public enum ColumnStrategy {
    NULLABLE, // mark the column as nullable value, only used for normal column
    NOT_NULLABLE, // mark the column as not nullable, only used for normal column
    STORED, // Computed and stored, but can not insert into
    VIRTUAL; // Computed and not stored, can not insert into

    public String asSerializableString() {
      switch (this) {
      case NULLABLE:
        return "NULL";
      case NOT_NULLABLE:
        return "NOT NULL";
      case STORED:
        return "STORED";
      case VIRTUAL:
        return "VIRTUAL";
      default:
        throw new TableException("Unexpected column strategy " + this);
      }
    }

/**
 * InitializerExpressionFactory supplies default values for INSERT, UPDATE, and NEW.
 */
public interface InitializerExpressionFactory {
  //~ Methods ----------------------------------------------------------------

  /**
   * Returns how a column is populated.
   *
   * @param table   the table containing the column
   * @param iColumn the 0-based offset of the column in the table
   *
   * @return generation strategy, never null
   *
   * @see RelOptTable#getColumnStrategies()
   */
  ColumnStrategy generationStrategy(
      RelOptTable table,
      int iColumn);

  /**
   * Creates an expression which evaluates to the default value for a
   * particular column.
   *
   * @param table   the table containing the column
   * @param iColumn the 0-based offset of the column in the table
   * @param context Context for creating the expression
   *
   * @return default value expression
   */
  RexNode newColumnDefaultValue(
      RelOptTable table,
      int iColumn,
      InitializerContext context);

  /**
   * Creates an expression which evaluates to the initializer expression for a
   * particular attribute of a structured type.
   *
   * @param type            the structured type
   * @param constructor     the constructor invoked to initialize the type
   * @param iAttribute      the 0-based offset of the attribute in the type
   * @param constructorArgs arguments passed to the constructor invocation
   * @param context Context for creating the expression
   *
   * @return default value expression
   */
  RexNode newAttributeInitializer(
      RelDataType type,
      SqlFunction constructor,
      int iAttribute,
      List<RexNode> constructorArgs,
      InitializerContext context);
}

// End InitializerExpressionFactory.java

/**
 * A table column represents a table's column structure with field name, data type
 * and column strategies.
 */
public class TableColumn {
  private String fieldName;
  private DataType fieldDataType;
  private Expression expression; // the computed column expression.
  private ColumnStrategy columnStrategy;

  private TableColumn(String fieldName,
      DataType fieldDataType,
      ColumnStrategy columnStrategy) {
    this.fieldName = Objects.requireNonNull(fieldName, "Column name can not be null.");
    this.fieldDataType = Objects.requireNonNull(fieldDataType,
      "Column data type can not be null.");
    this.expression = expression;
    this.columnStrategy = Objects.requireNonNull(columnStrategy);
  }

  public enum ColumnStrategy {
        ...
  }
  ...
}

/**
 * A table schema that represents a table's structure with field names and data types.
 */
public class TableSchema {
  private final TableColumn[] tableColumns;
    ...
}