更新：创建工单：https://jira.mongodb.org/browse/SERVER-48777

请参见下面的示例。为什么第二个查询执行文档查找？

db.createCollection('inventory')
db.getCollection('inventory').insertMany([

    { type: 'food', color: 'yellow', name: 'banana' },
    { type: 'food', color: 'red',    name: 'cherry' },
    { type: 'car',  color: 'yellow', name: 'taxi' }

])

db.getCollection('inventory').createIndex({ type: 1, name: 1 }, { name: 'byType' })
db.getCollection('inventory').createIndex({ name: 1 },          { name: 'onlyRed', partialFilterExpression: {  color: 'red' } })

// find cherry by type -> totalDocsExamined = 0 (as expected)
db.getCollection('inventory').find({ type: 'food', name: /c/ }, { name: 1, _id: 0 }).explain('executionStats').executionStats.totalDocsExamined

// find cherry by color -> totalDocsExamined = 1 :-(
db.getCollection('inventory').find({ color: 'red', name: /c/ }, { name: 1, _id: 0 }).explain('executionStats').executionStats.totalDocsExamined

以上示例的索引是：

db.getCollection('inventory').getIndexes()
[
    {
        "v" : 2,
        "key" : {
            "_id" : 1
        },
        "name" : "_id_"
    },
    {
        "v" : 2,
        "key" : {
            "type" : 1.0,
            "name" : 1.0
        },
        "name" : "byType"
    },
    {
        "v" : 2,
        "key" : {
            "name" : 1.0
        },
        "name" : "onlyRed",
        "partialFilterExpression" : {
            "color" : "red"
        }
    }
]

以下所有版本均显示此行为：4.2.1、4.2.7、4.4.0-rc9
我在文档中找不到任何关于Covered Queries和Partial Indexes的内容，为什么不应该支持它。
query 1的解释：

{
    "queryPlanner" : {
        "plannerVersion" : 1,
        "namespace" : "demo.inventory",
        "indexFilterSet" : false,
        "parsedQuery" : {
            "$and" : [ 
                {
                    "type" : {
                        "$eq" : "food"
                    }
                }, 
                {
                    "name" : {
                        "$regex" : "c"
                    }
                }
            ]
        },
        "winningPlan" : {
            "stage" : "PROJECTION_COVERED",
            "transformBy" : {
                "name" : 1.0,
                "_id" : 0.0
            },
            "inputStage" : {
                "stage" : "IXSCAN",
                "filter" : {
                    "name" : {
                        "$regex" : "c"
                    }
                },
                "keyPattern" : {
                    "type" : 1.0,
                    "name" : 1.0
                },
                "indexName" : "byType",
                "isMultiKey" : false,
                "multiKeyPaths" : {
                    "type" : [],
                    "name" : []
                },
                "isUnique" : false,
                "isSparse" : false,
                "isPartial" : false,
                "indexVersion" : 2,
                "direction" : "forward",
                "indexBounds" : {
                    "type" : [ 
                        "[\"food\", \"food\"]"
                    ],
                    "name" : [ 
                        "[\"\", {})", 
                        "[/c/, /c/]"
                    ]
                }
            }
        },
        "rejectedPlans" : []
    },
    "executionStats" : {
        "executionSuccess" : true,
        "nReturned" : 1,
        "executionTimeMillis" : 0,
        "totalKeysExamined" : 2,
        "totalDocsExamined" : 0,
        "executionStages" : {
            "stage" : "PROJECTION_COVERED",
            "nReturned" : 1,
            "executionTimeMillisEstimate" : 0,
            "works" : 3,
            "advanced" : 1,
            "needTime" : 1,
            "needYield" : 0,
            "saveState" : 0,
            "restoreState" : 0,
            "isEOF" : 1,
            "transformBy" : {
                "name" : 1.0,
                "_id" : 0.0
            },
            "inputStage" : {
                "stage" : "IXSCAN",
                "filter" : {
                    "name" : {
                        "$regex" : "c"
                    }
                },
                "nReturned" : 1,
                "executionTimeMillisEstimate" : 0,
                "works" : 3,
                "advanced" : 1,
                "needTime" : 1,
                "needYield" : 0,
                "saveState" : 0,
                "restoreState" : 0,
                "isEOF" : 1,
                "keyPattern" : {
                    "type" : 1.0,
                    "name" : 1.0
                },
                "indexName" : "byType",
                "isMultiKey" : false,
                "multiKeyPaths" : {
                    "type" : [],
                    "name" : []
                },
                "isUnique" : false,
                "isSparse" : false,
                "isPartial" : false,
                "indexVersion" : 2,
                "direction" : "forward",
                "indexBounds" : {
                    "type" : [ 
                        "[\"food\", \"food\"]"
                    ],
                    "name" : [ 
                        "[\"\", {})", 
                        "[/c/, /c/]"
                    ]
                },
                "keysExamined" : 2,
                "seeks" : 1,
                "dupsTested" : 0,
                "dupsDropped" : 0
            }
        }
    },
    "serverInfo" : {
        "host" : "mongo",
        "port" : 27017,
        "version" : "4.4.0-rc9",
        "gitVersion" : "bea79f76addfe4b754c8696db029c5b3c762041c"
    },
    "ok" : 1.0,
    "$clusterTime" : {
        "clusterTime" : Timestamp(1591984674, 1),
        "signature" : {
            "hash" : { "$binary" : "AAAAAAAAAAAAAAAAAAAAAAAAAAA=", "$type" : "00" },
            "keyId" : NumberLong(0)
        }
    },
    "operationTime" : Timestamp(1591984674, 1)
}

query 2的解释（使用部分索引）：

{
    "queryPlanner" : {
        "plannerVersion" : 1,
        "namespace" : "demo.inventory",
        "indexFilterSet" : false,
        "parsedQuery" : {
            "$and" : [ 
                {
                    "color" : {
                        "$eq" : "red"
                    }
                }, 
                {
                    "name" : {
                        "$regex" : "c"
                    }
                }
            ]
        },
        "winningPlan" : {
            "stage" : "PROJECTION_SIMPLE",
            "transformBy" : {
                "name" : 1.0,
                "_id" : 0.0
            },
            "inputStage" : {
                "stage" : "FETCH",
                "filter" : {
                    "color" : {
                        "$eq" : "red"
                    }
                },
                "inputStage" : {
                    "stage" : "IXSCAN",
                    "filter" : {
                        "name" : {
                            "$regex" : "c"
                        }
                    },
                    "keyPattern" : {
                        "name" : 1.0
                    },
                    "indexName" : "onlyRed",
                    "isMultiKey" : false,
                    "multiKeyPaths" : {
                        "name" : []
                    },
                    "isUnique" : false,
                    "isSparse" : false,
                    "isPartial" : true,
                    "indexVersion" : 2,
                    "direction" : "forward",
                    "indexBounds" : {
                        "name" : [ 
                            "[\"\", {})", 
                            "[/c/, /c/]"
                        ]
                    }
                }
            }
        },
        "rejectedPlans" : []
    },
    "executionStats" : {
        "executionSuccess" : true,
        "nReturned" : 1,
        "executionTimeMillis" : 0,
        "totalKeysExamined" : 1,
        "totalDocsExamined" : 1,
        "executionStages" : {
            "stage" : "PROJECTION_SIMPLE",
            "nReturned" : 1,
            "executionTimeMillisEstimate" : 0,
            "works" : 2,
            "advanced" : 1,
            "needTime" : 0,
            "needYield" : 0,
            "saveState" : 0,
            "restoreState" : 0,
            "isEOF" : 1,
            "transformBy" : {
                "name" : 1.0,
                "_id" : 0.0
            },
            "inputStage" : {
                "stage" : "FETCH",
                "filter" : {
                    "color" : {
                        "$eq" : "red"
                    }
                },
                "nReturned" : 1,
                "executionTimeMillisEstimate" : 0,
                "works" : 2,
                "advanced" : 1,
                "needTime" : 0,
                "needYield" : 0,
                "saveState" : 0,
                "restoreState" : 0,
                "isEOF" : 1,
                "docsExamined" : 1,
                "alreadyHasObj" : 0,
                "inputStage" : {
                    "stage" : "IXSCAN",
                    "filter" : {
                        "name" : {
                            "$regex" : "c"
                        }
                    },
                    "nReturned" : 1,
                    "executionTimeMillisEstimate" : 0,
                    "works" : 2,
                    "advanced" : 1,
                    "needTime" : 0,
                    "needYield" : 0,
                    "saveState" : 0,
                    "restoreState" : 0,
                    "isEOF" : 1,
                    "keyPattern" : {
                        "name" : 1.0
                    },
                    "indexName" : "onlyRed",
                    "isMultiKey" : false,
                    "multiKeyPaths" : {
                        "name" : []
                    },
                    "isUnique" : false,
                    "isSparse" : false,
                    "isPartial" : true,
                    "indexVersion" : 2,
                    "direction" : "forward",
                    "indexBounds" : {
                        "name" : [ 
                            "[\"\", {})", 
                            "[/c/, /c/]"
                        ]
                    },
                    "keysExamined" : 1,
                    "seeks" : 1,
                    "dupsTested" : 0,
                    "dupsDropped" : 0
                }
            }
        }
    },
    "serverInfo" : {
        "host" : "mongo",
        "port" : 27017,
        "version" : "4.4.0-rc9",
        "gitVersion" : "bea79f76addfe4b754c8696db029c5b3c762041c"
    },
    "ok" : 1.0,
    "$clusterTime" : {
        "clusterTime" : Timestamp(1591984064, 1),
        "signature" : {
            "hash" : { "$binary" : "AAAAAAAAAAAAAAAAAAAAAAAAAAA=", "$type" : "00" },
            "keyId" : NumberLong(0)
        }
    },
    "operationTime" : Timestamp(1591984064, 1)
}