Connections will be automatically pulled out of the pool based on the connection string, so if you do not change the connection string for each user (or other purposes), you can reuse the connection from the pool as soon as you do this with one (note that I describe the behavior that the union code performs, not what you need to do or worry about).
To succeed in this, you need to make sure that your connections are open only if you need them, and that they are always, always closed (this is general advice, and not just specific to your situation).
If you find that you are working with a connection pool limit, you can catch this error and either ask the user to try again later, or scroll for a short period of time and try the connection again.
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