How does MongoDB index arrays?

Question

In MongoDB, if I were to store an array (say [\"red\", \"blue\"]) in a field \"color\", does it index \"red\" and \"blue\" so

Answer 1

You can simply test index usage by appending "explain" to your query:

> db.col1.save({'colors': ['red','blue']})

# without index
> db.col1.find({'colors': 'red'}).explain()
{
        "queryPlanner" : {
                "plannerVersion" : 1,
                "namespace" : "protrain.col1",
                "indexFilterSet" : false,
                "parsedQuery" : {
                        "colors" : {
                                "$eq" : "red"
                        }
                },
                "winningPlan" : {
                        "stage" : "COLLSCAN", <--- simple column scan
                        "filter" : {
                                "colors" : {
                                        "$eq" : "red"
                                }
                        },
                        "direction" : "forward"
                },
                "rejectedPlans" : [ ]
        },
        "serverInfo" : {
                "host" : "bee34f15fe28",
                "port" : 27017,
                "version" : "3.4.4",
                "gitVersion" : "888390515874a9debd1b6c5d36559ca86b44babd"
        },
        "ok" : 1
}

# query with index
> db.col1.createIndex( { "colors":1 } )
> db.col1.find({'colors': 'red'}).explain()
{
        "queryPlanner" : {
                "plannerVersion" : 1,
                "namespace" : "protrain.col1",
                "indexFilterSet" : false,
                "parsedQuery" : {
                        "colors" : {
                                "$eq" : "red"
                        }
                },
                "winningPlan" : {
                        "stage" : "FETCH",
                        "inputStage" : {
                                "stage" : "IXSCAN", 
     
 

        
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