Apache Calcite 通过解析SQL创建关系表达式

        final SchemaPlus rootSchema = Frameworks.createRootSchema(true);
        rootSchema.add("DB1", new ReflectiveSchema(new RowExpressionBuilder.Db1()));
        final FrameworkConfig config = Frameworks.newConfigBuilder()
                .defaultSchema(rootSchema).build();
        String sql = "";
        PlannerImpl planner = new PlannerImpl(config);//执行计划需要进行解析，验证，转换三步
        SqlNode parse = planner.parse(sql);
        SqlNode validate = planner.validate(parse);
        RelNode convert = planner.convert(validate);//
        String s = RelOptUtil.toString(convert);//输出可读的关系表达式