MongoDB_09——执行计划

索引的执行计划

• 1:分析查询性能(Analyze Query Performance)通常使用执行计划(解释计划,explain Plan)来查询的情况，如查询耗费的时间，是否基于索引查询等.

• 那么,通常,我们想知道，建立的索引是否有效,效果如何，都需要通过执行计划查看。

  语法:
      db.collection.find(query,options).explain(options)
  --------------------------------------------------------------------
  #根据userid查询数据的情况
  db.comment.find({userid:"1003"}).explain()
  
  执行:
  > db.comment.find({userid:"1003"})
  { "_id" : "3", "articleid" : "100003", "content" : "我爱你，绩憨憨3", "userid" : "1003", "nickname" : "hanhan", "createdatetime" : ISODate("2020-02-28T14:45:54.235Z"), "likenum" : 10, "state" : null }
      
      
    
  > db.comment.find({userid:"1003"}).explain()
  {
          "queryPlanner" : {
                  "plannerVersion" : 1,
                  "namespace" : "comment.comment",
                  "indexFilterSet" : false,
                  "parsedQuery" : {
                          "userid" : {
                                  "$eq" : "1003"
                          }
                  },
                  "queryHash" : "37A12FC3",
                  "planCacheKey" : "37A12FC3",
                  "winningPlan" : {
                          "stage" : "COLLSCAN",           #集合扫描，全部扫描
                          "filter" : {
                                  "userid" : {
                                          "$eq" : "1003"
                                  }
                          },
                          "direction" : "forward"
                  },
                  "rejectedPlans" : [ ]
          },
          "serverInfo" : {
                  "host" : "LAPTOP-ZJ",
                  "port" : 27017,
                  "version" : "4.2.3",
                  "gitVersion" : "6874650b362138df74be53d366bbefc321ea32d4"
          },
          "ok" : 1
  }
  #加上索引，看效果
  db.comment.createIndex({userid:1})
  > db.comment.createIndex({userid:1})
  {
          "createdCollectionAutomatically" : false,
          "numIndexesBefore" : 1,
          "numIndexesAfter" : 2,
          "ok" : 1
  }
  
  
  > db.comment.find({userid:"1003"}).explain({})
  {
          "queryPlanner" : {
                  "plannerVersion" : 1,
                  "namespace" : "comment.comment",
                  "indexFilterSet" : false,
                  "parsedQuery" : {
                          "userid" : {
                                  "$eq" : "1003"
                          }
                  },
                  "winningPlan" : {
                          "stage" : "FETCH",      #这里是FETCH,拉取，有用了索引
                          "inputStage" : {
                                  "stage" : "IXSCAN",
                                  "keyPattern" : {
                                          "userid" : 1
                                  },
                                  "indexName" : "userid_1",
                                  "isMultiKey" : false,
                                  "multiKeyPaths" : {
                                          "userid" : [ ]
                                  },
                                  "isUnique" : false,
                                  "isSparse" : false,
                                  "isPartial" : false,
                                  "indexVersion" : 2,
                                  "direction" : "forward",
                                  "indexBounds" : {
                                          "userid" : [
                                                  "[\"1003\", \"1003\"]"
                                          ]
                                  }
                          }
                  },
                  "rejectedPlans" : [ ]
          },
          "executionStats" : {
                  "executionSuccess" : true,
                  "nReturned" : 1,
                  "executionTimeMillis" : 3,
                  "totalKeysExamined" : 1,
                  "totalDocsExamined" : 1,
                  "executionStages" : {
                          "stage" : "FETCH",
                          "nReturned" : 1,
                          "executionTimeMillisEstimate" : 0,
                          "works" : 2,
                          "advanced" : 1,
                          "needTime" : 0,
                          "needYield" : 0,
                          "saveState" : 0,
                          "restoreState" : 0,
                          "isEOF" : 1,
                          "docsExamined" : 1,
                          "alreadyHasObj" : 0,
                          "inputStage" : {
                                  "stage" : "IXSCAN",
                                  "nReturned" : 1,
                                  "executionTimeMillisEstimate" : 0,
                                  "works" : 2,
                                  "advanced" : 1,
                                  "needTime" : 0,
                                  "needYield" : 0,
                                  "saveState" : 0,
                                  "restoreState" : 0,
                                  "isEOF" : 1,
                                  "keyPattern" : {
                                          "userid" : 1
                                  },
                                  "indexName" : "userid_1",
                                  "isMultiKey" : false,
                                  "multiKeyPaths" : {
                                          "userid" : [ ]
                                  },
                                  "isUnique" : false,
                                  "isSparse" : false,
                                  "isPartial" : false,
                                  "indexVersion" : 2,
                                  "direction" : "forward",
                                  "indexBounds" : {
                                          "userid" : [
                                                  "[\"1003\", \"1003\"]"
                                          ]
                                  },
                                  "keysExamined" : 1,
                                  "seeks" : 1,
                                  "dupsTested" : 0,
                                  "dupsDropped" : 0
                          }
                  },
                  "allPlansExecution" : [ ]
          },
          "serverInfo" : {
                  "host" : "LAPTOP-ZJ",
                  "port" : 27017,
                  "version" : "4.2.3",
                  "gitVersion" : "6874650b362138df74be53d366bbefc321ea32d4"
          },
          "ok" : 1
  }
  
  #先通过IXSCAN,再FETCH