Ref T13587. Currently, when a document is reindexed by Ferret, the old document is completely discarded and a new version is inserted to replace it.
This approach is simple to implement, but can lead to exhaustion of the ngram AUTO_INCREMENT id column in reasonable circumstances.
Conceptually, this approach "should" be fine and this exhaustion is an awkard implementation detail. However, since it's easy to be less wasteful when performing document updates and all the other approaches are awkward or leaky in other ways that are probably worse, use a more complex implementation to avoid executing unnecessary INSERT statements.