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Type: Bug
Resolution: Duplicate
Priority: Major - P3
Fix Version/s: None
Affects Version/s: 4.2.7, 4.2.8, 4.4.0-rc9
Component/s: Performance
Labels:
None

Operating System:
ALL

Steps To Reproduce:

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Please see the example below. Why does 2nd query perform document lookups?

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' }

])
// create partial index, which "should" cover name & color field
db.getCollection('inventory').createIndex({ name: 1 },          { name: 'onlyRed', partialFilterExpression: {  color: 'red' } })

// totalDocsExamined is one but should be zero
db.getCollection('inventory').find({ color: 'red', name: /c/ }, { name: 1, _id: 0 }).explain('executionStats').executionStats.totalDocsExamined

Here is the full explain result:

{
    "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)
}

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Please see the example below. Why does 2nd query perform document lookups? 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' } ]) // create partial index, which "should" cover name & color field db.getCollection( 'inventory' ).createIndex({ name: 1 }, { name: 'onlyRed' , partialFilterExpression: { color: 'red' } }) // totalDocsExamined is one but should be zero db.getCollection( 'inventory' ).find({ color: 'red' , name: /c/ }, { name: 1, _id: 0 }).explain( 'executionStats' ).executionStats.totalDocsExamined Here is the full explain result: { "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) }

A query matching a partial index performs document lookups, although all fields could be covered by the index.

This issue was observed with the following versions of MongoDB: 4.2.1, 4.2.7, 4.4.0-rc9

I raised this already on StackOverflow: https://stackoverflow.com/questions/62342543/covered-queries-do-not-work-with-partial-indices

duplicates

SERVER-28889 Partial index shouldn't do fetch on conditions that are true by the definition of the index

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