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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: Querying
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None
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Query Optimization
In the find() layer, we attempt to push projects before sorts, in the case where the project makes the document smaller. See here. There is no equivalent optimization in the agg layer.
So, running:
db.c.explain().aggregate([{$sort: {a:1}}, {$project: {a: 1}}])
We use the following plan (PROJECT then SORT)
"winningPlan" : {
"stage" : "SORT",
"sortPattern" : {
"a" : 1
},
"memLimit" : 104857600,
"type" : "simple",
"inputStage" : {
"stage" : "PROJECTION_SIMPLE",
"transformBy" : {
"_id" : true,
"a" : true
},
"inputStage" : {
"stage" : "COLLSCAN",
"direction" : "forward"
}
}
},
On the other hand, this query (same as above, but only using the agg layer):
db.c.explain().aggregate([{$_internalInhibitOptimization: {}}, {$sort: {a:1}}, {$project: {a: 1}}])
Will use the following plan (SORT then PROJECT):
"stages" : [
{
"$cursor" : {
"queryPlanner" : {
"namespace" : "test.c",
"indexFilterSet" : false,
"parsedQuery" : {
},
"queryHash" : "8B3D4AB8",
"planCacheKey" : "8B3D4AB8",
"maxIndexedOrSolutionsReached" : false,
"maxIndexedAndSolutionsReached" : false,
"maxScansToExplodeReached" : false,
"winningPlan" : {
"stage" : "COLLSCAN",
"direction" : "forward"
},
"rejectedPlans" : [ ]
}
}
},
{
"$_internalInhibitOptimization" : {
}
},
{
"$sort" : {
"sortKey" : {
"a" : 1
}
}
},
{
"$project" : {
"_id" : true,
"a" : true
}
}
]
- is related to
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SERVER-54128 Computed projection should not be pushed down past sort+limit
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- Closed
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- related to
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- Open
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