[SERVER-78878] Avoid adding a project before a group when there are too many group dependencies. Created: 11/Jul/23  Updated: 27/Nov/23  Resolved: 27/Nov/23

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

Type: Improvement Priority: Major - P3
Reporter: Mickey Winters Assignee: Carlos Alonso Pérez
Resolution: Won't Do Votes: 0
Labels: None
Remaining Estimate: Not Specified
Time Spent: Not Specified
Original Estimate: Not Specified

Assigned Teams:
Query Optimization
Sprint: QO 2023-11-27, QO 2023-12-11
Participants:

 Description   

in certain cases when in the classic engine the rewrite that adds a project before a group is more expensive to evaluate then it is worth. perhaps we should not apply this optimization when we have more than N fields to project before the group. We noticed this when comparing 4.2 to 4.4



 Comments   
Comment by Mickey Winters [ 27/Nov/23 ]

When I originally opened this I thought the heuristic of if there are more than X fields in the group key then it is probably projecting the whole document. but even with the case of the customer that prompted me to open this there could be a situation like 400/700 fields projected so I doubt there is a safe value of X now. Realistically we might need statistics to figure out what X is, but bonsai is cost based and is already planned to determine if a project stage is actually worth it in this situation so no extra work is needed.

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