HOMEWORK: HOMEWORK 4.2
Suppose you have a collection called tweets whose documents contain information about thecreated_at time of the tweet and the user’s followers_count at the time they issued the tweet. What can you infer from the following explain output?
> db.tweets.explain("executionStats").find({"user.followers_count":{$gt:1000}}).limit(10).skip(5000).sort( { created_at : 1 } )
{
"queryPlanner" : {
"plannerVersion" : 1,
"namespace" : "twitter.tweets",
"indexFilterSet" : false,
"parsedQuery" : {
"user.followers_count" : {
"$gt" : 1000
}
},
"winningPlan" : {
"stage" : "LIMIT",
"limitAmount" : 0,
"inputStage" : {
"stage" : "SKIP",
"skipAmount" : 0,
"inputStage" : {
"stage" : "FETCH",
"filter" : {
"user.followers_count" : {
"$gt" : 1000
}
},
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"created_at" : -1
},
"indexName" : "created_at_-1",
"isMultiKey" : false,
"direction" : "backward",
"indexBounds" : {
"created_at" : [
"[MinKey, MaxKey]"
]
}
}
}
}
},
"rejectedPlans" : [ ]
},
"executionStats" : {
"executionSuccess" : true,
"nReturned" : 10,
"executionTimeMillis" : 563,
"totalKeysExamined" : 251120,
"totalDocsExamined" : 251120,
"executionStages" : {
"stage" : "LIMIT",
"nReturned" : 10,
"executionTimeMillisEstimate" : 500,
"works" : 251121,
"advanced" : 10,
"needTime" : 251110,
"needFetch" : 0,
"saveState" : 1961,
"restoreState" : 1961,
"isEOF" : 1,
"invalidates" : 0,
"limitAmount" : 0,
"inputStage" : {
"stage" : "SKIP",
"nReturned" : 10,
"executionTimeMillisEstimate" : 500,
"works" : 251120,
"advanced" : 10,
"needTime" : 251110,
"needFetch" : 0,
"saveState" : 1961,
"restoreState" : 1961,
"isEOF" : 0,
"invalidates" : 0,
"skipAmount" : 0,
"inputStage" : {
"stage" : "FETCH",
"filter" : {
"user.followers_count" : {
"$gt" : 1000
}
},
"nReturned" : 5010,
"executionTimeMillisEstimate" : 490,
"works" : 251120,
"advanced" : 5010,
"needTime" : 246110,
"needFetch" : 0,
"saveState" : 1961,
"restoreState" : 1961,
"isEOF" : 0,
"invalidates" : 0,
"docsExamined" : 251120,
"alreadyHasObj" : 0,
"inputStage" : {
"stage" : "IXSCAN",
"nReturned" : 251120,
"executionTimeMillisEstimate" : 100,
"works" : 251120,
"advanced" : 251120,
"needTime" : 0,
"needFetch" : 0,
"saveState" : 1961,
"restoreState" : 1961,
"isEOF" : 0,
"invalidates" : 0,
"keyPattern" : {
"created_at" : -1
},
"indexName" : "created_at_-1",
"isMultiKey" : false,
"direction" : "backward",
"indexBounds" : {
"created_at" : [
"[MinKey, MaxKey]"
]
},
"keysExamined" : 251120,
"dupsTested" : 0,
"dupsDropped" : 0,
"seenInvalidated" : 0,
"matchTested" : 0
}
}
}
}
},
"serverInfo" : {
"host" : "generic-name.local",
"port" : 27017,
"version" : "3.0.1",
"gitVersion" : "534b5a3f9d10f00cd27737fbcd951032248b5952"
},
"ok" : 1
}
Solution:
The query uses an index to determine the order in which to return result documents
The query examines 251120 documents.
