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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3.4.4
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None
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None
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Query
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ALL
Description
Performing a count on a collection with a partial index leads to IXSCAN/FETCH/COUNT, rather than the more efficient COUNT_SCAN. By contrast, a count on a regular index does lead to a COUNT_SCAN.
For example:
db.coll.createIndex({"value":1}, {"partialFilterExpression":{"flag":true}})
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db.coll.explain(true).count({"flag":true,"value":10})
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...
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"winningPlan" : {
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"stage" : "COUNT",
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"inputStage" : {
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"stage" : "FETCH",
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"filter" : {
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"flag" : {
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"$eq" : true
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}
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},
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"inputStage" : {
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"stage" : "IXSCAN",
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"keyPattern" : {
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"value" : 1
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},
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"indexName" : "value_1",
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"isMultiKey" : false,
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"multiKeyPaths" : {
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"value" : [ ]
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},
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"isUnique" : false,
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"isSparse" : false,
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"isPartial" : true,
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"indexVersion" : 2,
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"direction" : "forward",
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"indexBounds" : {
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"value" : [
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"[10.0, 10.0]"
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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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Here is the same example with a compound index on (flag, value):
db.coll.createIndex({"flag":1, "value":1})
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db.coll.explain(true).count({"flag":true,"value":10})
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...
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"winningPlan" : {
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"stage" : "COUNT",
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"inputStage" : {
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"stage" : "COUNT_SCAN",
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"keyPattern" : {
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"flag" : 1,
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"value" : 1
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},
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"indexName" : "flag_1_value_1",
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"isMultiKey" : false,
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"multiKeyPaths" : {
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"flag" : [ ],
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"value" : [ ]
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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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"indexBounds" : {
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"startKey" : {
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"flag" : true,
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"value" : 10
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},
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"startKeyInclusive" : true,
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"endKey" : {
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"flag" : true,
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"value" : 10
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},
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"endKeyInclusive" : true
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}
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}
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},
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This issue is related to, but distinct from, other cases where partial indexes are not utilized to the full extent, such as SERVER-23808, SERVER-28889.
Attachments
Issue Links
- duplicates
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SERVER-28889 Partial index shouldn't do fetch on conditions that are true by the definition of the index
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- Open
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- is related to
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SERVER-23808 Implement Index Choice based on Partial Filter Expression
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- Backlog
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- Open
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