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Type:
Bug
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Resolution: Unresolved
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Priority:
Minor - P4
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None
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Affects Version/s: None
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Component/s: None
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Labels:None
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Query Optimization
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ALL
For example, running
db.foo.explain().aggregate([{$redact: "$$KEEP"}, {$match: {a: "A"}}]).stages
Gives the following output:
[
{
"$cursor" : {
"queryPlanner" : {
"namespace" : "test.foo",
"indexFilterSet" : false,
"parsedQuery" : {
"a" : {
"$eq" : "A"
}
},
"queryHash" : "4B53BE76",
"planCacheKey" : "4B53BE76",
"maxIndexedOrSolutionsReached" : false,
"maxIndexedAndSolutionsReached" : false,
"maxScansToExplodeReached" : false,
"winningPlan" : {
"stage" : "COLLSCAN",
"filter" : {
"a" : {
"$eq" : "A"
}
},
"direction" : "forward"
},
"rejectedPlans" : [ ]
}
}
},
{
"$match" : {
"a" : {
"$eq" : "A"
}
}
},
{
"$redact" : "$$KEEP"
},
{
"$match" : {
"a" : {
"$eq" : "A"
}
}
}
]
where the $match after $cursor and before $redact is unexpected.
This is happening because Pipeline::optimizePipeline() is run more than once for each pipeline, and DocumentSourceRedact::doOptimizeAt() adds a $match to the pipeline each time. See here.