Understanding bitmap indexes in postgresql

The bitmap of pages is created dynamically for each query. It is not cached or re-used, and is discarded at the end of the bitmap index scan. It doesn't make sense to create the page bitmap in advance because its contents depend on the query predicates. Say you're searching for x=1 and y=2. You have b-tree indexes on x and y. PostgreSQL doesn't combine x and y into a bitmap then search the bitmap.