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Type:
Question
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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: Aggregation Framework
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Labels:None
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Query Optimization
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Query Execution 2021-03-08, Query Execution 2021-03-22
We currently reject {$limit: 0}:
> db.foo.explain().aggregate({$limit:0})
uncaught exception: Error: command failed: {
"ok" : 0,
"errmsg" : "the limit must be positive",
"code" : 15958,
"codeName" : "Location15958"
} with original command request: {
"aggregate" : "foo",
"pipeline" : [
{
"$limit" : 0
}
],
"explain" : true,
"cursor" : {
},
"lsid" : {
"id" : UUID("ffba48f6-ae33-4784-9a87-28962126ebdb")
}
}
But we allow it to happen after optimizations:
> db.foo.explain().aggregate({$limit:2}, {$skip:3})
{
"explainVersion" : "1",
"queryPlanner" : {
"namespace" : "blah.foo",
"indexFilterSet" : false,
"parsedQuery" : {
},
"queryHash" : "8B3D4AB8",
"planCacheKey" : "8B3D4AB8",
"optimizedPipeline" : true,
"maxIndexedOrSolutionsReached" : false,
"maxIndexedAndSolutionsReached" : false,
"maxScansToExplodeReached" : false,
"winningPlan" : {
"stage" : "LIMIT",
"limitAmount" : 0,
"inputStage" : {
"stage" : "SKIP",
"skipAmount" : 2,
"inputStage" : {
"stage" : "COLLSCAN",
"direction" : "forward"
}
}
},
"rejectedPlans" : [ ]
},
"command" : {
"aggregate" : "foo",
"pipeline" : [
{
"$limit" : 2
},
{
"$skip" : 3
}
],
"explain" : true,
"cursor" : {
},
"lsid" : {
"id" : UUID("ffba48f6-ae33-4784-9a87-28962126ebdb")
},
"$db" : "blah"
},
"serverInfo" : {
"host" : "ip-10-122-10-16",
"port" : 27017,
"version" : "4.9.0-alpha4-13-gbed3256",
"gitVersion" : "bed32560b4ef8df1eb6635c6d756119ab0e685a4"
},
"ok" : 1
}
This seems inconsistent. Either we should reject everything like this since it is likely to indicate user error, or we should allow both.
I haven't tested on a sharded system, but I could imagine the current behavior causing an issue there since we would serialize a {$limit:0} stage after optimization on mongos.