ISSUE DESCRIPTION AND IMPACT

On MongoDB Server version 8.0 - 8.0.4, it is possible under certain circumstances for a simple equality (non-_id) predicate query on sharded collections to report no results when a valid result exists.

The bug can arise when all of the following are true:
The query is executed on a sharded cluster with two or more shards running MongoDB versions 8.0 - 8.0.4
The query is of the form:
find({'fieldA':'value'}).limit(1) or
aggregate([{$match: {'fieldA':'value'}},{$limit:1}])
(where 'fieldA' is not the _id field)
The sharded collection has a compound unique index that includes the field used in the query, for example, {key:{'fieldA':1,'fieldB':1}, unique:true}. And it doesn't have a unique index on just that one field, like {key:{'fieldA': 1}, unique:true }.
While executing the query described above, the query engine may improperly apply an optimization. In certain circumstances involving the placement of orphaned documents, this may result in no documents being returned instead of one.
DIAGNOSIS AND AFFECTED VERSIONS

This bug affects users in 8.0 - 8.0.4.

REMEDIATION AND WORKAROUNDS

Customers are recommended to upgrade to 8.0.6. For MongoDB Atlas Customers, your Atlas 8.0 clusters will be upgraded automatically to the version containing the fix.

Workaround exists without upgrading the cluster:
Add a hint() to the affected query, which causes the fault path to be bypassed.
Alternatively the user can set the query knob internalQueryDisableSingleFieldExpressExecutor to true to turn off the fault path for all non-_id queries.

OTHER AFFECTED TOOLS

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