When the 'Use Connection Pool' option is checked, what will happen is that the Test/VSE that is executing the step will check for the existence of a connection pool based on the following 4 items
. Class Driver
. JDBC Connect String
If a connection pool is not available, it will create a new connection pool using the same four items that were defined in the step.
You should be able to use the connection pool option in the JDBC step, but if you are running into any issue, what we would recommend is for you to reach out to your DBA, to find out if you have run out of connections in the database, and to allow the Test/VSE to create more connections to support connection pooling.
You might also want to check with your end users who are sending transactions to make sure they are using a generic SQL account to make better use of the connection pool. If users are using their own accounts for the JDBC step and not a generic account, that will increase the connection count for the database, and most likely cause additional overhead from the database to support the execution of your requests. Your DBA should also be able to help you identify if this is the case.