The number of purges on a database is tracked using a purge sequence. This is used by the view indexer to optimize the updating of views that contain the purged documents.

When the indexer identifies that the purge sequence on a database has changed, it compares the purge sequence of the database with that stored in the view index. If the difference between the stored sequence and database is sequence is only 1, then the indexer uses a cached list of the most recently purged documents, and then removes these documents from the index individually. This prevents completely rebuilding the index from scratch.

If the difference between the stored sequence number and current database sequence is greater than 1, then the view index is entirely rebuilt. This is an expensive operation as every document in the database must be examined.