Details
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Improvement
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Status: Closed
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Major - P3
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Resolution: Works as Designed
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None
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None
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Query
Description
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:
{
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"nReturned" : 1,
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"executionTimeMillisEstimate" : 0,
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"totalKeysExamined" : 123,
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"totalDocsExamined" : 1,
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"executionStages" : {
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"stage" : "PROJECTION",
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"nReturned" : 1,
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"executionTimeMillisEstimate" : 0,
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"works" : 123,
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"advanced" : 1,
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"needTime" : 122,
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"needYield" : 0,
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"saveState" : 2,
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"restoreState" : 2,
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"isEOF" : 0,
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"invalidates" : 0,
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"transformBy" : {
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"_id" : 0,
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"x" : 1,
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"class" : 1
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},
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"inputStage" : {
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"stage" : "FETCH",
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"filter" : {
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"$and" : [
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{
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"x" : {
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"$eq" : 1
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}
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},
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{
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"class" : {
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"$eq" : 0.027375694861415
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}
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}
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]
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},
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"nReturned" : 1,
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"executionTimeMillisEstimate" : 0,
|
"works" : 123,
|
"advanced" : 1,
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"needTime" : 122,
|
"needYield" : 0,
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"saveState" : 2,
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"restoreState" : 2,
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"isEOF" : 0,
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"invalidates" : 0,
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"docsExamined" : 1,
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"alreadyHasObj" : 0,
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"inputStage" : {
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"stage" : "AND_SORTED",
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"nReturned" : 1,
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"executionTimeMillisEstimate" : 0,
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"works" : 123,
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"advanced" : 1,
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"needTime" : 122,
|
"needYield" : 0,
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"saveState" : 2,
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"restoreState" : 2,
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"isEOF" : 0,
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"invalidates" : 0,
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"flagged" : 0,
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"failedAnd_0" : 2,
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"failedAnd_1" : 1,
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"inputStages" : [
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{
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"stage" : "IXSCAN",
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"nReturned" : 116,
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"executionTimeMillisEstimate" : 0,
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"works" : 116,
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"advanced" : 116,
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"needTime" : 0,
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"needYield" : 0,
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"saveState" : 2,
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"restoreState" : 2,
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"isEOF" : 0,
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"invalidates" : 0,
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"keyPattern" : {
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"x" : 1
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},
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"indexName" : "x_1",
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"isMultiKey" : false,
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"multiKeyPaths" : {
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"x" : [ ]
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},
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"isUnique" : false,
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"isSparse" : false,
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"isPartial" : false,
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"indexVersion" : 2,
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"direction" : "forward",
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"indexBounds" : {
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"x" : [
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"[1.0, 1.0]"
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]
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},
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"keysExamined" : 116,
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"seeks" : 1,
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"dupsTested" : 0,
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"dupsDropped" : 0,
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"seenInvalidated" : 0
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},
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{
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"stage" : "IXSCAN",
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"nReturned" : 7,
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"executionTimeMillisEstimate" : 0,
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"works" : 7,
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"advanced" : 7,
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"needTime" : 0,
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"needYield" : 0,
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"saveState" : 2,
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"restoreState" : 2,
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"isEOF" : 0,
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"invalidates" : 0,
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"keyPattern" : {
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"class" : 1
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},
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"indexName" : "class_1",
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"isMultiKey" : false,
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"multiKeyPaths" : {
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"class" : [ ]
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},
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"isUnique" : false,
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"isSparse" : false,
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"isPartial" : false,
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"indexVersion" : 2,
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"direction" : "forward",
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"indexBounds" : {
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"class" : [
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"[0.027375694861415, 0.027375694861415]"
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]
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},
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"keysExamined" : 7,
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"seeks" : 1,
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"dupsTested" : 0,
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"dupsDropped" : 0,
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"seenInvalidated" : 0
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}
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]
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}
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}
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}
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},
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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).