The pg_loose_count extension looks at the table statistics in the PostgreSQL system tables to get a fast approximate count of the number of rows in a given table:
DB.loose_count(:table) # => 123456
It can also support schema qualified tables:
DB.loose_count(Sequel[:schema][:table]) # => 123456
How accurate this count is depends on the number of rows added/deleted from the table since the last time it was analyzed.
To load the extension into the database: