Have a collection with three indexes: {a:1}, {b:1}, {a:1, b:1}. Run explain for a query that would consider all three indexes, e.g. find({a:7, b:9}). Result: the output for rejectedPlans would look similar to the below one. Notice, that the IXSCAN stage refers to index a_1 while slotBasedPlan.stages refers to a_1_b_1 (which is the winning plan in this case). "rejectedPlans" : [ { "queryPlan" : { "stage" : "FETCH", "planNodeId" : 2, "filter" : { "b" : { "$eq" : 42 } }, "inputStage" : { "stage" : "IXSCAN", "planNodeId" : 1, "keyPattern" : { "a" : 1 } , "indexName" : "a_1", "isMultiKey" : false, "multiKeyPaths" : { "a" : [ ] } , "isUnique" : false, "isSparse" : false, "isPartial" : false, "indexVersion" : 2, "direction" : "forward", "indexBounds" : { "a" : [ "[7.0, 7.0]" ] } } }, "slotBasedPlan" : { "slots" : "$$RESULT=s12 $$RID=s13 env: \{ s2 = Nothing (SEARCH_META), s1 = TimeZoneDatabase(Africa/Monrovia...Mexico/BajaSur) (timeZoneDB), s3 = 1636757269766 (NOW) } ", "stages" : " [2] nlj [] [s8, s4, s5, s6, s7] \n left \n [1] nlj [s5, s7] [s9, s10] \n left \n [1] project [s5 = \"a_1_b_1\", s7 = \{\"a\" : 1, \"b\" : 1}, s9 = KS(2B0E2B540104), s10 = KS(2B0E2B54FE04)] \n [1] limit 1 \n [1] coscan \n right \n [1] project [s4 = s11] \n [1] ixseek s9 s10 s6 s8 s11 [] @\"9f965641-651e-48d6-859b-e7b494a16b5f\" @\"a_1_b_1\" true \n \n \n right \n [2] limit 1 \n [2] seek s8 s12 s13 s4 s5 s6 s7 [] @\"9f965641-651e-48d6-859b-e7b494a16b5f\" true false \n \n"