• Define an enum type to assign to each Expression-derived class
• Implement a visitor method that, using the above enum, invokes the proper callback method exposed by a type (provided as a template argument)
• Ensure that the performance of the switch operation is comparable with the performance of the equivalent virtual method dispatch (e.g. execute(), optimize() or visit())

See https://gist.github.com/dpercy/b411a4fa92af1a5bb5bcd2790fc893a9 and the branch alby/SERVER-97130