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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2.6.3
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None
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Query Optimization
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ALL
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Query 2019-04-08, Query 2019-07-29, Query 2019-08-12, Query 2019-08-26, Query 2019-09-09
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(copied to CRM)
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0
Description
The nscanned value is the same for both plans, but we choose the one with a much higher nscannedObjects:
t.drop();
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t.ensureIndex({type: 1});
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t.ensureIndex({type: 1, folder: 1});
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for (var i = 0; i < 110; i++) { t.insert({type: "folder", folder: "b"}); }
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t.insert({type: "folder", folder: "m"});
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t.find({type: "folder", folder: /m/}).explain(true)
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I get the following explain output from a 2.6.3 server:
{
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"cursor" : "BtreeCursor type_1",
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"isMultiKey" : false,
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"n" : 1,
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"nscannedObjects" : 111,
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"nscanned" : 111,
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"nscannedObjectsAllPlans" : 112,
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"nscannedAllPlans" : 222,
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"scanAndOrder" : false,
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"indexOnly" : false,
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"nYields" : 1,
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"nChunkSkips" : 0,
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"millis" : 1,
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"indexBounds" : {
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"type" : [
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[
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"folder",
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"folder"
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]
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]
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},
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"allPlans" : [
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{
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"cursor" : "BtreeCursor type_1",
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"isMultiKey" : false,
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"n" : 1,
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"nscannedObjects" : 111,
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"nscanned" : 111,
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"scanAndOrder" : false,
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"indexOnly" : false,
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"nChunkSkips" : 0,
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"indexBounds" : {
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"type" : [
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[
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"folder",
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"folder"
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]
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]
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}
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},
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{
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"cursor" : "BtreeCursor type_1_folder_1",
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"isMultiKey" : false,
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"n" : 1,
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"nscannedObjects" : 1,
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"nscanned" : 111,
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"scanAndOrder" : false,
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"indexOnly" : false,
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"nChunkSkips" : 0,
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"indexBounds" : {
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"type" : [
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[
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"folder",
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"folder"
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]
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],
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"folder" : [
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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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/m/,
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/m/
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]
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]
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}
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}
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],
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"server" : "Macintosh.local:27017",
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"filterSet" : false,
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"stats" : {
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"type" : "KEEP_MUTATIONS",
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"works" : 113,
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"yields" : 1,
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"unyields" : 1,
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"invalidates" : 0,
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"advanced" : 1,
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"needTime" : 110,
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"needFetch" : 0,
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"isEOF" : 1,
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"children" : [
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{
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"type" : "FETCH",
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"works" : 112,
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"yields" : 1,
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"unyields" : 1,
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"invalidates" : 0,
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"advanced" : 1,
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"needTime" : 110,
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"needFetch" : 0,
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"isEOF" : 1,
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"alreadyHasObj" : 0,
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"forcedFetches" : 0,
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"matchTested" : 1,
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"children" : [
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{
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"type" : "IXSCAN",
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"works" : 111,
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"yields" : 1,
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"unyields" : 1,
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"invalidates" : 0,
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"advanced" : 111,
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"needTime" : 0,
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"needFetch" : 0,
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"isEOF" : 1,
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"keyPattern" : "{ type: 1.0 }",
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"boundsVerbose" : "field #0['type']: [\"folder\", \"folder\"]",
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"isMultiKey" : 0,
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"yieldMovedCursor" : 0,
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"dupsTested" : 0,
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"dupsDropped" : 0,
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"seenInvalidated" : 0,
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"matchTested" : 0,
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"keysExamined" : 111,
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"children" : [ ]
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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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This happens because the plans tie: both return the same number of documents in the same amount of work cycles, even though one plan has to fetch more documents.
Attachments
Issue Links
- is duplicated by
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- Closed
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SERVER-79400 Implement number of documents tie breaking heuristics
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- Closed
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- is related to
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- Closed
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- Closed
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SERVER-37194 Number of docsExamined is not considered for index selection
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- Closed
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- links to