聊聊flink Table的OrderBy及Limit

序

本文主要研究一下flink Table的OrderBy及Limit

实例

Table in = tableEnv.fromDataSet(ds, "a, b, c");
Table result = in.orderBy("a.asc");

Table in = tableEnv.fromDataSet(ds, "a, b, c");

// returns the first 5 records from the sorted result
Table result1 = in.orderBy("a.asc").fetch(5); 

// skips the first 3 records and returns all following records from the sorted result
Table result2 = in.orderBy("a.asc").offset(3);

// skips the first 10 records and returns the next 5 records from the sorted result
Table result3 = in.orderBy("a.asc").offset(10).fetch(5);

• orderBy方法类似 sql 的order by；limit则由offset及fetch两个方法构成，类似sql的offset及fetch

Table

flink-table_2.11-1.7.0-sources.jar!/org/apache/flink/table/api/table.scala

class Table(
    private[flink] val tableEnv: TableEnvironment,
    private[flink] val logicalPlan: LogicalNode) {

  //......

  def orderBy(fields: String): Table = {
    val parsedFields = ExpressionParser.parseExpressionList(fields)
    orderBy(parsedFields: _*)
  }

  def orderBy(fields: Expression*): Table = {
    val order: Seq[Ordering] = fields.map {
      case o: Ordering => o
      case e => Asc(e)
    }
    new Table(tableEnv, Sort(order, logicalPlan).validate(tableEnv))
  }

  def offset(offset: Int): Table = {
    new Table(tableEnv, Limit(offset, -1, logicalPlan).validate(tableEnv))
  }

  def fetch(fetch: Int): Table = {
    if (fetch < 0) {
      throw new ValidationException("FETCH count must be equal or larger than 0.")
    }
    this.logicalPlan match {
      case Limit(o, -1, c) =>
        // replace LIMIT without FETCH by LIMIT with FETCH
        new Table(tableEnv, Limit(o, fetch, c).validate(tableEnv))
      case Limit(_, _, _) =>
        throw new ValidationException("FETCH is already defined.")
      case _ =>
        new Table(tableEnv, Limit(0, fetch, logicalPlan).validate(tableEnv))
    }
  }

  //......
}

• Table的orderBy方法，支持String或Expression类型的参数，其中String类型最终是转为Expression类型；orderBy方法最后使用Sort重新创建了Table；offset及fetch方法，使用Limit重新创建了Table( offset方法创建的Limit其fetch为-1；fetch方法如果之前没有指定offset则创建的Limit的offset为0 )

Sort

flink-table_2.11-1.7.0-sources.jar!/org/apache/flink/table/plan/logical/operators.scala

case class Sort(order: Seq[Ordering], child: LogicalNode) extends UnaryNode {
  override def output: Seq[Attribute] = child.output

  override protected[logical] def construct(relBuilder: RelBuilder): RelBuilder = {
    child.construct(relBuilder)
    relBuilder.sort(order.map(_.toRexNode(relBuilder)).asJava)
  }

  override def validate(tableEnv: TableEnvironment): LogicalNode = {
    if (tableEnv.isInstanceOf[StreamTableEnvironment]) {
      failValidation(s"Sort on stream tables is currently not supported.")
    }
    super.validate(tableEnv)
  }
}

• Sort继承了UnaryNode，它的构造器接收Set类型的Ordering，其construct方法使用relBuilder.sort来构建sort条件

Ordering

flink-table_2.11-1.7.0-sources.jar!/org/apache/flink/table/expressions/ordering.scala

abstract class Ordering extends UnaryExpression {
  override private[flink] def validateInput(): ValidationResult = {
    if (!child.isInstanceOf[NamedExpression]) {
      ValidationFailure(s"Sort should only based on field reference")
    } else {
      ValidationSuccess
    }
  }
}

case class Asc(child: Expression) extends Ordering {
  override def toString: String = s"($child).asc"

  override private[flink] def toRexNode(implicit relBuilder: RelBuilder): RexNode = {
    child.toRexNode
  }

  override private[flink] def resultType: TypeInformation[_] = child.resultType
}

case class Desc(child: Expression) extends Ordering {
  override def toString: String = s"($child).desc"

  override private[flink] def toRexNode(implicit relBuilder: RelBuilder): RexNode = {
    relBuilder.desc(child.toRexNode)
  }

  override private[flink] def resultType: TypeInformation[_] = child.resultType
}

• Ordering是一个抽象类，它有Asc及Desc两个子类

Limit

flink-table_2.11-1.7.0-sources.jar!/org/apache/flink/table/plan/logical/operators.scala

case class Limit(offset: Int, fetch: Int = -1, child: LogicalNode) extends UnaryNode {
  override def output: Seq[Attribute] = child.output

  override protected[logical] def construct(relBuilder: RelBuilder): RelBuilder = {
    child.construct(relBuilder)
    relBuilder.limit(offset, fetch)
  }

  override def validate(tableEnv: TableEnvironment): LogicalNode = {
    if (tableEnv.isInstanceOf[StreamTableEnvironment]) {
      failValidation(s"Limit on stream tables is currently not supported.")
    }
    if (!child.isInstanceOf[Sort]) {
      failValidation(s"Limit operator must be preceded by an OrderBy operator.")
    }
    if (offset < 0) {
      failValidation(s"Offset should be greater than or equal to zero.")
    }
    super.validate(tableEnv)
  }
}

• Limit继承了UnaryNode，它的构造器接收offset及fetch参数，它的construct方法通过relBuilder.limit来设置offset及fetch

小结

offset方法创建的Limit其fetch为-1；fetch方法如果之前没有指定offset则创建的Limit的offset为0

doc

• OrderBy, Offset & Fetch