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Analyze Query Performance

When we start programming, one thing come in our mind which is performance. There are many factors of performance but query optimization/analyzing and database structure is the more important.

In this article we will evaluate the performance of the query, Analyzing query is aspect of measurement of database and indexing effectiveness.

db.collection.explain("executionStats")  methods provide statistics about performance of query. This data output can be useful in measuring if and how a query uses an index.

Using $explain
It's provides statistics about performance of query.

Evaluate the performance of query


{ "_id" : 1, "ISBN" : "22345654562349", type: " Paper", quantity: 500 }
{ "_id" : 2, " ISBN " : "3345678765678", type: " Paper ", quantity: 100 }
{ "_id" : 3, " ISBN " : "1232234543551", type: " Paper ", quantity: 200 }
{ "_id" : 4, " ISBN " : "5545533344555", type: " Paper ", quantity: 150 }
{ "_id" : 5, " ISBN " : "9988844545457", type: " Paper ", quantity: 300 }


Query With No Index


db.Book.find(
   { quantity: { $gte: 100, $lte: 200 } }
)


Query Returns result


{ "_id" : 2, " ISBN " : "3345678765678", type: " Paper ", quantity: 100 }
{ "_id" : 3, " ISBN " : "1232234543551", type: " Paper ", quantity: 200 }
{ "_id" : 4, " ISBN " : "5545533344555", type: " Paper ", quantity: 150 }



$explain on the following query:


db.Book.find(
   { quantity: { $gte: 100, $lte: 200 } }
).explain("executionStats")


The above explain() query returns the following analyzed result:


{
   "queryPlanner" : {
         "plannerVersion" : 1,
         ...
         "winningPlan" : {
            "stage" : "COLLSCAN",
            ...
         }
   },
   "executionStats" : {
      "executionSuccess" : true,
      "nReturned" : 3,
      "executionTimeMillis" : 0,
      "totalKeysExamined" : 0,
      "totalDocsExamined" : 5,
      "executionStages" : {
         "stage" : "COLLSCAN",
         ...
      },
      ...
   },
   ...
}

The fields in this result set:

queryPlanner.winningPlan.stage : COLLSCAN which indicate a collection is scanned.

executionStats.nReturned : 3 which indicate that the query matches and returns three documents.

executionStats.totalDocsExamined : 5 which indicate that MongoDB had to scan five documents (i.e. all documents in the collection) to find the three matching documents.


Now we indexing field quantity


db.book.createIndex( { quantity: 1 } )


To view the query plan statistics, use the explain("executionStats") method:


db.Book.find(
   { quantity: { $gte: 100, $lte: 200 } }
).explain("executionStats")



The explain() method returns the following results:


{
   "queryPlanner" : {
         "plannerVersion" : 1,
         ...
         "winningPlan" : {
               "stage" : "FETCH",
               "inputStage" : {
                  "stage" : "IXSCAN",
                  "keyPattern" : {
                     "quantity" : 1
                  },
                  ...
               }
         },
         "rejectedPlans" : [ ]
   },
   "executionStats" : {
         "executionSuccess" : true,
         "nReturned" : 3,
         "executionTimeMillis" : 0,
         "totalKeysExamined" : 3,
         "totalDocsExamined" : 3,
         "executionStages" : {
            ...
         },
         ...
   },
   ...
}


queryPlanner.winningPlan.stage : IXSCAN which indicate a collection is scanned.

executionStats.nReturned : 3 which indicate that the query matches and returns three documents.

executionStats.totalDocsExamined : 3 which indicate that MongoDB had to scan three documents (i.e. all documents in the index) to find the three matching documents.


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