Details
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Bug
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Status: Closed
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Major - P3
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Resolution: Duplicate
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2.6.10, 3.0.0
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None
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None
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ALL
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QuInt A (10/12/15)
Description
The pattern of using key/value attributes and querying using $all and $elemMatch used to work really efficiently even when the cardinality of values for specific keys was not known. Since 2.6.10 and 3.0.* this pattern now only uses the first predicate regardless.
SERVER-16256 appears to have introduced this new behavior, essentially returning it to 2.4.* status.
Shown below are two explain(true) outputs, the first is what is seen when run on 2.6.0 -> 2.6.9... notice there are 2 possible plans in the allPlans list:
>db.kv.find({attr:{$all:[{$elemMatch: {k: "low", v: 1}}, {$elemMatch: {k: "high", v: 1024}}]}}).explain(true); |
{
|
"cursor" : "BtreeCursor attr.k_1_attr.v_1", |
"isMultiKey" : true, |
"n" : 2, |
"nscannedObjects" : 6, |
"nscanned" : 6, |
"nscannedObjectsAllPlans" : 17, |
"nscannedAllPlans" : 17, |
"scanAndOrder" : false, |
"indexOnly" : false, |
"nYields" : 8, |
"nChunkSkips" : 0, |
"millis" : 0, |
"indexBounds" : { |
"attr.k" : [ |
[
|
"high", |
"high" |
]
|
],
|
"attr.v" : [ |
[
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1024,
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1024
|
]
|
]
|
},
|
"allPlans" : [ |
{
|
"cursor" : "BtreeCursor attr.k_1_attr.v_1", |
"isMultiKey" : true, |
"n" : 2, |
"nscannedObjects" : 6, |
"nscanned" : 6, |
"scanAndOrder" : false, |
"indexOnly" : false, |
"nChunkSkips" : 0, |
"indexBounds" : { |
"attr.k" : [ |
[
|
"high", |
"high" |
]
|
],
|
"attr.v" : [ |
[
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1024,
|
1024
|
]
|
]
|
}
|
},
|
{
|
"cursor" : "BtreeCursor attr.k_1_attr.v_1", |
"isMultiKey" : true, |
"n" : 0, |
"nscannedObjects" : 11, |
"nscanned" : 11, |
"scanAndOrder" : false, |
"indexOnly" : false, |
"nChunkSkips" : 0, |
"indexBounds" : { |
"attr.k" : [ |
[
|
"low", |
"low" |
]
|
],
|
"attr.v" : [ |
[
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1,
|
1
|
]
|
]
|
}
|
}
|
],
|
"server" : "Boomtime:27017", |
"filterSet" : false, |
"stats" : { |
"type" : "KEEP_MUTATIONS", |
"works" : 14, |
"yields" : 8, |
"unyields" : 8, |
"invalidates" : 0, |
"advanced" : 2, |
"needTime" : 4, |
"needFetch" : 6, |
"isEOF" : 1, |
"children" : [ |
{
|
"type" : "FETCH", |
"works" : 13, |
"yields" : 8, |
"unyields" : 8, |
"invalidates" : 0, |
"advanced" : 2, |
"needTime" : 4, |
"needFetch" : 6, |
"isEOF" : 1, |
"alreadyHasObj" : 0, |
"forcedFetches" : 0, |
"matchTested" : 2, |
"children" : [ |
{
|
"type" : "IXSCAN", |
"works" : 7, |
"yields" : 8, |
"unyields" : 8, |
"invalidates" : 0, |
"advanced" : 6, |
"needTime" : 0, |
"needFetch" : 0, |
"isEOF" : 1, |
"keyPattern" : "{ attr.k: 1.0, attr.v: 1.0 }", |
"isMultiKey" : 1, |
"boundsVerbose" : "field #0['attr.k']: [\"high\", \"high\"], field #1['attr.v']: [1024.0, 1024.0]", |
"yieldMovedCursor" : 0, |
"dupsTested" : 6, |
"dupsDropped" : 0, |
"seenInvalidated" : 0, |
"matchTested" : 0, |
"keysExamined" : 6, |
"children" : [ ] |
}
|
]
|
}
|
]
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}
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}
|
The second explain is what is seen for the same query on 2.6.10+ (including all of 3.0). Notice that there is now only 1 plan considered, based exclusively on the first item in the $all array:
17:46:50@test>db.kv.find({attr:{$all:[{$elemMatch: {k: "low", v: 1}}, {$elemMatch: {k: "high", v: 1024}}]}}).explain(true); |
{
|
"cursor" : "BtreeCursor attr.k_1_attr.v_1", |
"isMultiKey" : true, |
"n" : 2, |
"nscannedObjects" : 5138, |
"nscanned" : 5138, |
"nscannedObjectsAllPlans" : 5138, |
"nscannedAllPlans" : 5138, |
"scanAndOrder" : false, |
"indexOnly" : false, |
"nYields" : 174, |
"nChunkSkips" : 0, |
"millis" : 26, |
"indexBounds" : { |
"attr.k" : [ |
[
|
"low", |
"low" |
]
|
],
|
"attr.v" : [ |
[
|
1,
|
1
|
]
|
]
|
},
|
"allPlans" : [ |
{
|
"cursor" : "BtreeCursor attr.k_1_attr.v_1", |
"isMultiKey" : true, |
"n" : 2, |
"nscannedObjects" : 5138, |
"nscanned" : 5138, |
"scanAndOrder" : false, |
"indexOnly" : false, |
"nChunkSkips" : 0, |
"indexBounds" : { |
"attr.k" : [ |
[
|
"low", |
"low" |
]
|
],
|
"attr.v" : [ |
[
|
1,
|
1
|
]
|
]
|
}
|
}
|
],
|
"server" : "Boomtime:27017", |
"filterSet" : false, |
"stats" : { |
"type" : "KEEP_MUTATIONS", |
"works" : 5313, |
"yields" : 174, |
"unyields" : 174, |
"invalidates" : 0, |
"advanced" : 2, |
"needTime" : 5136, |
"needFetch" : 174, |
"isEOF" : 1, |
"children" : [ |
{
|
"type" : "FETCH", |
"works" : 5313, |
"yields" : 174, |
"unyields" : 174, |
"invalidates" : 0, |
"advanced" : 2, |
"needTime" : 5136, |
"needFetch" : 174, |
"isEOF" : 1, |
"alreadyHasObj" : 0, |
"forcedFetches" : 0, |
"matchTested" : 2, |
"children" : [ |
{
|
"type" : "IXSCAN", |
"works" : 5139, |
"yields" : 174, |
"unyields" : 174, |
"invalidates" : 0, |
"advanced" : 5138, |
"needTime" : 0, |
"needFetch" : 0, |
"isEOF" : 1, |
"keyPattern" : "{ attr.k: 1.0, attr.v: 1.0 }", |
"isMultiKey" : 1, |
"boundsVerbose" : "field #0['attr.k']: [\"low\", \"low\"], field #1['attr.v']: [1.0, 1.0]", |
"yieldMovedCursor" : 0, |
"dupsTested" : 5138, |
"dupsDropped" : 0, |
"seenInvalidated" : 0, |
"matchTested" : 0, |
"keysExamined" : 5138, |
"children" : [ ] |
}
|
]
|
}
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]
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}
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}
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Attachments
Issue Links
- duplicates
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SERVER-16042 Optimise $all/$and to select smallest subset as initial index bounds
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- Closed
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- is related to
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- Closed
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