Details
-
Bug
-
Status: Closed
-
Major - P3
-
Resolution: Fixed
-
2.6.0-rc0
-
ALL
-
Description
In the attached 'Test 1' case, v2.6.0-rc0 selects a different query plan than v2.4.9, and v2.6.0-rc0 takes significantly longer to execute. The indexed key is a string with many duplicates.
In 'Test 2', the indexed key is an integer with many duplicates, and v2.6.0-rc0 is faster than v2.4.9, and selects the same query plan.
Note: the two repro cases attached are identical, except for the value type of 'a'
Explain for test 1:
> db.version();
|
2.6.0-rc1-pre-
|
> db.t.find({a:"1"}).sort({d:1}).explain();
|
{
|
"cursor" : "BtreeCursor a_1_b_1",
|
"isMultiKey" : false,
|
"n" : 10000,
|
"nscannedObjects" : 10000,
|
"nscanned" : 10000,
|
"nscannedObjectsAllPlans" : 10098,
|
"nscannedAllPlans" : 10098,
|
"scanAndOrder" : true,
|
"indexOnly" : false,
|
"nYields" : 158,
|
"nChunkSkips" : 0,
|
"millis" : 50,
|
"indexBounds" : {
|
"a" : [
|
[
|
"1",
|
"1"
|
]
|
],
|
"b" : [
|
[
|
{
|
"$minElement" : 1
|
},
|
{
|
"$maxElement" : 1
|
}
|
]
|
]
|
},
|
"server" : "zzyzx:27018",
|
"filterSet" : false
|
}
|
|
|
> db.version();
|
2.4.9
|
> db.t.find({a:"1"}).sort({d:1}).explain();
|
{
|
"cursor" : "BtreeCursor d_1_e_1",
|
"isMultiKey" : false,
|
"n" : 10000,
|
"nscannedObjects" : 11000,
|
"nscanned" : 11000,
|
"nscannedObjectsAllPlans" : 13203,
|
"nscannedAllPlans" : 13203,
|
"scanAndOrder" : false,
|
"indexOnly" : false,
|
"nYields" : 0,
|
"nChunkSkips" : 0,
|
"millis" : 35,
|
"indexBounds" : {
|
"d" : [
|
[
|
{
|
"$minElement" : 1
|
},
|
{
|
"$maxElement" : 1
|
}
|
]
|
],
|
"e" : [
|
[
|
{
|
"$minElement" : 1
|
},
|
{
|
"$maxElement" : 1
|
}
|
]
|
]
|
},
|
"server" : "zzyzx:27018"
|
}
|
Explain for test 2:
> db.version();
|
2.6.0-rc1-pre-
|
> db.t.find({a:1}).sort({d:1}).explain();
|
{
|
"cursor" : "BtreeCursor d_1_e_1",
|
"isMultiKey" : false,
|
"n" : 10001,
|
"nscannedObjects" : 11000,
|
"nscanned" : 11000,
|
"nscannedObjectsAllPlans" : 11197,
|
"nscannedAllPlans" : 11198,
|
"scanAndOrder" : false,
|
"indexOnly" : false,
|
"nYields" : 87,
|
"nChunkSkips" : 0,
|
"millis" : 14,
|
"indexBounds" : {
|
"d" : [
|
[
|
{
|
"$minElement" : 1
|
},
|
{
|
"$maxElement" : 1
|
}
|
]
|
],
|
"e" : [
|
[
|
{
|
"$minElement" : 1
|
},
|
{
|
"$maxElement" : 1
|
}
|
]
|
]
|
},
|
"server" : "zzyzx:27018",
|
"filterSet" : false
|
}
|
|
|
> db.version();
|
2.4.9
|
> db.t.find({a:1}).sort({d:1}).explain();
|
{
|
"cursor" : "BtreeCursor d_1_e_1",
|
"isMultiKey" : false,
|
"n" : 10001,
|
"nscannedObjects" : 11000,
|
"nscanned" : 11000,
|
"nscannedObjectsAllPlans" : 13201,
|
"nscannedAllPlans" : 13201,
|
"scanAndOrder" : false,
|
"indexOnly" : false,
|
"nYields" : 0,
|
"nChunkSkips" : 0,
|
"millis" : 24,
|
"indexBounds" : {
|
"d" : [
|
[
|
{
|
"$minElement" : 1
|
},
|
{
|
"$maxElement" : 1
|
}
|
]
|
],
|
"e" : [
|
[
|
{
|
"$minElement" : 1
|
},
|
{
|
"$maxElement" : 1
|
}
|
]
|
]
|
},
|
"server" : "zzyzx:27018"
|
}
|
FWIW, the delta in the NLTK c++ test is an order of magnitude, though the dataset varies widely (see QA/QA-401/QA401.cpp for details):
> db.regressions.findOne({$nor:[{qry:/where/}, {qry:/exists/}]})
|
{
|
"_id" : ObjectId("530f6bad7e6c53525919dfad"),
|
"deltaRatio" : "9.55",
|
"deltaUs" : 917.67,
|
"v249Avg" : "107.33",
|
"v249Results" : [
|
106,
|
108,
|
109,
|
106,
|
107,
|
108
|
],
|
"rc0Avg" : "1025.00",
|
"rc0Results" : [
|
1005,
|
1081,
|
1011,
|
1026,
|
1013,
|
1014
|
],
|
"testId" : "GeneratedTests::GeneratedQuery607[Compound1]",
|
"qry" : "{ query: { a: \"\" }, orderby: { d: 1 } }",
|
"proj" : "{ NONE: 1 }",
|
"idx" : "Compound1",
|
"rc0Count" : 0,
|
"v249Count" : 0,
|
"countsMatch" : true
|
}
|