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  1. Core Server
  2. SERVER-15221

Planner sort analysis should not add FETCH stage if sort key included in index key pattern

    • Type: Icon: Improvement Improvement
    • Resolution: Done
    • Priority: Icon: Major - P3 Major - P3
    • 4.1.10
    • Affects Version/s: None
    • Component/s: Querying
    • None
    • Fully Compatible
    • Query 2019-01-28, Query 2019-03-11, Query 2019-03-25

      Queries that require a blocking sort always fetch the full document from the collection to retrieve the sort key, even if the query plan includes an index scan over an index that has the sort key as a part of its key pattern. These queries should be covered instead.

      To reproduce:

      > db.foo.drop()
      true
      > db.foo.ensureIndex({a: 1, b: 1})
      > db.foo.find({a: {$gt: 0}}, {a: 1, b: 1, _id: 0}).sort({b: 1}).explain(true).stats
      {
      	"stage" : "PROJECTION",
      	"nReturned" : 0,
      	"executionTimeMillis" : 2,
      	"works" : 4,
      	"advanced" : 0,
      	"needTime" : 0,
      	"isEOF" : 1,
      	"invalidates" : 0,
      	"transformBy" : {
      		"_id" : 0,
      		"a" : 1,
      		"b" : 1
      	},
      	"inputStage" : {
      		"stage" : "SORT",
      		"nReturned" : 0,
      		"executionTimeMillis" : 2,
      		"works" : 4,
      		"advanced" : 0,
      		"needTime" : 2,
      		"isEOF" : 1,
      		"invalidates" : 0,
      		"sortPattern" : {
      			"b" : 1
      		},
      		"memUsage" : 0,
      		"memLimit" : 33554432,
      		"inputStage" : {
      			"stage" : "KEEP_MUTATIONS",
      			"nReturned" : 0,
      			"executionTimeMillis" : 0,
      			"works" : 2,
      			"advanced" : 0,
      			"needTime" : 1,
      			"isEOF" : 1,
      			"invalidates" : 0,
      			"inputStage" : {
      				"stage" : "FETCH",
      				"nReturned" : 0,
      				"executionTimeMillis" : 0,
      				"works" : 2,
      				"advanced" : 0,
      				"needTime" : 1,
      				"isEOF" : 1,
      				"invalidates" : 0,
      				"docsExamined" : 0,
      				"alreadyHasObj" : 0,
      				"inputStage" : {
      					"stage" : "IXSCAN",
      					"nReturned" : 0,
      					"executionTimeMillis" : 0,
      					"works" : 1,
      					"advanced" : 0,
      					"needTime" : 1,
      					"isEOF" : 1,
      					"invalidates" : 0,
      					"keysExamined" : 0,
      					"keyPattern" : {
      						"a" : 1,
      						"b" : 1
      					},
      					"isMultiKey" : false,
      					"indexBounds" : "field #0['a']: (0.0, inf.0], field #1['b']: [MinKey, MaxKey]",
      					"dupsTested" : 0,
      					"dupsDropped" : 0,
      					"seenInvalidated" : 0,
      					"matchTested" : 0
      				}
      			}
      		}
      	}
      }
      

            Assignee:
            james.wahlin@mongodb.com James Wahlin
            Reporter:
            rassi J Rassi
            Votes:
            1 Vote for this issue
            Watchers:
            9 Start watching this issue

              Created:
              Updated:
              Resolved: