Often DataFlows contain many processors which deal with database - select, update or delete different data in different tables.
Yet database is same and connection pooling helps to speed up connecting to database (open connection is fairly expensive). Also configuration must be done only in one place.
Database Connection Pooling Controller Service helps to solve this in consistent way.
https://issues.apache.org/jira/browse/NIFI-293 : Add a JDBC Processor for executing arbitrary SQL queries.