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MongoDB - Analyzing Queries


MongoDB - Analyzing Queries In this chapter will learn the frequently used $explain and $hint queries

Using $explain

The $explain operator provides information on the query, indexes used in a query and other statistics. It is pretty easy use when analyzing how well your indexes are optimized.

We will now use $explain on the following query
>db.users.ensureIndex({gender:1,user_name:1})
>db.users.find({gender:"F"},{user_name:1,_id:0}).explain()
The above explain() query returns the following analyzed result
{
    "cursor" : "BtreeCursor gender_1_user_name_1",
    "isMultiKey" : false,
    "n" : 1,
    "nscannedObjects" : 0,
    "nscanned" : 1,
    "nscannedObjectsAllPlans" : 0,
    "nscannedAllPlans" : 1,
    "scanAndOrder" : false,
    "indexOnly" : true,
    "nYields" : 0,
    "nChunkSkips" : 0,
    "millis" : 0,
    "indexBounds" : {
        "gender" : [
            [
                "M",
                "M"
            ]
        ],
        "user_name" : [
            [
                {
                    "$minElement" : 1
                },
                {
                    "$maxElement" : 1
                }
            ]
        ]
    }
}

We will now look at the fields in this result set indexOnly indicates that this query has used indexing. cursor field specifies the type of cursor used.BTreeCursor type indicates that an index was used and also gives the name of the index used n indicates the number of documents matching returned.nscannedObjects indicates the total number of documents scanned.nscanned indicates the total number of documents or index entries scanned.

Using $hint

The $hint operator forces the query optimizer to use the specified index to run a query.

>db.users.find({gender:"F"},{user_name:1,_id:0}).hint({gender:1,user_name:1})
        




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