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  1. Core Server
  2. SERVER-13324

Sparse index is not used to filter {field: {$ne: null}} queries

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    • Type: Icon: Bug Bug
    • Resolution: Done
    • Priority: Icon: Major - P3 Major - P3
    • None
    • Affects Version/s: 2.6.0-rc2
    • Component/s: Querying
    • Labels:
    • ALL
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      > db.foo.drop()
true
> db.foo.ensureIndex({bar: 1}, {sparse: 1})
{
	"createdCollectionAutomatically" : true,
	"numIndexesBefore" : 1,
	"numIndexesAfter" : 2,
	"ok" : 1
}
> db.foo.find({bar: {$ne: null}}).explain()
{
	"cursor" : "BasicCursor",
	"isMultiKey" : false,
	"n" : 0,
	"nscannedObjects" : 0,
	"nscanned" : 0,
	"nscannedObjectsAllPlans" : 0,
	"nscannedAllPlans" : 0,
	"scanAndOrder" : false,
	"indexOnly" : false,
	"nYields" : 0,
	"nChunkSkips" : 0,
	"millis" : 0,
	"server" : "lucien.lic.songza.com:27017",
	"filterSet" : false
}
> db.foo.find({bar: {$exists: true}}).explain()
{
	"cursor" : "BtreeCursor bar_1",
	"isMultiKey" : false,
	"n" : 0,
	"nscannedObjects" : 0,
	"nscanned" : 0,
	"nscannedObjectsAllPlans" : 0,
	"nscannedAllPlans" : 0,
	"scanAndOrder" : false,
	"indexOnly" : false,
	"nYields" : 0,
	"nChunkSkips" : 0,
	"millis" : 0,
	"indexBounds" : {
		"bar" : [
			[
				{
					"$minElement" : 1
				},
				{
					"$maxElement" : 1
				}
			]
		]
	},
	"server" : "lucien.lic.songza.com:27017",
	"filterSet" : false
}
      Show
      > db.foo.drop() true > db.foo.ensureIndex({bar: 1}, {sparse: 1}) { "createdCollectionAutomatically" : true , "numIndexesBefore" : 1, "numIndexesAfter" : 2, "ok" : 1 } > db.foo.find({bar: {$ne: null }}).explain() { "cursor" : "BasicCursor" , "isMultiKey" : false , "n" : 0, "nscannedObjects" : 0, "nscanned" : 0, "nscannedObjectsAllPlans" : 0, "nscannedAllPlans" : 0, "scanAndOrder" : false , "indexOnly" : false , "nYields" : 0, "nChunkSkips" : 0, "millis" : 0, "server" : "lucien.lic.songza.com:27017" , "filterSet" : false } > db.foo.find({bar: {$exists: true }}).explain() { "cursor" : "BtreeCursor bar_1" , "isMultiKey" : false , "n" : 0, "nscannedObjects" : 0, "nscanned" : 0, "nscannedObjectsAllPlans" : 0, "nscannedAllPlans" : 0, "scanAndOrder" : false , "indexOnly" : false , "nYields" : 0, "nChunkSkips" : 0, "millis" : 0, "indexBounds" : { "bar" : [ [ { "$minElement" : 1 }, { "$maxElement" : 1 } ] ] }, "server" : "lucien.lic.songza.com:27017" , "filterSet" : false }

      In previous version of mongo a sparse index on a field could not be used to find those documents that only had the indexed field. Take the following collection:

      db.foo.ensureIndex({bar: 1}, {sparse: 1})

      If one were to naively write a query to find all documents that had the "bar" property one would start with:

      db.foo.find({bar: {$exists: true}})

      In mongo 2.4 and below this particular query would result in a full collection scan with a BasicCursor. For an application in which "bar" would never be null this particular query could be rewritten as:

      db.foo.find({bar: {$ne: null}})

      This query would use the sparse index. It appears that while mongo 2.5 can now correctly use the sparse index for the $exists: true query it will fail to use any index for the {$ne: null} variety unless an explicit hint is given.

            Assignee:
            david.storch@mongodb.com David Storch
            Reporter:
            michael@songza.com Michael Henson
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              Created:
              Updated:
              Resolved: