We run Postgres for a product with millions of users. Along the way I've broken it in every possible way. Here are the lessons I wish I'd known on day one.
Autovacuum is not optional
You can ignore autovacuum for a while. You cannot ignore it forever. Dead tuples accumulate. Query plans go bad. Eventually a query that used to take 10ms takes 3 seconds and nobody knows why.
Tune autovacuum earlier than you think. autovacuum_vacuum_scale_factor = 0.05 on big tables is a good default.
Connection pooling is not optional
