ISSUE DESCRIPTION AND IMPACT

Users who upgraded from MongoDB 4.0 to 4.2+ (featureCompatibility 4.2+) may experience index inconsistencies within unique partial indexes (unique indexes which specify a partialFilterExpression), in the form of missing index keys. Unique primary (_id) indexes are not affected.

Affected versions of MongoDB incorrectly remove index entries from the unique partial index when all of the following take place:

• prior to upgrading to FCV 4.2+, a document has a key in a unique partial index because it matches the index's partialFilterExpression.
• prior to upgrading to FCV 4.2+, another document with a matching unique field value exists, but is not contained in the unique partial index (because it does not match the index's partialFilterExpression).
• The un-indexed document is deleted after the upgrade to FCV 4.2+.

When the un-indexed document is deleted, affected versions of MongoDB incorrectly delete the key for the indexed document.

Missing index entries in unique partial index in v4.2+ can have the following effects:

• Queries using the affected index may return incomplete results.
• MongoDB will incorrectly allow the insertion of a new document that matches the partialFilterExpression, even though the insert should fail with a duplicate key error. As a result, queries that do not use an affected unique index may return documents with duplicate values that should not have been allowed.
• Attempts to drop and rebuild the unique partial index may fail due to duplicate keys existing in the collection.

DIAGNOSIS AND AFFECTED VERSIONS

Affected Versions: 4.2.0, 4.4.0, 5.0.0, 6.0.0, 6.3.0-rc3
Fixed Versions: 4.4.23, 5.0.19, 6.0.7, 7.0.0-rc3, 7.1.0-rc0

Users who upgraded to v4.2 and are still running in FCV 4.0 are not impacted. Even after upgrading to a fixed version, users can still be impacted from missing index entries and documents with duplicate index keys.

In FCV 4.2+, users who have collections that rely on unique indexes with partialFilterExpression may be impacted by this bug. Users can verify if a unique partial index is set by calling `getIndexes` on all the collections. Missing unique index entries can be checked by calling validate.

REMEDIATION AND WORKAROUNDS

User action is required in order to remediate this issue. Impacted users should follow the below steps:

1. Upgrade to a fixed version.
2. Check for “missingIndexEntries” by running the validate() command.
   • If there are no missing index entries, no more remediation measures are needed.
   • If there are missing index entries, drop and rebuild the index

Rebuilding the index may fail due to existing documents with duplicate keys in the collection.

1. Build a new valid index.
2. Query using that index to find the duplicate values in the collection
   Example:

   //bad index
   db.coll.createIndex({ “user” : 1 }, { “partialFilterExpression” : { “active” : true }, “unique” : true })
   
   //new index
   db.coll.createIndex({ “user” : 1, “foo” : 1 }, { “partialFilterExpression” : { “active” : true } })
   
   > db.coll.aggregate(
   ... [
   ...     {
   ...         $group: {
   ...                 _id: oldindex, 	//{"user": "x"}
   ...                 count: { $sum: 1 },
   ...             },
   ...     },
   ...     {
   ...         $match: {
   ...             count: { $gt: 1 }
   ...         }
   ...     },
   ... ],
   ... {hint: (newindex)}  			//"user_1_foo_1"
   ... )
   
   Returns:
   { "_id" : { "user" : "x" }, "count" : 2 }
   
   > db.coll.find({ "user" : "x" })
   { "_id" : 2, "user" : "x", "active" : true }
   { "_id" : 3, "user" : "x", "active" : true }
   

1. The duplicate documents may be removed based on the application logic.
2. Once the user action is taken for the affected documents, drop and recreate the unique index for all collections