$skip and $limit in aggregation framework

Question

When I read the document I found the following notes:

When a $sort immediately precedes a $limit in the pipeline, the $sort operation only maintains t

Answer 1

I found that it seems the sequence of limit and skip is immaterial. If I specify skip before limit, the mongoDB will make limit before skip under the hood.

> db.system.profile.find().limit(1).sort( { ts : -1 } ).pretty()
{
    "op" : "command",
    "ns" : "archiprod.userinfos",
    "command" : {
        "aggregate" : "userinfos",
        "pipeline" : [
            {
                "$sort" : {
                    "updatedAt" : -1
                }
            },
            {
                "$limit" : 625
            },
            {
                "$skip" : 600
            }
        ],
    },
    "keysExamined" : 625,
    "docsExamined" : 625,
    "cursorExhausted" : true,
    "numYield" : 4,
    "nreturned" : 25,
    "millis" : 25,
    "planSummary" : "IXSCAN { updatedAt: -1 }",
    /* Some fields are omitted */
}

What happens if I swtich $skip and $limit? I got the same result in terms of keysExamined and docsExamined.

> db.system.profile.find().limit(1).sort( { ts : -1 } ).pretty()
{
    "op" : "command",
    "ns" : "archiprod.userinfos",
    "command" : {
        "aggregate" : "userinfos",
        "pipeline" : [
            {
                "$sort" : {
                    "updatedAt" : -1
                }
            },
            {
                "$skip" : 600
            },
            {
                "$limit" : 25
            }
        ],
    },
    "keysExamined" : 625,
    "docsExamined" : 625,
    "cursorExhausted" : true,
    "numYield" : 5,
    "nreturned" : 25,
    "millis" : 71,
    "planSummary" : "IXSCAN { updatedAt: -1 }",
}

I then checked the explain result of the query. I found that totalDocsExamined is already 625 in the limit stage.

> db.userinfos.explain('executionStats').aggregate([ { "$sort" : { "updatedAt" : -1 } }, { "$limit" : 625 }, { "$skip" : 600 } ])
{
    "stages" : [
        {
            "$cursor" : {
                "sort" : {
                    "updatedAt" : -1
                },
                "limit" : NumberLong(625),
                "queryPlanner" : {
                    "winningPlan" : {
                        "stage" : "FETCH",
                        "inputStage" : {
                            "stage" : "IXSCAN",
                            "keyPattern" : {
                                "updatedAt" : -1
                            },
                            "indexName" : "updatedAt_-1",
                        }
                    },
                },
                "executionStats" : {
                    "executionSuccess" : true,
                    "nReturned" : 625,
                    "executionTimeMillis" : 22,
                    "totalKeysExamined" : 625,
                    "totalDocsExamined" : 625,
                    "executionStages" : {
                        "stage" : "FETCH",
                        "nReturned" : 625,
                        "executionTimeMillisEstimate" : 0,
                        "works" : 625,
                        "advanced" : 625,
                        "docsExamined" : 625,
                        "inputStage" : {
                            "stage" : "IXSCAN",
                            "nReturned" : 625,
                            "works" : 625,
                            "advanced" : 625,
                            "keyPattern" : {
                                "updatedAt" : -1
                            },
                            "indexName" : "updatedAt_-1",
                            "keysExamined" : 625,
                        }
                    }
                }
            }
        },
        {
            "$skip" : NumberLong(600)
        }
    ]
}

And surprisingly, I found switching the $skip and $limit results in the same explain result.

> db.userinfos.explain('executionStats').aggregate([ { "$sort" : { "updatedAt" : -1 } }, { "$skip" : 600 }, { "$limit" : 25 } ])
{
    "stages" : [
        {
            "$cursor" : {
                "sort" : {
                    "updatedAt" : -1
                },
                "limit" : NumberLong(625),
                "queryPlanner" : {
                    /* Omitted */
                },
                "executionStats" : {
                    "executionSuccess" : true,
                    "nReturned" : 625,
                    "executionTimeMillis" : 31,
                    "totalKeysExamined" : 625,
                    "totalDocsExamined" : 625,
                    /* Omitted */
                }
            }
        },
        {
            "$skip" : NumberLong(600)
        }
    ]
}

As you can see, even though I specified $skip before $limit, in the explain result, it's still $limit before $skip.