Is each connection backed by one Thread in Database Connection Pooling?

Question

In Database connection Pooling,

• does each connection correspond to 1 thread in the database ?
• Would it really matter ? I mean, there could be a threadPool executor, that execute whatever a connection Object has to execute ?

I wonder how it works, because that would help in understanding how this things is actually tuned.

My understanding so far is "one connection - one thread". Otherwise why most database would be blocking ?

Answer 1

Connection pooling is what you have on client side (i.e. in Java).

The connection pool is just that - a pool of open connections to the database. These are not bound to threads, any number of threads can request connection from the pool at any given time - the pool will grant the request if a connection is available, and if not, it will either create a new one, block or deny the request (depends on implementation). The main idea here is to have less connections than threads, another purpose is to keep those connections open if there's many short DB operations (the creation of DB connection is an expensive operation).

On the server side, this depends on the DB implementation. I would expect most DB servers to use one thread per connection - someone has to listen on the open socket after all. For many DB engines this can be much more complex though, e.g. there may be one module listening on the socket, and in turn sending the queries to another module that may have different number of threads running.