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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: None
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Query Optimization
This would work similarly to distinct('x'), but for each distinct x value discovered, it should then seek to the first entry with that x and a y that would match the query. If you land in a matching x and y, then record the value and jump. Either way, after that jump to the next possible x and repeat. This should do roughly 2x(number of distinct x values) seeks into the index, and no record fetches.
Right now it just does a table scan which is likely to be much more expensive:
> db.foo.createIndex({x:1, y:1})
{
"numIndexesBefore" : 2,
"numIndexesAfter" : 3,
"createdCollectionAutomatically" : false,
"ok" : 1
}
> db.foo.explain().distinct('x', {y:1})
{
"explainVersion" : "1",
"queryPlanner" : {
"namespace" : "test.foo",
"parsedQuery" : {
"y" : {
"$eq" : 1
}
},
"indexFilterSet" : false,
"queryHash" : "3F4E3077",
"planCacheKey" : "75804815",
"optimizationTimeMillis" : 0,
"maxIndexedOrSolutionsReached" : false,
"maxIndexedAndSolutionsReached" : false,
"maxScansToExplodeReached" : false,
"prunedSimilarIndexes" : false,
"winningPlan" : {
"isCached" : false,
"stage" : "COLLSCAN",
"filter" : {
"y" : {
"$eq" : 1
}
},
"direction" : "forward"
},
"rejectedPlans" : [ ]
},
"command" : {
"distinct" : "foo",
"key" : "x",
"query" : {
"y" : 1
},
"$db" : "test"
},
"serverInfo" : {
"host" : "ws",
"port" : 27017,
"version" : "8.1.0-alpha-1988-gb141e74",
"gitVersion" : "b141e743215ced3bfea98f2353355b9f3a982369"
},
"serverParameters" : {
"internalQueryFacetBufferSizeBytes" : 104857600,
"internalQueryFacetMaxOutputDocSizeBytes" : 104857600,
"internalLookupStageIntermediateDocumentMaxSizeBytes" : 104857600,
"internalDocumentSourceGroupMaxMemoryBytes" : 104857600,
"internalQueryMaxBlockingSortMemoryUsageBytes" : 104857600,
"internalQueryProhibitBlockingMergeOnMongoS" : 0,
"internalQueryMaxAddToSetBytes" : 104857600,
"internalDocumentSourceSetWindowFieldsMaxMemoryBytes" : 104857600,
"internalQueryFrameworkControl" : "trySbeRestricted",
"internalQueryPlannerIgnoreIndexWithCollationForRegex" : 1
},
"ok" : 1
}