Major - P3 The example below shows a query that returns "n" larger than the size of the collection.
Set up:
> for(i=0;i<10000;i++){db.foo.insert({a:1,b:1,c:1})}
Issue a long-running series of updates:
> for(i=0;;i++){eval("db.foo.update({},{$set:{f"+i+":true}},false,true)");}
Meanwhile, run the following query (and note that itcount() will return values greater than 10000, the number of documents in the collection):
> db.foo.find({$or:[{a:1}, {b:1}]}).sort({c:1}).itcount()
7979
> db.foo.find({$or:[{a:1}, {b:1}]}).sort({c:1}).itcount()
10000
> db.foo.find({$or:[{a:1}, {b:1}]}).sort({c:1}).itcount()
11038
Full explain:
> db.foo.find({$or:[{a:1}, {b:1}]}).sort({c:1}).explain(true)
{
"clauses" : [
{
"cursor" : "BtreeCursor a_1_c_1",
"isMultiKey" : false,
"n" : 11126,
"nscannedObjects" : 0,
"nscanned" : 11126,
"scanAndOrder" : false,
"indexOnly" : false,
"nChunkSkips" : 0,
"indexBounds" : {
"a" : [
[
1,
1
]
],
"c" : [
[
{
"$minElement" : 1
},
{
"$maxElement" : 1
}
]
]
}
},
{
"cursor" : "BtreeCursor b_1_c_1",
"isMultiKey" : false,
"n" : 11126,
"nscannedObjects" : 0,
"nscanned" : 11126,
"scanAndOrder" : false,
"indexOnly" : false,
"nChunkSkips" : 0,
"indexBounds" : {
"b" : [
[
1,
1
]
],
"c" : [
[
{
"$minElement" : 1
},
{
"$maxElement" : 1
}
]
]
}
}
],
"cursor" : "QueryOptimizerCursor",
"n" : 11126,
"nscannedObjects" : 0,
"nscanned" : 22252,
"nscannedObjectsAllPlans" : 0,
"nscannedAllPlans" : 22252,
"scanAndOrder" : false,
"nYields" : 267,
"nChunkSkips" : 0,
"millis" : 2559,
"allPlans" : [
{
"clauses" : [
{
"cursor" : "BtreeCursor a_1_c_1",
"isMultiKey" : false,
"n" : 11126,
"nscannedObjects" : 0,
"nscanned" : 11126,
"scanAndOrder" : false,
"indexOnly" : false,
"nChunkSkips" : 0,
"indexBounds" : {
"a" : [
[
1,
1
]
],
"c" : [
[
{
"$minElement" : 1
},
{
"$maxElement" : 1
}
]
]
}
},
{
"cursor" : "BtreeCursor b_1_c_1",
"isMultiKey" : false,
"n" : 11126,
"nscannedObjects" : 0,
"nscanned" : 11126,
"scanAndOrder" : false,
"indexOnly" : false,
"nChunkSkips" : 0,
"indexBounds" : {
"b" : [
[
1,
1
]
],
"c" : [
[
{
"$minElement" : 1
},
{
"$maxElement" : 1
}
]
]
}
}
],
"cursor" : "QueryOptimizerCursor",
"n" : 11126,
"nscannedObjects" : 0,
"nscanned" : 22252,
"scanAndOrder" : false,
"nChunkSkips" : 0
}
],
"server" : "Rassi-MacBook-Pro.local:27017",
"filterSet" : false,
"stats" : {
"type" : "FETCH",
"works" : 33394,
"yields" : 267,
"unyields" : 267,
"invalidates" : 3365,
"advanced" : 11126,
"needTime" : 22254,
"needFetch" : 13,
"isEOF" : 1,
"alreadyHasObj" : 0,
"forcedFetches" : 0,
"matchTested" : 0,
"children" : [
{
"type" : "SORT_MERGE",
"works" : 33380,
"yields" : 267,
"unyields" : 267,
"invalidates" : 3365,
"advanced" : 11126,
"needTime" : 22254,
"needFetch" : 0,
"isEOF" : 1,
"dupsTested" : 22252,
"dupsDropped" : 11126,
"forcedFetches" : 0,
"children" : [
{
"type" : "IXSCAN",
"works" : 11127,
"yields" : 267,
"unyields" : 267,
"invalidates" : 3365,
"advanced" : 11126,
"needTime" : 0,
"needFetch" : 0,
"isEOF" : 1,
"keyPattern" : "{ a: 1.0, c: 1.0 }",
"boundsVerbose" : "field #0['a']: [1.0, 1.0], field #1['c']: [MinKey, MaxKey]",
"isMultiKey" : 0,
"yieldMovedCursor" : 0,
"dupsTested" : 0,
"dupsDropped" : 0,
"seenInvalidated" : 0,
"matchTested" : 0,
"keysExamined" : 11126,
"children" : [ ]
},
{
"type" : "IXSCAN",
"works" : 11127,
"yields" : 267,
"unyields" : 267,
"invalidates" : 3365,
"advanced" : 11126,
"needTime" : 0,
"needFetch" : 0,
"isEOF" : 1,
"keyPattern" : "{ b: 1.0, c: 1.0 }",
"boundsVerbose" : "field #0['b']: [1.0, 1.0], field #1['c']: [MinKey, MaxKey]",
"isMultiKey" : 0,
"yieldMovedCursor" : 0,
"dupsTested" : 0,
"dupsDropped" : 0,
"seenInvalidated" : 0,
"matchTested" : 0,
"keysExamined" : 11126,
"children" : [ ]
}
]
}
]
}
}