There are a few calls to KeyString::getKeySize() in WiredTigerIndex such as here to determine if the KeyString has a RecordId appended. We should avoid calling this method when possible because it reconstructs the entire KeyString which is an expensive operation.

There are a couple known cases when we know whether the RecordId is appended to the KeyString or not.

• _id indexes: these never have the RecordId appended.
• Non-unique indexes: they always have the RecordId appended.