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  1. Core Server
  2. SERVER-12297

Distinct - nscanned for $in and $or greater than nscannedObjects

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    • Type: Icon: Bug Bug
    • Resolution: Done
    • Priority: Icon: Minor - P4 Minor - P4
    • None
    • Affects Version/s: 2.5.4
    • Component/s: Querying
    • Labels:
    • Fully Compatible
    • ALL
    • Hide

      The following queries generates an anomaly with nscanned:

      t = db.distinct_index1
t.drop();

for ( i=0; i < 100; ++i ) {
      t.insert( {a: i%10} );
}
t.ensureIndex({a:1});
t.runCommand( "distinct" , { key : 'a' , query : { a : { $in : [2, 4, 6, 8] } } } );

{
	"values" : [
		2,
		4,
		6,
		8
	],
	"stats" : {
		"n" : 40,
		"nscanned" : 43,
		"nscannedObjects" : 40,
		"timems" : 12,
		"cursor" : "BtreeCursor a_1"
	},
	"ok" : 1
}

t.runCommand( "distinct" , { key : 'a' , query :{ $or: [{a:3}, {a:5}] } } );

{
	"values" : [
		3,
		5
	],
	"stats" : {
		"n" : 20,
		"nscanned" : 21,
		"nscannedObjects" : 20,
		"timems" : 0,
		"cursor" : "BtreeCursor a_1"
	},
	"ok" : 1
}
      Show
      The following queries generates an anomaly with nscanned: t = db.distinct_index1 t.drop(); for ( i=0; i < 100; ++i ) { t.insert( {a: i%10} ); } t.ensureIndex({a:1}); t.runCommand( "distinct" , { key : 'a' , query : { a : { $in : [2, 4, 6, 8] } } } ); { "values" : [ 2, 4, 6, 8 ], "stats" : { "n" : 40, "nscanned" : 43, "nscannedObjects" : 40, "timems" : 12, "cursor" : "BtreeCursor a_1" }, "ok" : 1 } t.runCommand( "distinct" , { key : 'a' , query :{ $or: [{a:3}, {a:5}] } } ); { "values" : [ 3, 5 ], "stats" : { "n" : 20, "nscanned" : 21, "nscannedObjects" : 20, "timems" : 0, "cursor" : "BtreeCursor a_1" }, "ok" : 1 }

      nscanned for certain indexed queries ($or & $in) returns a different result than nscannedObjects. This is not an issue when using $gt, $lt, $and, etc.

        1. server12297.js
          0.8 kB

            Assignee:
            benety.goh@mongodb.com Benety Goh
            Reporter:
            jonathan.abrahams Jonathan Abrahams
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              Created:
              Updated:
              Resolved: