Server does not appropriately respect setting internalQueryEnumerationMaxOrSolutions to 0

db.foo.drop() db.foo.createIndex({common:1}) db.foo.createIndex({one:1}) db.foo.createIndex({two:1})   // Confirming that with the default setting of 10, a total of 5 plans (4 rejectedPlans) are generated: db.adminCommand({setParameter:1, internalQueryEnumerationMaxOrSolutions:10}) db.foo.find({common:1, $or:[{one:0, two:0},{one:1, two:1}]}).explain().queryPlanner.rejectedPlans.length     // Setting this to 0 should result in only a single plan, using the { common: 1 } index, being generated.  However there is a rejected "OR" plan: db.adminCommand({setParameter:1, internalQueryEnumerationMaxOrSolutions:0}) db.foo.find({common:1, $or:[{one:0, two:0},{one:1, two:1}]}).explain().queryPlanner.rejectedPlans.length db.foo.find({common:1, $or:[{one:0, two:0},{one:1, two:1}]}).explain().queryPlanner.rejectedPlans[0]     // With a value of 1, we get the same result as when it is 0.  This is output seems correct for this setting: db.adminCommand({setParameter:1, internalQueryEnumerationMaxOrSolutions:1}) db.foo.find({common:1, $or:[{one:0, two:0},{one:1, two:1}]}).explain().queryPlanner.rejectedPlans.length db.foo.find({common:1, $or:[{one:0, two:0},{one:1, two:1}]}).explain().queryPlanner.rejectedPlans[0]   // Further increases to the parameter have the expected increase to the number of plans: db.adminCommand({setParameter:1, internalQueryEnumerationMaxOrSolutions:2}) db.foo.find({common:1, $or:[{one:0, two:0},{one:1, two:1}]}).explain().queryPlanner.rejectedPlans.length db.adminCommand({setParameter:1, internalQueryEnumerationMaxOrSolutions:3}) db.foo.find({common:1, $or:[{one:0, two:0},{one:1, two:1}]}).explain().queryPlanner.rejectedPlans.length db.adminCommand({setParameter:1, internalQueryEnumerationMaxOrSolutions:4}) db.foo.find({common:1, $or:[{one:0, two:0},{one:1, two:1}]}).explain().queryPlanner.rejectedPlans.length Full output from a mongo shell session against a 4.2.2 mongod : > db.foo.drop() true > > db.foo.createIndex({common:1}) { "createdCollectionAutomatically" : true, "numIndexesBefore" : 1, "numIndexesAfter" : 2, "ok" : 1 } > db.foo.createIndex({one:1}) { "createdCollectionAutomatically" : false, "numIndexesBefore" : 2, "numIndexesAfter" : 3, "ok" : 1 } > db.foo.createIndex({two:1}) { "createdCollectionAutomatically" : false, "numIndexesBefore" : 3, "numIndexesAfter" : 4, "ok" : 1 } > > > // Confirming that with the default setting of 10, a total of 5 plans (4 rejectedPlans) are generated: > db.adminCommand({setParameter:1, internalQueryEnumerationMaxOrSolutions:10}) { "was" : 10, "ok" : 1 } > db.foo.find({common:1, $or:[{one:0, two:0},{one:1, two:1}]}).explain().queryPlanner.rejectedPlans.length 4 > > > // Setting this to 0 should result in only a single plan, using the { common: 1 } index, being generated.  However there is a rejected "OR" plan: > db.adminCommand({setParameter:1, internalQueryEnumerationMaxOrSolutions:0}) { "was" : 10, "ok" : 1 } > db.foo.find({common:1, $or:[{one:0, two:0},{one:1, two:1}]}).explain().queryPlanner.rejectedPlans.length 1 > db.foo.find({common:1, $or:[{one:0, two:0},{one:1, two:1}]}).explain().queryPlanner.rejectedPlans[0] { "stage" : "FETCH", "filter" : { "common" : { "$eq" : 1 } }, "inputStage" : { "stage" : "OR", "inputStages" : [ { "stage" : "FETCH", "filter" : { "two" : { "$eq" : 0 } }, "inputStage" : { "stage" : "IXSCAN", "keyPattern" : { "one" : 1 }, "indexName" : "one_1", "isMultiKey" : false, "multiKeyPaths" : { "one" : [ ] }, "isUnique" : false, "isSparse" : false, "isPartial" : false, "indexVersion" : 2, "direction" : "forward", "indexBounds" : { "one" : [ "[0.0, 0.0]" ] } } }, { "stage" : "FETCH", "filter" : { "one" : { "$eq" : 1 } }, "inputStage" : { "stage" : "IXSCAN", "keyPattern" : { "two" : 1 }, "indexName" : "two_1", "isMultiKey" : false, "multiKeyPaths" : { "two" : [ ] }, "isUnique" : false, "isSparse" : false, "isPartial" : false, "indexVersion" : 2, "direction" : "forward", "indexBounds" : { "two" : [ "[1.0, 1.0]" ] } } } ] } } > > // With a value of 1, we get the same result as when it is 0.  But the output now seems correct based on the updated parameter value: > db.adminCommand({setParameter:1, internalQueryEnumerationMaxOrSolutions:1}) { "was" : 0, "ok" : 1 } > db.foo.find({common:1, $or:[{one:0, two:0},{one:1, two:1}]}).explain().queryPlanner.rejectedPlans.length 1 > db.foo.find({common:1, $or:[{one:0, two:0},{one:1, two:1}]}).explain().queryPlanner.rejectedPlans[0] { "stage" : "FETCH", "filter" : { "common" : { "$eq" : 1 } }, "inputStage" : { "stage" : "OR", "inputStages" : [ { "stage" : "FETCH", "filter" : { "two" : { "$eq" : 1 } }, "inputStage" : { "stage" : "IXSCAN", "keyPattern" : { "one" : 1 }, "indexName" : "one_1", "isMultiKey" : false, "multiKeyPaths" : { "one" : [ ] }, "isUnique" : false, "isSparse" : false, "isPartial" : false, "indexVersion" : 2, "direction" : "forward", "indexBounds" : { "one" : [ "[1.0, 1.0]" ] } } }, { "stage" : "FETCH", "filter" : { "one" : { "$eq" : 0 } }, "inputStage" : { "stage" : "IXSCAN", "keyPattern" : { "two" : 1 }, "indexName" : "two_1", "isMultiKey" : false, "multiKeyPaths" : { "two" : [ ] }, "isUnique" : false, "isSparse" : false, "isPartial" : false, "indexVersion" : 2, "direction" : "forward", "indexBounds" : { "two" : [ "[0.0, 0.0]" ] } } } ] } } > > > // Further increases to the parameter have the expected corresponding increase to the number of plans: > db.adminCommand({setParameter:1, internalQueryEnumerationMaxOrSolutions:2}) { "was" : 1, "ok" : 1 } > db.foo.find({common:1, $or:[{one:0, two:0},{one:1, two:1}]}).explain().queryPlanner.rejectedPlans.length 2 > db.adminCommand({setParameter:1, internalQueryEnumerationMaxOrSolutions:3}) { "was" : 2, "ok" : 1 } > db.foo.find({common:1, $or:[{one:0, two:0},{one:1, two:1}]}).explain().queryPlanner.rejectedPlans.length 3 > db.adminCommand({setParameter:1, internalQueryEnumerationMaxOrSolutions:4}) { "was" : 3, "ok" : 1 } > db.foo.find({common:1, $or:[{one:0, two:0},{one:1, two:1}]}).explain().queryPlanner.rejectedPlans.length 4