Indexes can become a problem in many database applications that involve a high proportion of INSERT/DELETE commands. Just as tables can become bloated, so can indexes.
In the previous recipe, you saw that non-HOT updates can cause bloated indexes.
Non-primary-key indexes are also prone to some bloat from normal INSERT commands, as is common in most relational databases.
Autovacuum does not detect bloated indexes, nor does it do anything to rebuild indexes. So we need to look at other ways to maintain indexes.