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Type: Bug
Resolution: Duplicate
Priority: Major - P3
Fix Version/s: None
Affects Version/s: None
Component/s: None
Labels:
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Operating System:
ALL
CAR Domain/s:
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Aha! Reference:
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Tracking Level:
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Risk Status:
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Exec Notes:
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Goal Name(s):
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Goal Link:
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We are running a sharded mongodb cluster (v2.6.1).
Counting and iterating returns different results.
For example:

mongos> db.offer.count({shopId:6160})
520472
mongos> db.offer.find({shopId:6160}).itcount()
516564

Projection of _id returns correctly the same:

mongos> db.offer.find({shopId:6160},{_id:1}).itcount()
516564

The indexes are as follows:

mongos> db.offer.getIndexKeys()
[
	{
		"_id" : 1
	},
	{
		"bokey" : 1
	},
	{
		"categoryBokey" : 1
	},
	{
		"mappedCatalogCategory" : 1
	},
	{
		"clickCount" : 1
	},
	{
		"missingSince" : 1
	},
	{
		"shopId" : 1,
		"merchantId" : 1
	},
	{
		"asin" : 1
	},
	{
		"importGroup" : 1,
		"importId" : 1,
		"missingSince" : 1
	},
	{
		"shopId" : 1,
		"_id" : 1
	}
]

Using explicitly either index shopId, _id or shopId, merchantId returns the same number of documents as itcount:

mongos> db.offer.find({shopId:6160},{_id:1}).hint({shopId:1, _id:1}).explain()
{
	"clusteredType" : "ParallelSort",
	"shards" : {
		"offerStoreDE1/s479:27017,s480:27017,s483:27017" : [
			{
				"cursor" : "BtreeCursor shopId_1__id_1",
				"isMultiKey" : false,
				"n" : 94115,
				"nscannedObjects" : 94115,
				"nscanned" : 94115,
				"nscannedObjectsAllPlans" : 94115,
				"nscannedAllPlans" : 94115,
				"scanAndOrder" : false,
				"indexOnly" : false,
				"nYields" : 735,
				"nChunkSkips" : 0,
				"millis" : 695,
				"indexBounds" : {
					"shopId" : [
						[
							6160,
							6160
						]
					],
					"_id" : [
						[
							{
								"$minElement" : 1
							},
							{
								"$maxElement" : 1
							}
						]
					]
				},
				"server" : "s480:27017",
				"filterSet" : false
			}
		],
		"offerStoreDE2/s127:27017,s131:27017,s136:27017" : [
			{
				"cursor" : "BtreeCursor shopId_1__id_1",
				"isMultiKey" : false,
				"n" : 46393,
				"nscannedObjects" : 48801,
				"nscanned" : 48801,
				"nscannedObjectsAllPlans" : 48801,
				"nscannedAllPlans" : 48801,
				"scanAndOrder" : false,
				"indexOnly" : false,
				"nYields" : 381,
				"nChunkSkips" : 2408,
				"millis" : 454,
				"indexBounds" : {
					"shopId" : [
						[
							6160,
							6160
						]
					],
					"_id" : [
						[
							{
								"$minElement" : 1
							},
							{
								"$maxElement" : 1
							}
						]
					]
				},
				"server" : "s131:27017",
				"filterSet" : false
			}
		],
		"offerStoreDE3/s124:27017,s129:27017,s132:27017" : [
			{
				"cursor" : "BtreeCursor shopId_1__id_1",
				"isMultiKey" : false,
				"n" : 55811,
				"nscannedObjects" : 57100,
				"nscanned" : 57100,
				"nscannedObjectsAllPlans" : 57100,
				"nscannedAllPlans" : 57100,
				"scanAndOrder" : false,
				"indexOnly" : false,
				"nYields" : 446,
				"nChunkSkips" : 1289,
				"millis" : 526,
				"indexBounds" : {
					"shopId" : [
						[
							6160,
							6160
						]
					],
					"_id" : [
						[
							{
								"$minElement" : 1
							},
							{
								"$maxElement" : 1
							}
						]
					]
				},
				"server" : "s129:27017",
				"filterSet" : false
			}
		],
		"offerStoreDE4/s125:27017,s191:27017,s216:27017" : [
			{
				"cursor" : "BtreeCursor shopId_1__id_1",
				"isMultiKey" : false,
				"n" : 46573,
				"nscannedObjects" : 46784,
				"nscanned" : 46784,
				"nscannedObjectsAllPlans" : 46784,
				"nscannedAllPlans" : 46784,
				"scanAndOrder" : false,
				"indexOnly" : false,
				"nYields" : 365,
				"nChunkSkips" : 211,
				"millis" : 397,
				"indexBounds" : {
					"shopId" : [
						[
							6160,
							6160
						]
					],
					"_id" : [
						[
							{
								"$minElement" : 1
							},
							{
								"$maxElement" : 1
							}
						]
					]
				},
				"server" : "s216:27017",
				"filterSet" : false
			}
		],
		"offerStoreDE5/s484:27017,s487:27017,s488:27017" : [
			{
				"cursor" : "BtreeCursor shopId_1__id_1",
				"isMultiKey" : false,
				"n" : 125006,
				"nscannedObjects" : 125006,
				"nscanned" : 125006,
				"nscannedObjectsAllPlans" : 125006,
				"nscannedAllPlans" : 125006,
				"scanAndOrder" : false,
				"indexOnly" : false,
				"nYields" : 976,
				"nChunkSkips" : 0,
				"millis" : 1577,
				"indexBounds" : {
					"shopId" : [
						[
							6160,
							6160
						]
					],
					"_id" : [
						[
							{
								"$minElement" : 1
							},
							{
								"$maxElement" : 1
							}
						]
					]
				},
				"server" : "s487:27017",
				"filterSet" : false
			}
		],
		"offerStoreDE6/s561:27017,s562:27017,s563:27017" : [
			{
				"cursor" : "BtreeCursor shopId_1__id_1",
				"isMultiKey" : false,
				"n" : 148666,
				"nscannedObjects" : 148666,
				"nscanned" : 148666,
				"nscannedObjectsAllPlans" : 148666,
				"nscannedAllPlans" : 148666,
				"scanAndOrder" : false,
				"indexOnly" : false,
				"nYields" : 1161,
				"nChunkSkips" : 0,
				"millis" : 2274,
				"indexBounds" : {
					"shopId" : [
						[
							6160,
							6160
						]
					],
					"_id" : [
						[
							{
								"$minElement" : 1
							},
							{
								"$maxElement" : 1
							}
						]
					]
				},
				"server" : "s563:27017",
				"filterSet" : false
			}
		]
	},
	"cursor" : "BtreeCursor shopId_1__id_1",
	"n" : 516564,
	"nChunkSkips" : 3908,
	"nYields" : 4064,
	"nscanned" : 520472,
	"nscannedAllPlans" : 520472,
	"nscannedObjects" : 520472,
	"nscannedObjectsAllPlans" : 520472,
	"millisShardTotal" : 5923,
	"millisShardAvg" : 987,
	"numQueries" : 6,
	"numShards" : 6,
	"millis" : 2276
}
mongos> db.offer.find({shopId:6160},{_id:1}).hint({shopId:1, merchantId:1}).explain()
{
	"clusteredType" : "ParallelSort",
	"shards" : {
		"offerStoreDE1/s479:27017,s480:27017,s483:27017" : [
			{
				"cursor" : "BtreeCursor shopId_1_merchantId_1",
				"isMultiKey" : false,
				"n" : 94115,
				"nscannedObjects" : 94115,
				"nscanned" : 94115,
				"nscannedObjectsAllPlans" : 94115,
				"nscannedAllPlans" : 94115,
				"scanAndOrder" : false,
				"indexOnly" : false,
				"nYields" : 735,
				"nChunkSkips" : 0,
				"millis" : 723,
				"indexBounds" : {
					"shopId" : [
						[
							6160,
							6160
						]
					],
					"merchantId" : [
						[
							{
								"$minElement" : 1
							},
							{
								"$maxElement" : 1
							}
						]
					]
				},
				"server" : "s480:27017",
				"filterSet" : false
			}
		],
		"offerStoreDE2/s127:27017,s131:27017,s136:27017" : [
			{
				"cursor" : "BtreeCursor shopId_1_merchantId_1",
				"isMultiKey" : false,
				"n" : 46393,
				"nscannedObjects" : 48801,
				"nscanned" : 48801,
				"nscannedObjectsAllPlans" : 48801,
				"nscannedAllPlans" : 48801,
				"scanAndOrder" : false,
				"indexOnly" : false,
				"nYields" : 381,
				"nChunkSkips" : 2408,
				"millis" : 438,
				"indexBounds" : {
					"shopId" : [
						[
							6160,
							6160
						]
					],
					"merchantId" : [
						[
							{
								"$minElement" : 1
							},
							{
								"$maxElement" : 1
							}
						]
					]
				},
				"server" : "s131:27017",
				"filterSet" : false
			}
		],
		"offerStoreDE3/s124:27017,s129:27017,s132:27017" : [
			{
				"cursor" : "BtreeCursor shopId_1_merchantId_1",
				"isMultiKey" : false,
				"n" : 55811,
				"nscannedObjects" : 57100,
				"nscanned" : 57100,
				"nscannedObjectsAllPlans" : 57100,
				"nscannedAllPlans" : 57100,
				"scanAndOrder" : false,
				"indexOnly" : false,
				"nYields" : 446,
				"nChunkSkips" : 1289,
				"millis" : 526,
				"indexBounds" : {
					"shopId" : [
						[
							6160,
							6160
						]
					],
					"merchantId" : [
						[
							{
								"$minElement" : 1
							},
							{
								"$maxElement" : 1
							}
						]
					]
				},
				"server" : "s129:27017",
				"filterSet" : false
			}
		],
		"offerStoreDE4/s125:27017,s191:27017,s216:27017" : [
			{
				"cursor" : "BtreeCursor shopId_1_merchantId_1",
				"isMultiKey" : false,
				"n" : 46573,
				"nscannedObjects" : 46784,
				"nscanned" : 46784,
				"nscannedObjectsAllPlans" : 46784,
				"nscannedAllPlans" : 46784,
				"scanAndOrder" : false,
				"indexOnly" : false,
				"nYields" : 365,
				"nChunkSkips" : 211,
				"millis" : 389,
				"indexBounds" : {
					"shopId" : [
						[
							6160,
							6160
						]
					],
					"merchantId" : [
						[
							{
								"$minElement" : 1
							},
							{
								"$maxElement" : 1
							}
						]
					]
				},
				"server" : "s216:27017",
				"filterSet" : false
			}
		],
		"offerStoreDE5/s484:27017,s487:27017,s488:27017" : [
			{
				"cursor" : "BtreeCursor shopId_1_merchantId_1",
				"isMultiKey" : false,
				"n" : 125006,
				"nscannedObjects" : 125006,
				"nscanned" : 125006,
				"nscannedObjectsAllPlans" : 125006,
				"nscannedAllPlans" : 125006,
				"scanAndOrder" : false,
				"indexOnly" : false,
				"nYields" : 976,
				"nChunkSkips" : 0,
				"millis" : 965,
				"indexBounds" : {
					"shopId" : [
						[
							6160,
							6160
						]
					],
					"merchantId" : [
						[
							{
								"$minElement" : 1
							},
							{
								"$maxElement" : 1
							}
						]
					]
				},
				"server" : "s487:27017",
				"filterSet" : false
			}
		],
		"offerStoreDE6/s561:27017,s562:27017,s563:27017" : [
			{
				"cursor" : "BtreeCursor shopId_1_merchantId_1",
				"isMultiKey" : false,
				"n" : 148666,
				"nscannedObjects" : 148666,
				"nscanned" : 148666,
				"nscannedObjectsAllPlans" : 148666,
				"nscannedAllPlans" : 148666,
				"scanAndOrder" : false,
				"indexOnly" : false,
				"nYields" : 1161,
				"nChunkSkips" : 0,
				"millis" : 2496,
				"indexBounds" : {
					"shopId" : [
						[
							6160,
							6160
						]
					],
					"merchantId" : [
						[
							{
								"$minElement" : 1
							},
							{
								"$maxElement" : 1
							}
						]
					]
				},
				"server" : "s563:27017",
				"filterSet" : false
			}
		]
	},
	"cursor" : "BtreeCursor shopId_1_merchantId_1",
	"n" : 516564,
	"nChunkSkips" : 3908,
	"nYields" : 4064,
	"nscanned" : 520472,
	"nscannedAllPlans" : 520472,
	"nscannedObjects" : 520472,
	"nscannedObjectsAllPlans" : 520472,
	"millisShardTotal" : 5537,
	"millisShardAvg" : 922,
	"numQueries" : 6,
	"numShards" : 6,
	"millis" : 2500
}
mongos>

So why count counts too much documents?

duplicates

SERVER-3645 Sharded collection counts (on primary) can report too many results

Closed

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