Asset Manager query without/with index

explain-with-index.rb

{"queryPlanner"=>
  {"plannerVersion"=>1,
   "namespace"=>"govuk_assets_development.assets",
   "indexFilterSet"=>false,
   "parsedQuery"=>
    {"$and"=>
      [{"_type"=>{"$eq"=>"WhitehallAsset"}},
       {"deleted_at"=>{"$eq"=>nil}},
       {"legacy_url_path"=>{"$eq"=>"foo"}}]},
   "winningPlan"=>
    {"stage"=>"FETCH",
     "filter"=>
      {"$and"=>
        [{"_type"=>{"$eq"=>"WhitehallAsset"}}, {"deleted_at"=>{"$eq"=>nil}}]},
     "inputStage"=>
      {"stage"=>"IXSCAN",
       "keyPattern"=>{"legacy_url_path"=>1},
       "indexName"=>"legacy_url_path_1",
       "isMultiKey"=>false,
       "multiKeyPaths"=>{"legacy_url_path"=>[]},
       "isUnique"=>false,
       "isSparse"=>false,
       "isPartial"=>false,
       "indexVersion"=>2,
       "direction"=>"forward",
       "indexBounds"=>{"legacy_url_path"=>["[\"foo\", \"foo\"]"]}}},
   "rejectedPlans"=>[]},
 "executionStats"=>
  {"executionSuccess"=>true,
   "nReturned"=>0,
   "executionTimeMillis"=>0,
   "totalKeysExamined"=>0,
   "totalDocsExamined"=>0,
   "executionStages"=>
    {"stage"=>"FETCH",
     "filter"=>
      {"$and"=>
        [{"_type"=>{"$eq"=>"WhitehallAsset"}}, {"deleted_at"=>{"$eq"=>nil}}]},
     "nReturned"=>0,
     "executionTimeMillisEstimate"=>0,
     "works"=>1,
     "advanced"=>0,
     "needTime"=>0,
     "needYield"=>0,
     "saveState"=>0,
     "restoreState"=>0,
     "isEOF"=>1,
     "invalidates"=>0,
     "docsExamined"=>0,
     "alreadyHasObj"=>0,
     "inputStage"=>
      {"stage"=>"IXSCAN",
       "nReturned"=>0,
       "executionTimeMillisEstimate"=>0,
       "works"=>1,
       "advanced"=>0,
       "needTime"=>0,
       "needYield"=>0,
       "saveState"=>0,
       "restoreState"=>0,
       "isEOF"=>1,
       "invalidates"=>0,
       "keyPattern"=>{"legacy_url_path"=>1},
       "indexName"=>"legacy_url_path_1",
       "isMultiKey"=>false,
       "multiKeyPaths"=>{"legacy_url_path"=>[]},
       "isUnique"=>false,
       "isSparse"=>false,
       "isPartial"=>false,
       "indexVersion"=>2,
       "direction"=>"forward",
       "indexBounds"=>{"legacy_url_path"=>["[\"foo\", \"foo\"]"]},
       "keysExamined"=>0,
       "seeks"=>1,
       "dupsTested"=>0,
       "dupsDropped"=>0,
       "seenInvalidated"=>0}},
   "allPlansExecution"=>[]},
 "serverInfo"=>
  {"host"=>"trooz",
   "port"=>27017,
   "version"=>"3.4.9",
   "gitVersion"=>"876ebee8c7dd0e2d992f36a848ff4dc50ee6603e"},
 "ok"=>1.0}

explain-without-index.rb

{"queryPlanner"=>
  {"plannerVersion"=>1,
   "namespace"=>"govuk_assets_development.assets",
   "indexFilterSet"=>false,
   "parsedQuery"=>
    {"$and"=>
      [{"_type"=>{"$eq"=>"WhitehallAsset"}},
       {"deleted_at"=>{"$eq"=>nil}},
       {"legacy_url_path"=>{"$eq"=>"foo"}}]},
   "winningPlan"=>
    {"stage"=>"COLLSCAN",
     "filter"=>
      {"$and"=>
        [{"_type"=>{"$eq"=>"WhitehallAsset"}},
         {"deleted_at"=>{"$eq"=>nil}},
         {"legacy_url_path"=>{"$eq"=>"foo"}}]},
     "direction"=>"forward"},
   "rejectedPlans"=>[]},
 "executionStats"=>
  {"executionSuccess"=>true,
   "nReturned"=>0,
   "executionTimeMillis"=>39,
   "totalKeysExamined"=>0,
   "totalDocsExamined"=>65660,
   "executionStages"=>
    {"stage"=>"COLLSCAN",
     "filter"=>
      {"$and"=>
        [{"_type"=>{"$eq"=>"WhitehallAsset"}},
         {"deleted_at"=>{"$eq"=>nil}},
         {"legacy_url_path"=>{"$eq"=>"foo"}}]},
     "nReturned"=>0,
     "executionTimeMillisEstimate"=>50,
     "works"=>65662,
     "advanced"=>0,
     "needTime"=>65661,
     "needYield"=>0,
     "saveState"=>513,
     "restoreState"=>513,
     "isEOF"=>1,
     "invalidates"=>0,
     "direction"=>"forward",
     "docsExamined"=>65660},
   "allPlansExecution"=>[]},
 "serverInfo"=>
  {"host"=>"trooz",
   "port"=>27017,
   "version"=>"3.4.9",
   "gitVersion"=>"876ebee8c7dd0e2d992f36a848ff4dc50ee6603e"},
 "ok"=>1.0}