In FLINK-15595, CoreModule should contains all functions in FlinkSqlOperatorTable. Otherwise, resolution Order is chaotic. I think it is time to align blink built-in functions to BuiltInFunctionDefinitions.
Impact to legacy planner: user can not use the function name directly that he define function with the same name of blink built-in function in catalog. I think it is reasonable, since he will migrate his job to blink planner.