If I use pgbouncer, what are the reasons I should not just set Active Record's pool size to 99999, effectively disabling it, and leaving pgbouncer in charge of all pooling?
In my case, this is with Rails 5.2. pgbouncer uses transaction pooling.
I can think of some possible reasons:
If a runaway process somehow tries to open a high number of threads/connections, the AR pool would set a ceiling, preventing it from exhausting all connections.
Similarly, if AR doesn't close connections to pgbouncer correctly (e.g. if some code opens connections in threads without closing them), and AR's reaper does not run or does not run often enough, that code could exhaust all connections.
If Active Record itself has costly overhead per connection (does it?), perhaps it's preferable to wait and reuse connections instead of opening a higher number of connections, in situations where the same process tries to open a lot of connections.
Are those valid reasons? Are they the only reasons?
(I've seen Disabling Connection Pooling in Rails to use PgBouncer and think this is related but not quite the same question.)
