webMethods JDBC minimum pool size
Performance-wise a min connection > 0 will be "better" for the first set of transactions that is processed in IS using those connections. That is because the connections to the DB will already be established--the time cost of connecting and logging in was incurred during start-up rather than when a JDBC service was invoked.
With a min size of 0, the first JDBC service that is invoked will be delayed a bit while the connection is made and log-in accomplished. After that, there is no overhead (for that connection--each connection up to max pool size will connect and log-in when multiple connections are needed).
An idle pool will drop the connections back to 0 at some point, and then the next JDBC services will incur the connect/log-in overhead again.
For busy pools, the actual connections will not go back to 0. They will be maintained and connection/log-in overhead avoided.
The biggest problem with a min connection > 0 is the potential for stale sessions. This can cause a JDBC service failure if a session is kept and is idle long enough for the DB to close the session or if the DB is restarted.
As usual, it is a trade-off. You want to strike a balance between keeping too many idle connections (wasteful and risks stale connections) and establishing connections too often (time delay and impact on the DB server). It is likely rare that a value other than 0 should/must be used.