Import the data from the sample export Unable to find source-code formatter for language: shell. Available languages are: actionscript, ada, applescript, bash, c, c#, c++, cpp, css, erlang, go, groovy, haskell, html, java, javascript, js, json, lua, none, nyan, objc, perl, php, python, r, rainbow, ruby, scala, sh, sql, swift, visualbasic, xml, yaml gunzip -c records.json.gz |mongoimport --db index-test --collection workspaceFile Create indexes db.workspaceFile.createIndex( { "_id" : 1, // <---- this seems to cause a problem, see executionStats below "workspaceId" : 1, "parent._id" : 1 }, {name: 'testIndexA' } ); db.workspaceFile.createIndex( { "workspaceId" : 1, "parent._id" : 1 }, {name: 'testIndexB' } );   Define the query parameters var workspaceId = "ws-3" ; var fileId = ObjectId( "5bd33d003bff4aac38456157" ); var fileFamilyQuery = { "$and" : [{ "workspaceId" : workspaceId, "$or" : [{ "_id" : fileId, }, { "parent._id" : fileId }] }] };   Run query against testIndexA db.workspaceFile.find(fileFamilyQuery).sort({ "_id" :-1}).hint( "testIndexA" ).explain( 'executionStats' ) Inspect the indexBounds and see that the parameters are not set and the number of keys examined is too high "indexBounds" : { "_id" : [ "[MaxKey, MinKey]" ], "workspaceId" : [ "[MaxKey, MinKey]" ], "parent._id" : [ "[MaxKey, MinKey]" ] }, "keysExamined" : 500005, Run against testIndexB db.workspaceFile.find(fileFamilyQuery).sort({ "_id" :-1}).hint( "testIndexB" ).explain( 'executionStats' ) Inspect the indexBounds and see the parameters are partially at least partially set ( but still missing parent._id ) "indexBounds" : { "workspaceId" : [ "[\" ws-3\ ", \" ws-3\ "]" ], "parent._id" : [ "[MinKey, MaxKey]" ] }, "keysExamined" : 2,