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Type:
Improvement
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Resolution: Works as Designed
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Priority:
Major - P3
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None
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Affects Version/s: None
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Component/s: Querying
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Query
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None
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3
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None
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None
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None
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None
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None
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None
Trying to analyze cost calculation for index intersection I came across something that seems incorrect (suboptimal) - maybe it's related to SERVER-13740?
There is an index on x and class and query is equality on x and class - this section is from rejectedPlans in explain with execution stats:
{
"nReturned" : 1,
"executionTimeMillisEstimate" : 0,
"totalKeysExamined" : 123,
"totalDocsExamined" : 1,
"executionStages" : {
"stage" : "PROJECTION",
"nReturned" : 1,
"executionTimeMillisEstimate" : 0,
"works" : 123,
"advanced" : 1,
"needTime" : 122,
"needYield" : 0,
"saveState" : 2,
"restoreState" : 2,
"isEOF" : 0,
"invalidates" : 0,
"transformBy" : {
"_id" : 0,
"x" : 1,
"class" : 1
},
"inputStage" : {
"stage" : "FETCH",
"filter" : {
"$and" : [
{
"x" : {
"$eq" : 1
}
},
{
"class" : {
"$eq" : 0.027375694861415
}
}
]
},
"nReturned" : 1,
"executionTimeMillisEstimate" : 0,
"works" : 123,
"advanced" : 1,
"needTime" : 122,
"needYield" : 0,
"saveState" : 2,
"restoreState" : 2,
"isEOF" : 0,
"invalidates" : 0,
"docsExamined" : 1,
"alreadyHasObj" : 0,
"inputStage" : {
"stage" : "AND_SORTED",
"nReturned" : 1,
"executionTimeMillisEstimate" : 0,
"works" : 123,
"advanced" : 1,
"needTime" : 122,
"needYield" : 0,
"saveState" : 2,
"restoreState" : 2,
"isEOF" : 0,
"invalidates" : 0,
"flagged" : 0,
"failedAnd_0" : 2,
"failedAnd_1" : 1,
"inputStages" : [
{
"stage" : "IXSCAN",
"nReturned" : 116,
"executionTimeMillisEstimate" : 0,
"works" : 116,
"advanced" : 116,
"needTime" : 0,
"needYield" : 0,
"saveState" : 2,
"restoreState" : 2,
"isEOF" : 0,
"invalidates" : 0,
"keyPattern" : {
"x" : 1
},
"indexName" : "x_1",
"isMultiKey" : false,
"multiKeyPaths" : {
"x" : [ ]
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {
"x" : [
"[1.0, 1.0]"
]
},
"keysExamined" : 116,
"seeks" : 1,
"dupsTested" : 0,
"dupsDropped" : 0,
"seenInvalidated" : 0
},
{
"stage" : "IXSCAN",
"nReturned" : 7,
"executionTimeMillisEstimate" : 0,
"works" : 7,
"advanced" : 7,
"needTime" : 0,
"needYield" : 0,
"saveState" : 2,
"restoreState" : 2,
"isEOF" : 0,
"invalidates" : 0,
"keyPattern" : {
"class" : 1
},
"indexName" : "class_1",
"isMultiKey" : false,
"multiKeyPaths" : {
"class" : [ ]
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {
"class" : [
"[0.027375694861415, 0.027375694861415]"
]
},
"keysExamined" : 7,
"seeks" : 1,
"dupsTested" : 0,
"dupsDropped" : 0,
"seenInvalidated" : 0
}
]
}
}
}
},
After AND_SORTED there is the single matched document and it should be covered but there's a FETCH applying the query filter again before the PROJECTION stage. Seems like the FETCH is unnecessary (unless AND_SORTED isn't passed the index values? in which case this would be enhancement request to enable that).