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Type:
Improvement
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Resolution: Unresolved
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Priority:
Major - P3
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None
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Affects Version/s: 7.1.0-rc0
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Component/s: None
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Labels:None
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Query Optimization
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.