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.

hw4.2answer

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