeconds(Math.floor(Math.random() * 10000))) };
    }
    //不需寫入確認
    db.order.insert(orders, { writeConcern: { w: 0 } });
}

1.2 mongodb的查詢計劃

db.collection.explain().<method(...)>

通常使用執行統計模式，例如

db.order.explain("executionStats").find({orderNo:1000000})

返回的executionStats對象字段說明

部分字段說明

字段	說明
executionSuccess	是否執行成功
nReturned	返回匹配文檔數量
executionTimeMillis	執行時間，單位：毫秒
totalKeysExamined	索引檢索數目
totalDocsExamined	文檔檢索數目

查看未加索引前查詢計劃

db.order.explain("executionStats").find({orderNo:1000000})

截取部分返回結果,能夠看出

executionTimeMillis : 用時1437毫秒
totalDocsExamined : 掃描文檔200W
executionStages.stage : 集合掃描

"executionStats" : {
    "executionSuccess" : true,
    "nReturned" : 1,
    "executionTimeMillis" : 1437,
    "totalKeysExamined" : 0,
    "totalDocsExamined" : 2000000,
    "executionStages" : {
            "stage" : "COLLSCAN",

1.3 查看當前集合統計信息

db.order.stats()

截取部分信息，能夠看出如今存儲文件大小大概爲72M

{
        "ns" : "mongo.order",
        "size" : 204000000,
        "count" : 2000000,
        "avgObjSize" : 102,
        "storageSize" : 74473472,

2. 建立索引

db.order.createIndex({ orderNo: 1 }, { name: "ix_orderNo" })

索引名稱不是必須，若不指定，按字段名稱_排序類型組合自動生成，索引名稱一旦建立不能修改，若要修改，只能刪除索引從新生成索引，建議仍是建索引的時候就把索引名稱設置好。

2.1 執行查詢計劃

db.order.explain("executionStats").find({orderNo:1000000})

截取部分結果，直觀就能夠感受查詢速度有了質的提高，再看查詢計劃更加驚訝

nReturned : 匹配到1個文檔
executionTimeMillis : 0，呃。。
totalKeysExamined : 總共檢索了1個索引
totalDocsExamined : 總共檢索了1個文檔
executionStages.stage : FETCH，根據索引去檢索指定文檔，像SQL的Index Seek

 "executionStats" : {
                "executionSuccess" : true,
                "nReturned" : 1,
                "executionTimeMillis" : 0,
                "totalKeysExamined" : 1,
                "totalDocsExamined" : 1,
                "executionStages" : {
                        "stage" : "FETCH"