Drill does not allow the number of connections in the pool to configured
When a JDBC storage plugin is created, it creates 8 connections per plugin which happen to be the default for dbcp connection pool. Currently, there is no way to configure these parameters using storage configuration. Max Idle is set to 8 leaving all connections open even when unused. This situation creates an unnecessary connection to DB.
These parameters must be made configuration in the JDBC storage plugin configuration.
DRILL-7639 BasicDataSource was replaced with Hikari, sourceParameters sections remains and allows to configure data source, Hikari related configuration properties should be used instead. List of Hikari properties: https://github.com/brettwooldridge/HikariCP.