Details
-
Improvement
-
Resolution: Unresolved
-
Major - P3
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None
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7.1.0-rc0
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None
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None
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Query Optimization
Description
The setup:
assert.commandWorked(coll.createIndex({_id: 1, a: 1})); |
coll.insert({_id: 1, a: 1}); |
coll.insert({_id: 2, a: 2}); |
coll.insert({_id: 3, a: 2}); |
When running an explain for `coll.find({_id: 2, a: 2})` we get only one winning plan, that is using an _id field. The newly introduced index is not taken into account and is not part of the rejected plans.
{
|
"explainVersion" : "2", |
"queryPlanner" : { |
"namespace" : "test.query_settings_cmds", |
"indexFilterSet" : false, |
"parsedQuery" : { |
"$and" : [ |
{
|
"_id" : { |
"$eq" : 2 |
}
|
},
|
{
|
"a" : { |
"$eq" : 2 |
}
|
}
|
]
|
},
|
"queryHash" : "FA27A693", |
"planCacheKey" : "7E57B88A", |
"maxIndexedOrSolutionsReached" : false, |
"maxIndexedAndSolutionsReached" : false, |
"maxScansToExplodeReached" : false, |
"winningPlan" : { |
"queryPlan" : { |
"stage" : "FETCH", |
"planNodeId" : 2, |
"filter" : { |
"a" : { |
"$eq" : 2 |
}
|
},
|
"inputStage" : { |
"stage" : "IXSCAN", |
"planNodeId" : 1, |
"keyPattern" : { |
"_id" : 1 |
},
|
"indexName" : "_id_", |
"isMultiKey" : false, |
"isUnique" : true, |
"isSparse" : false, |
"isPartial" : false, |
"indexVersion" : 2, |
"direction" : "forward", |
"indexBounds" : { |
"_id" : [ |
"[2.0, 2.0]" |
]
|
}
|
}
|
},
|
"slotBasedPlan" : { |
"slots" : "$$RESULT=s8 env: { s3 = KS(2B04FE04), s11 = 2, s2 = KS(2B040104), s7 = {\"_id\" : 1} }", |
"stages" : "[2] filter {traverseF(s10, lambda(l1.0) { ((l1.0 == s11) ?: false) }, false)} \n[2] nlj inner [] [s1, s4, s5, s6, s7] \n left \n [1] cfilter {(exists(s2) && exists(s3))} \n [1] ixseek s2 s3 s6 s1 s4 s5 [] @\"1ae72221-0cb8-4cf5-8162-53f0b93c8ced\" @\"_id_\" true \n right \n [2] limit 1 \n [2] seek s1 s8 s9 s4 s5 s6 s7 none none [s10 = a] @\"1ae72221-0cb8-4cf5-8162-53f0b93c8ced\" true false \n" |
}
|
},
|
"rejectedPlans" : [ ] |
},
|
"command" : { |
"find" : "query_settings_cmds", |
"filter" : { |
"_id" : 2, |
"a" : 2 |
},
|
"$db" : "test" |
},
|
"serverInfo" : { |
"host" : "ip-10-122-5-229", |
"port" : 20000, |
"version" : "7.1.0-alpha", |
"gitVersion" : "unknown" |
},
|
"serverParameters" : { |
"internalQueryFacetBufferSizeBytes" : 104857600, |
"internalQueryFacetMaxOutputDocSizeBytes" : 104857600, |
"internalLookupStageIntermediateDocumentMaxSizeBytes" : 104857600, |
"internalDocumentSourceGroupMaxMemoryBytes" : 104857600, |
"internalQueryMaxBlockingSortMemoryUsageBytes" : 104857600, |
"internalQueryProhibitBlockingMergeOnMongoS" : 0, |
"internalQueryMaxAddToSetBytes" : 104857600, |
"internalDocumentSourceSetWindowFieldsMaxMemoryBytes" : 104857600, |
"internalQueryFrameworkControl" : "trySbeEngine" |
},
|
"ok" : 1 |
}
|
With the index hint, however, we can generate a plan with the given index (`coll.find({_id: 2, a: 2}).hint({_id: 1, a: 1})`):
{
|
"explainVersion" : "2", |
"queryPlanner" : { |
"namespace" : "test.query_settings_cmds", |
"indexFilterSet" : false, |
"parsedQuery" : { |
"$and" : [ |
{
|
"_id" : { |
"$eq" : 2 |
}
|
},
|
{
|
"a" : { |
"$eq" : 2 |
}
|
}
|
]
|
},
|
"queryHash" : "E67D31E6", |
"planCacheKey" : "E7B1B151", |
"maxIndexedOrSolutionsReached" : false, |
"maxIndexedAndSolutionsReached" : false, |
"maxScansToExplodeReached" : false, |
"winningPlan" : { |
"queryPlan" : { |
"stage" : "FETCH", |
"planNodeId" : 2, |
"inputStage" : { |
"stage" : "IXSCAN", |
"planNodeId" : 1, |
"keyPattern" : { |
"_id" : 1, |
"a" : 1 |
},
|
"indexName" : "_id_1_a_1", |
"isMultiKey" : false, |
"multiKeyPaths" : { |
"_id" : [ ], |
"a" : [ ] |
},
|
"isUnique" : false, |
"isSparse" : false, |
"isPartial" : false, |
"indexVersion" : 2, |
"direction" : "forward", |
"indexBounds" : { |
"_id" : [ |
"[2.0, 2.0]" |
],
|
"a" : [ |
"[2.0, 2.0]" |
]
|
}
|
}
|
},
|
"slotBasedPlan" : { |
"slots" : "$$RESULT=s8 env: { s3 = KS(2B042B04FE04), s7 = {\"_id\" : 1, \"a\" : 1}, s2 = KS(2B042B040104) }", |
"stages" : "[2] nlj inner [] [s1, s4, s5, s6, s7] \n left \n [1] cfilter {(exists(s2) && exists(s3))} \n [1] ixseek s2 s3 s6 s1 s4 s5 [] @\"1ae72221-0cb8-4cf5-8162-53f0b93c8ced\" @\"_id_1_a_1\" true \n right \n [2] limit 1 \n [2] seek s1 s8 s9 s4 s5 s6 s7 none none [] @\"1ae72221-0cb8-4cf5-8162-53f0b93c8ced\" true false \n" |
}
|
},
|
"rejectedPlans" : [ ] |
},
|
"command" : { |
"find" : "query_settings_cmds", |
"filter" : { |
"_id" : 2, |
"a" : 2 |
},
|
"hint" : { |
"_id" : 1, |
"a" : 1 |
},
|
"$db" : "test" |
},
|
"serverInfo" : { |
"host" : "ip-10-122-5-229", |
"port" : 20000, |
"version" : "7.1.0-alpha", |
"gitVersion" : "unknown" |
},
|
"serverParameters" : { |
"internalQueryFacetBufferSizeBytes" : 104857600, |
"internalQueryFacetMaxOutputDocSizeBytes" : 104857600, |
"internalLookupStageIntermediateDocumentMaxSizeBytes" : 104857600, |
"internalDocumentSourceGroupMaxMemoryBytes" : 104857600, |
"internalQueryMaxBlockingSortMemoryUsageBytes" : 104857600, |
"internalQueryProhibitBlockingMergeOnMongoS" : 0, |
"internalQueryMaxAddToSetBytes" : 104857600, |
"internalDocumentSourceSetWindowFieldsMaxMemoryBytes" : 104857600, |
"internalQueryFrameworkControl" : "trySbeEngine" |
},
|
"ok" : 1 |
}
|
In theory the composite index should be preferred, as in case of a mismatch, no FETCH is needed to evaluate the predicate.
While the affected version is 7.1, the assumption is that this is and old bug present on other branches as well.