in the query projection list.
With this patch, we enable SELECT/DERIVED handler execution for
queries involving 'get_user_var' function. Whereas for 'set_user_var'
function, the handlers are still disabled and the query execution
fallsback to the server, except for some edges case described in
the test file mcol-4410.test, in which case, an appropriate error
message is returned to the client.
