regex prefix on unique and nonunique indices

mongo_index

cureatr:PRIMARY> use test
cureatr:PRIMARY> db.foo.createIndex({prefix: 1, unique: 1}, {unique: true})
cureatr:PRIMARY> db.foo.createIndex({prefix : 1, nonunique: 1})

cureatr:PRIMARY> db.foo.explain().find({prefix: "a", nonunique: /^a/i})
{
    "queryPlanner" : {
        "plannerVersion" : 1,
        "namespace" : "test.foo",
        "indexFilterSet" : false,
        "parsedQuery" : {
            "$and" : [
                {
                    "prefix" : {
                        "$eq" : "a"
                    }
                },
                {
                    "nonunique" : /^a/i
                }
            ]
        },
        "winningPlan" : {
            "stage" : "FETCH",
            "inputStage" : {
                "stage" : "IXSCAN",
                "filter" : {
                    "nonunique" : /^a/i
                },
                "keyPattern" : {
                    "prefix" : 1,
                    "nonunique" : 1
                },
                "indexName" : "prefix_1_nonunique_1",
                "isMultiKey" : false,
                "direction" : "forward",
                "indexBounds" : {
                    "prefix" : [
                        "[\"a\", \"a\"]"
                    ],
                    "nonunique" : [
                        "[\"\", {})",
                        "[/^a/i, /^a/i]"
                    ]
                }
            }
        },
        "rejectedPlans" : [
            {
                "stage" : "KEEP_MUTATIONS",
                "inputStage" : {
                    "stage" : "FETCH",
                    "filter" : {
                        "nonunique" : /^a/i
                    },
                    "inputStage" : {
                        "stage" : "IXSCAN",
                        "keyPattern" : {
                            "prefix" : 1,
                            "unique" : 1
                        },
                        "indexName" : "prefix_1_unique_1",
                        "isMultiKey" : false,
                        "direction" : "forward",
                        "indexBounds" : {
                            "prefix" : [
                                "[\"a\", \"a\"]"
                            ],
                            "unique" : [
                                "[MinKey, MaxKey]"
                            ]
                        }
                    }
                }
            }
        ]
    },
    "serverInfo" : {
        "host" : "cureatr-vm",
        "port" : 27017,
        "version" : "3.0.5",
        "gitVersion" : "8bc4ae20708dbb493cb09338d9e7be6698e4a3a3"
    },
    "ok" : 1
}

cureatr:PRIMARY> db.foo.explain().find({prefix: "a", unique: /^a/i})
{
    "queryPlanner" : {
        "plannerVersion" : 1,
        "namespace" : "test.foo",
        "indexFilterSet" : false,
        "parsedQuery" : {
            "$and" : [
                {
                    "prefix" : {
                        "$eq" : "a"
                    }
                },
                {
                    "unique" : /^a/i
                }
            ]
        },
        "winningPlan" : {
            "stage" : "KEEP_MUTATIONS",
            "inputStage" : {
                "stage" : "FETCH",
                "filter" : {
                    "unique" : /^a/i
                },
                "inputStage" : {
                    "stage" : "IXSCAN",
                    "keyPattern" : {
                        "prefix" : 1,
                        "nonunique" : 1
                    },
                    "indexName" : "prefix_1_nonunique_1",
                    "isMultiKey" : false,
                    "direction" : "forward",
                    "indexBounds" : {
                        "prefix" : [
                            "[\"a\", \"a\"]"
                        ],
                        "nonunique" : [
                            "[MinKey, MaxKey]"
                        ]
                    }
                }
            }
        },
        "rejectedPlans" : [
            {
                "stage" : "FETCH",
                "inputStage" : {
                    "stage" : "IXSCAN",
                    "filter" : {
                        "unique" : /^a/i
                    },
                    "keyPattern" : {
                        "prefix" : 1,
                        "unique" : 1
                    },
                    "indexName" : "prefix_1_unique_1",
                    "isMultiKey" : false,
                    "direction" : "forward",
                    "indexBounds" : {
                        "prefix" : [
                            "[\"a\", \"a\"]"
                        ],
                        "unique" : [
                            "[\"\", {})",
                            "[/^a/i, /^a/i]"
                        ]
                    }
                }
            }
        ]
    },
    "serverInfo" : {
        "host" : "cureatr-vm",
        "port" : 27017,
        "version" : "3.0.5",
        "gitVersion" : "8bc4ae20708dbb493cb09338d9e7be6698e4a3a3"
    },
    "ok" : 1
}