Understanding mongo db explain

Question

I fired a query and tried to explain it on mongo console and got

"isMultiKey" : true, "n" : 8, "nscannedObjects" : 17272, "nscanned" : 17272, "nscannedObjectsAllPlans" : 21836, "nscannedAllPlans" : 21836, "scanAndOrder" : true, "indexOnly" : false, "nYields" : 0, "nChunkSkips" : 0, "millis" : 184, 

Most of the things are explained in http://www.mongodb.org/display/DOCS/Explain, but I cannot understand what does nscannedObjectsAllPlans, nscannedAllPlans means. Can anyone help?

Thanks

Answer 1

nscanned and nscannedObjects report results for the winning query plan.

nscannedAllPlans and nscannedObjectsAllPlans report results for all plans.

Doc

Number of index entries scanned. totalKeysExamined corresponds to the nscanned field returned by cursor.explain() in earlier versions of MongoDB.

For example:

t = db.jstests_explainb; t.drop();  t.ensureIndex( { a:1, b:1 } ); t.ensureIndex( { b:1, a:1 } );  t.save( { a:0, b:1 } ); t.save( { a:1, b:0 } );  // Older mongodb (< 3.0? ) t.find( { a:{ $gte:0 }, b:{ $gte:0 } } ).explain( true );     {       "isMultiKey": false,       "n": 2,       "nscannedObjects": 2,       "nscanned": 2,       "nscannedObjectsAllPlans": 6,       "nscannedAllPlans": 6,       "scanAndOrder": false,       "indexOnly": false,       "nYields": 0,       "nChunkSkips": 0,       "millis": 2,     ...     }  // MongoDB 4.4 t.find( { a:{ $gte:0 }, b:{ $gte:0 } } ).explain( true ); {     "queryPlanner" : {         "plannerVersion" : 1,         "namespace" : "test.jstests_explainb",         ...         "queryHash" : "CB67518C",         "planCacheKey" : "5E76CDD1",         "winningPlan" : {             "stage" : "FETCH",             "inputStage" : {                 "stage" : "IXSCAN",                 "keyPattern" : {                     "a" : 1,                     "b" : 1                 },                 "indexName" : "a_1_b_1",             }         },         "rejectedPlans" : [             {                 "stage" : "FETCH",                 "inputStage" : {                     "stage" : "IXSCAN",                     "keyPattern" : {                         "b" : 1,                         "a" : 1                     },                     "indexName" : "b_1_a_1",                 }             }         ],         ...     },     "executionStats" : {         "executionSuccess" : true,         "nReturned" : 2,         "executionTimeMillis" : 0,         "totalKeysExamined" : 2, // <-- same as `nscanned`         "totalDocsExamined" : 2, // <--         "executionStages" : { ... }         "allPlansExecution" : [             {...},             {...}         ]     }