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) }