[DOCS-15522] [SERVER] Clarify contraints for uniqueness of `_id` for sharded collections Created: 04/Aug/22  Updated: 22/Jan/24

Status: Backlog
Project: Documentation
Component/s: manual, Server
Affects Version/s: 4.2.0, 4.4.0, 5.0.0, 6.0.0
Fix Version/s: None

Type: Task Priority: Major - P3
Reporter: Pierlauro Sciarelli Assignee: Unassigned
Resolution: Unresolved Votes: 1
Labels: backlog, proactive, sharding
Remaining Estimate: Not Specified
Time Spent: Not Specified
Original Estimate: Not Specified

Participants:
Days since reply: 1 year, 26 weeks, 6 days ago
Epic Link: DOCSP-11702

 Description   

The documentation states the following, that can be confusing:

the `_id` index only enforces the uniqueness constraint per shard if the `_id` field is not the shard key or the prefix of the shard key.

I propose to change this paragraph with the following (or similar phrasing):

The `_id` index guarantees uniqueness across all shards only when the shard key is exactly `_id` . In any other case - even when the `_id` field is part of a compound shard key - uniqueness is only guaranteed on a per-shard basis.


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