Details
-
Task
-
Resolution: Fixed
-
Major - P3
-
None
-
None
Description
Description
Description:
We should document that explain of an $out pipeline with "executionStats" or "allPlansExecution" is disallowed in 4.2. This was not enforced in 3.6 or 4.0, the two existing stable versions that support execution-level explain of aggregate (see SERVER-19758).
If users want explain information from an $out pipeline, they have two options:
1) Use the 'queryPlanner' explain verbosity. This will not report execution stats.
2) Run at 'executionStats' or 'allPlansExecution' verbosity, but omit the $out stage from the end of the pipeline. This will display execution stats for the pipeline that feeds the $out stage. Further work is required for $out to supply execution stats itself, without actually performing any writes.
Engineering Ticket Description:
We allow explaining write operations such as the update and delete commands. However, these operations are never supposed to perform writes. An explain of an $out aggregation stage should behave similarly. However, an "executionStats" or "allPlansExecution" verbosity explain of an $out will actually persist the results to the target collection:
> db.inputColl.drop()
|
false
|
> db.inputColl.insert({_id: 1})
|
WriteResult({ "nInserted" : 1 })
|
> db.inputColl.explain("executionStats").aggregate([{$out: "outputColl"}])
|
{
|
"stages" : [
|
{
|
"$cursor" : {
|
"query" : {
|
|
|
},
|
"queryPlanner" : {
|
"plannerVersion" : 1,
|
"namespace" : "test.inputColl",
|
"indexFilterSet" : false,
|
"parsedQuery" : {
|
|
|
},
|
"queryHash" : "8B3D4AB8",
|
"planCacheKey" : "8B3D4AB8",
|
"winningPlan" : {
|
"stage" : "COLLSCAN",
|
"direction" : "forward"
|
},
|
"rejectedPlans" : [ ]
|
},
|
"executionStats" : {
|
"executionSuccess" : true,
|
"nReturned" : 1,
|
"executionTimeMillis" : 59,
|
"totalKeysExamined" : 0,
|
"totalDocsExamined" : 1,
|
"executionStages" : {
|
"stage" : "COLLSCAN",
|
"nReturned" : 1,
|
"executionTimeMillisEstimate" : 0,
|
"works" : 3,
|
"advanced" : 1,
|
"needTime" : 1,
|
"needYield" : 0,
|
"saveState" : 2,
|
"restoreState" : 2,
|
"isEOF" : 1,
|
"direction" : "forward",
|
"docsExamined" : 1
|
}
|
}
|
}
|
},
|
{
|
"$out" : {
|
"to" : "outputColl",
|
"db" : "test",
|
"mode" : "replaceCollection",
|
"uniqueKey" : {
|
"_id" : 1
|
}
|
}
|
}
|
],
|
"ok" : 1
|
}
|
> db.outputColl.find()
|
{ "_id" : 1 }
|
This behavior was most likely introduced by the fixes to executionStats agg explain under SERVER-29421.
Instead, we should probably raise an error if a user attempts to perform this operation with an error message such as "$out cannot be explain with 'executionStats' level verbosity or higher". Users who wish to explain an $out operation with execution stats can simply drop the $out stage from the tail of their pipeline.
Scope of changes
includes:
- the 4.2 compatibility notes, as it is a minor breaking change.
- source/reference/command/explain.txt
- source/reference/method/cursor.explain.txt
- source/reference/method/db.collection.aggregate.txt
- source/reference/method/db.collection.explain.txt
Impact to Other Docs
MVP (Work and Date)
Resources (Scope or Design Docs, Invision, etc.)
Attachments
Issue Links
- documents
-
SERVER-38063 executionStats or allPlansExecution level explain of an aggregate with a $out does writes
-
- Closed
-