下面是一个示例代码，展示了如何在RelBuilder中注册UDFs：

import org.apache.calcite.sql.SqlFunction;
import org.apache.calcite.sql.type.SqlReturnTypeInference;
import org.apache.calcite.sql.type.SqlTypeName;
import org.apache.calcite.sql.validate.SqlMonotonicity;
import org.apache.calcite.sql.validate.SqlValidator;
import org.apache.calcite.sql.validate.SqlValidatorScope;
import org.apache.calcite.util.Optionality;
import org.apache.calcite.util.Static;

public class MyUDF extends SqlFunction {
  
  public MyUDF() {
    super("MY_UDF",
        new SqlReturnTypeInference() {
          @Override
          public RelDataType inferReturnType(SqlOperatorBinding opBinding) {
            // Define the return type of the UDF
            return opBinding.getTypeFactory().createSqlType(SqlTypeName.INTEGER);
          }
        },
        null,
        new ScalarOperandTypeInference() {
          @Override
          public void inferOperandTypes(SqlCallBinding callBinding, RelDataType returnType, RelDataType[] operandTypes) {
            // Define the input types of the UDF
            operandTypes[0] = callBinding.getTypeFactory().createSqlType(SqlTypeName.INTEGER);
          }
        },
        OperandTypes.family(SqlTypeFamily.INTEGER),
        SqlFunctionCategory.USER_DEFINED_FUNCTION);
  }

  @Override
  public RelDataType deriveType(SqlValidator validator, SqlValidatorScope scope, SqlCall call) {
    // Perform type derivation for the UDF
    return getReturnTypeInference().inferReturnType(call.getOperatorBinding());
  }

  @Override
  public SqlMonotonicity getMonotonicity(SqlOperatorBinding call) {
    return SqlMonotonicity.NOT_MONOTONIC;
  }

  @Override
  public boolean isDeterministic() {
    return true;
  }

  @Override
  public Optionality getKind() {
    return Optionality.FORBIDDEN;
  }

  @Override
  public boolean argumentMustBeScalar(int ordinal) {
    return false;
  }

  @Override
  public boolean[] getNullableArguments() {
    return null;
  }

  @Override
  public boolean isCallTypeChecked() {
    return false;
  }

  @Override
  public boolean isDynamicFunction() {
    return false;
  }

  @Override
  public boolean isDeterministic(SqlOperatorBinding call) {
    return true;
  }

  @Override
  public boolean isDeterministic(SqlCall call, SqlValidatorScope scope) {
    return true;
  }
}

import org.apache.calcite.tools.RelBuilder;
import org.apache.calcite.sql.SqlOperatorTable;

public class Main {
  
  public static void main(String[] args) {
    RelBuilder builder = RelBuilder.create();
    
    // Step 1: Create a UDF class
    MyUDF myUdf = new MyUDF();
    
    // Step 2: Create a UDF instance
    
    // Step 3: Create a SqlOperatorTable object
    SqlOperatorTable operatorTable = builder.getOperatorTable();
    
    // Step 4: Add UDF instance to SqlOperatorTable
    operatorTable.register(myUdf);
    
    // Step 5: Set SqlOperatorTable to RelBuilder
    builder.setOperatorTable(operatorTable);
    
    // Use the registered UDF in RelBuilder to build queries
    // ...
  }
}

在这个示例中，我们创建了一个名为MY_UDF的UDF，并将其注册到Calcite的RelBuilder中。用户可以根据自己的需求实现自定义的UDF类，并根据具体情况定义返回类型、输入类型等。然后，将UDF实例注册到SqlOperatorTable中，并将其设置给RelBuilder以便在构建查询时使用。