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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: None
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Component/s: Performance, Querying
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None
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Query Optimization
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Right now I don't think we are getting any advantage from using the index for $exists:true queries since we are always going to the row store. In fact, it is significantly slower because we are doing a bunch of point queries into the row store rather than using a (relatively) efficient collection scan. However, for covering indexes, we only need to go to the row store when the index says null to disambiguate missing fields from explicit null storage. For non-covering indexes we should probably just always use a collection scan because we need to go to the row store regardless of whether there is a match or not:
> db.products.explain('executionStats').find({type: {$exists:true}}, {_id:0, type: 1})
{
"explainVersion" : "1",
"queryPlanner" : {
"namespace" : "bestbuy.products",
"indexFilterSet" : false,
"parsedQuery" : {
"type" : {
"$exists" : true
}
},
"maxIndexedOrSolutionsReached" : false,
"maxIndexedAndSolutionsReached" : false,
"maxScansToExplodeReached" : false,
"winningPlan" : {
"stage" : "PROJECTION_SIMPLE",
"transformBy" : {
"_id" : 0,
"type" : 1
},
"inputStage" : {
"stage" : "FETCH",
"filter" : {
"type" : {
"$exists" : true
}
},
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"type" : 1,
"customerReviewCount" : -1
},
"indexName" : "type_1_customerReviewCount_-1",
"isMultiKey" : false,
"multiKeyPaths" : {
"type" : [ ],
"customerReviewCount" : [ ]
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {
"type" : [
"[MinKey, MaxKey]"
],
"customerReviewCount" : [
"[MaxKey, MinKey]"
]
}
}
}
},
"rejectedPlans" : [
{
"stage" : "PROJECTION_SIMPLE",
"transformBy" : {
"_id" : 0,
"type" : 1
},
"inputStage" : {
"stage" : "FETCH",
"filter" : {
"type" : {
"$exists" : true
}
},
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"type" : 1,
"name" : 1
},
"indexName" : "type_1_name_1",
"isMultiKey" : false,
"multiKeyPaths" : {
"type" : [ ],
"name" : [ ]
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {
"type" : [
"[MinKey, MaxKey]"
],
"name" : [
"[MinKey, MaxKey]"
]
}
}
}
},
{
"stage" : "PROJECTION_SIMPLE",
"transformBy" : {
"_id" : 0,
"type" : 1
},
"inputStage" : {
"stage" : "FETCH",
"filter" : {
"type" : {
"$exists" : true
}
},
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"type" : 1,
"subclass" : 1,
"genre" : 1,
"format" : 1
},
"indexName" : "type_1_subclass_1_genre_1_format_1",
"isMultiKey" : false,
"multiKeyPaths" : {
"type" : [ ],
"subclass" : [ ],
"genre" : [ ],
"format" : [ ]
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {
"type" : [
"[MinKey, MaxKey]"
],
"subclass" : [
"[MinKey, MaxKey]"
],
"genre" : [
"[MinKey, MaxKey]"
],
"format" : [
"[MinKey, MaxKey]"
]
}
}
}
},
{
"stage" : "PROJECTION_SIMPLE",
"transformBy" : {
"_id" : 0,
"type" : 1
},
"inputStage" : {
"stage" : "FETCH",
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"$_path" : 1,
"type" : 1
},
"indexName" : "$**_1",
"isMultiKey" : false,
"multiKeyPaths" : {
"$_path" : [ ],
"type" : [ ]
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {
"$_path" : [
"[\"type\", \"type\"]",
"[\"type.\", \"type/\")"
],
"type" : [
"[MinKey, MaxKey]"
]
}
}
}
}
]
},
"executionStats" : {
"executionSuccess" : true,
"nReturned" : 1901574,
"executionTimeMillis" : 3035,
"totalKeysExamined" : 1901574,
"totalDocsExamined" : 1901574,
"executionStages" : {
"stage" : "PROJECTION_SIMPLE",
"nReturned" : 1901574,
"executionTimeMillisEstimate" : 460,
"works" : 1901575,
"advanced" : 1901574,
"needTime" : 0,
"needYield" : 0,
"saveState" : 1901,
"restoreState" : 1901,
"isEOF" : 1,
"transformBy" : {
"_id" : 0,
"type" : 1
},
"inputStage" : {
"stage" : "FETCH",
"filter" : {
"type" : {
"$exists" : true
}
},
"nReturned" : 1901574,
"executionTimeMillisEstimate" : 403,
"works" : 1901575,
"advanced" : 1901574,
"needTime" : 0,
"needYield" : 0,
"saveState" : 1901,
"restoreState" : 1901,
"isEOF" : 1,
"docsExamined" : 1901574,
"alreadyHasObj" : 0,
"inputStage" : {
"stage" : "IXSCAN",
"nReturned" : 1901574,
"executionTimeMillisEstimate" : 141,
"works" : 1901575,
"advanced" : 1901574,
"needTime" : 0,
"needYield" : 0,
"saveState" : 1901,
"restoreState" : 1901,
"isEOF" : 1,
"keyPattern" : {
"type" : 1,
"customerReviewCount" : -1
},
"indexName" : "type_1_customerReviewCount_-1",
"isMultiKey" : false,
"multiKeyPaths" : {
"type" : [ ],
"customerReviewCount" : [ ]
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {
"type" : [
"[MinKey, MaxKey]"
],
"customerReviewCount" : [
"[MaxKey, MinKey]"
]
},
"keysExamined" : 1901574,
"seeks" : 1,
"dupsTested" : 0,
"dupsDropped" : 0
}
}
}
},
"command" : {
"find" : "products",
"filter" : {
"type" : {
"$exists" : true
}
},
"projection" : {
"_id" : 0,
"type" : 1
},
"$db" : "bestbuy"
},
"serverInfo" : {
"host" : "ip-10-122-10-16",
"port" : 27017,
"version" : "4.9.0-alpha4-13-gbed3256",
"gitVersion" : "bed32560b4ef8df1eb6635c6d756119ab0e685a4"
},
"ok" : 1
}
Compare that to forcing a table scan:
> db.products.explain('executionStats').find({type: {$exists:true}}, {_id:0, type: 1}).hint({$natural:1})
{
"explainVersion" : "1",
"queryPlanner" : {
"namespace" : "bestbuy.products",
"indexFilterSet" : false,
"parsedQuery" : {
"type" : {
"$exists" : true
}
},
"maxIndexedOrSolutionsReached" : false,
"maxIndexedAndSolutionsReached" : false,
"maxScansToExplodeReached" : false,
"winningPlan" : {
"stage" : "PROJECTION_SIMPLE",
"transformBy" : {
"_id" : 0,
"type" : 1
},
"inputStage" : {
"stage" : "COLLSCAN",
"filter" : {
"type" : {
"$exists" : true
}
},
"direction" : "forward"
}
},
"rejectedPlans" : [ ]
},
"executionStats" : {
"executionSuccess" : true,
"nReturned" : 1901574,
"executionTimeMillis" : 1454,
"totalKeysExamined" : 0,
"totalDocsExamined" : 1901574,
"executionStages" : {
"stage" : "PROJECTION_SIMPLE",
"nReturned" : 1901574,
"executionTimeMillisEstimate" : 135,
"works" : 1901576,
"advanced" : 1901574,
"needTime" : 1,
"needYield" : 0,
"saveState" : 1901,
"restoreState" : 1901,
"isEOF" : 1,
"transformBy" : {
"_id" : 0,
"type" : 1
},
"inputStage" : {
"stage" : "COLLSCAN",
"filter" : {
"type" : {
"$exists" : true
}
},
"nReturned" : 1901574,
"executionTimeMillisEstimate" : 101,
"works" : 1901576,
"advanced" : 1901574,
"needTime" : 1,
"needYield" : 0,
"saveState" : 1901,
"restoreState" : 1901,
"isEOF" : 1,
"direction" : "forward",
"docsExamined" : 1901574
}
}
},
"command" : {
"find" : "products",
"filter" : {
"type" : {
"$exists" : true
}
},
"projection" : {
"_id" : 0,
"type" : 1
},
"hint" : {
"$natural" : 1
},
"$db" : "bestbuy"
},
"serverInfo" : {
"host" : "ip-10-122-10-16",
"port" : 27017,
"version" : "4.9.0-alpha4-13-gbed3256",
"gitVersion" : "bed32560b4ef8df1eb6635c6d756119ab0e685a4"
},
"ok" : 1
}
It takes 1.5s whereas using the index+fetch is 3s.
- is related to
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SERVER-67378 Index null values and missing values differently
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- Closed
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- Backlog
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