[SERVER-12775] explain() performance degraded when all potential plans added to results Created: 19/Feb/14  Updated: 29/Jan/15  Resolved: 29/Jan/15

Status: Closed
Project: Core Server
Component/s: Concurrency, Querying
Affects Version/s: 2.5.5
Fix Version/s: None

Type: Bug Priority: Major - P3
Reporter: Davide Italiano Assignee: David Storch
Resolution: Incomplete Votes: 0
Labels: 26qa, Performance
Remaining Estimate: Not Specified
Time Spent: Not Specified
Original Estimate: Not Specified

Attachments: PNG File chart_2.png    
Issue Links:
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Operating System: ALL
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 Description   

In a workload where explain() is called a lot I've lately seen a certain regression in throughput.
I was able to isolate the commit that caused the problem.
https://github.com/mongodb/mongo/commit/6dd40e54de4d62ed1c649f6f620a574e6de74fa8

The throughput looks like 20% less and I'm completely able to recover if I revert to the previous revision. See screenshot attached.



 Comments   
Comment by David Storch [ 29/Jan/15 ]

This refers to performance of explain in 2.6.x. Closing as incomplete for two reasons:

  1. explain doesn't have strict performance requirements like regular reads and writes do, and
  2. explain has been rewritten for 3.0.x under SERVER-10448, so this ticket refers to old code.
Comment by Daniel Pasette (Inactive) [ 19/Feb/14 ]

I don't think explain performance is something we're worried about, but the offending commit causes us to do more work in explain, so it's not unexpected.

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