Question

Let me explain what information I need for this:

I have several concurrent users hitting the same record at once. This means, there will be some queuing and locking going on, on the db end. How big is this buffer? Can queue and locking hold for 200+ concurrent users?

How can I determine the size of this buffer in our setup? Is there a default setting?

Answer 1

There is no query queue (“buffer”) in the database.

Each concurrent connection to the database can have one query in flight. Other queries cannot be queued up behind it^*.

Your application probably uses an internal connection pool, being Rails, so you can have however many queries waiting as you have slots in the connection pool.

If you have an external connection pool like PgBouncer proxying between your app and PostgreSQL then you can have more queries queued because you can have a much larger pool size in the app when connecting to pgbouncer as pgbouncer connections are so lightweight. PgBouncer will service those requests on a smaller number of real connections to PostgreSQL. That effectively makes PgBouncer a query queue (though not necessarily a FIFO queue) when used this way. HOWEVER because those queries don’t actually hit Pg when they’re issued they don’t take locks while waiting in PgBouncer. This could be important for some concurrency designs.

---

^* OK, so you can send multiple semicolon separated queries at once, but not in series like a queue.