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Type: Task
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Resolution: Fixed
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Priority: Major - P3
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Affects Version/s: None
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Labels:None
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.)
- documents
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SERVER-38063 executionStats or allPlansExecution level explain of an aggregate with a $out does writes
- Closed