[SERVER-42854] $avg with Infinity can incorrectly return NaN Created: 16/Aug/19  Updated: 06/Dec/22  Resolved: 19/Oct/21

Status: Closed
Project: Core Server
Component/s: None
Affects Version/s: None
Fix Version/s: None

Type: Bug Priority: Major - P3
Reporter: Nicholas Zolnierz Assignee: Backlog - Query Optimization
Resolution: Duplicate Votes: 0
Labels: query-44-grooming
Remaining Estimate: Not Specified
Time Spent: Not Specified
Original Estimate: Not Specified

Issue Links:
Depends
Duplicate
duplicates SERVER-37429 $avg of Infinity and null is NaN on d... Backlog
Assigned Teams:
Query Optimization
Operating System: ALL
Steps To Reproduce:

*On a debug build, to ensure we always spill to disk*

db.test.insert({_id: 0, value: Infinity});
db.test.insert({_id: 0, value: 5});
db.test.aggregate([{ $group: {
                _id: null,
                'num': { 
                    $avg: '$value'
                }
} }]);

Participants:
Linked BF Score: 50

 Description   

When adding Infinity to any value using the DoubleDoubleSummation class, the _addend double is calculated to be NaN. This is probably fine, however getting the result of the summation here results in a value of Infinity with an error of NaN.

If this happens when the $avg accumulator spills to disk, we will attempt to process the NaN error as if its a finite value and as a result the accumulator evaluates to NaN.



 Comments   
Comment by David Percy [ 19/Oct/21 ]

Looks like the same bug as SERVER-37429.

Comment by Geert Bosch [ 03/Sep/19 ]

nicholas.zolnierz, your fix looks correct to me. Indeed the idea behind the _special member was to correctly handle all the non-finite corner cases by doing the same that a simple (non-double-double) summation would return if that more accurate mechanism results in special (non-finite) values in any way.

Comment by Charlie Swanson [ 22/Aug/19 ]

Putting this on the backlog because our understanding is that we will soon disable testing of debug vs. non-debug builds and this will stop being a problem in the build.

It is still a bug though, so leaving it open.

cc robert.guo

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