作者：周志湖

以下的代码演示了通过Case Class进行表Schema定义的样例：

// sc is an existing SparkContext.val sqlContext = new org.apache.spark.sql.SQLContext(sc)// this is used to implicitly convert an RDD to a DataFrame.import sqlContext.implicits._// Define the schema using a case class.// Note: Case classes in Scala 2.10 can support only up to 22 fields. To work around this limit,// you can use custom classes that implement the Product interface.case class Person(name: String, age: Int)// Create an RDD of Person objects and register it as a table.val people = sc.textFile("examples/src/main/resources/people.txt").map(_.split(",")).map(p => Person(p(0), p(1).trim.toInt)).toDF()people.registerTempTable("people")// SQL statements can be run by using the sql methods provided by sqlContext.val teenagers = sqlContext.sql("SELECT name, age FROM people WHERE age >= 13 AND age <= 19")// The results of SQL queries are DataFrames and support all the normal RDD operations.// The columns of a row in the result can be accessed by field index:teenagers.map(t => "Name: " + t(0)).collect().foreach(println)// or by field name:teenagers.map(t => "Name: " + t.getAs[String]("name")).collect().foreach(println)// row.getValuesMap[T] retrieves multiple columns at once into a Map[String, T]teenagers.map(_.getValuesMap[Any](List("name", "age"))).collect().foreach(println)// Map("name" -> "Justin", "age" -> 19)

（1）sql方法返回DataFrame

def sql(sqlText: String): DataFrame = {    DataFrame(this, parseSql(sqlText))  }

当中parseSql(sqlText)方法生成对应的LogicalPlan得到，该方法源代码例如以下：

//依据传入的sql语句，生成LogicalPlanprotected[sql] def parseSql(sql: String): LogicalPlan = ddlParser.parse(sql, false)

ddlParser对象定义例如以下：

protected[sql] val sqlParser = new SparkSQLParser(getSQLDialect().parse(_))protected[sql] val ddlParser = new DDLParser(sqlParser.parse(_))

（2）然后调用DataFrame的apply方法

private[sql] object DataFrame {  def apply(sqlContext: SQLContext, logicalPlan: LogicalPlan): DataFrame = {    new DataFrame(sqlContext, logicalPlan)  }}

能够看到，apply方法參数有两个，各自是SQLContext和LogicalPlan，调用的是DataFrame的构造方法，详细源代码例如以下：

//DataFrame构造方法。该构造方法会自己主动对LogicalPlan进行分析，然后返回QueryExecution对象def this(sqlContext: SQLContext, logicalPlan: LogicalPlan) = {    this(sqlContext, {      val qe = sqlContext.executePlan(logicalPlan)      //推断是否已经创建。假设是则抛异常      if (sqlContext.conf.dataFrameEagerAnalysis) {        qe.assertAnalyzed()  // This should force analysis and throw errors if there are any      }      qe    })  }

（3）val qe = sqlContext.executePlan(logicalPlan) 返回QueryExecution， sqlContext.executePlan方法源代码例如以下：

protected[sql] def executePlan(plan: LogicalPlan) =    new sparkexecution.QueryExecution(this, plan)

QueryExecution类中表达了Spark运行SQL的主要工作流程，详细例如以下

class QueryExecution(val sqlContext: SQLContext, val logical: LogicalPlan) {
        @VisibleForTesting  def assertAnalyzed(): Unit = sqlContext.analyzer.checkAnalysis(analyzed)  lazy val analyzed: LogicalPlan = sqlContext.analyzer.execute(logical)  lazy val withCachedData: LogicalPlan = {    assertAnalyzed()    sqlContext.cacheManager.useCachedData(analyzed)  }  lazy val optimizedPlan: LogicalPlan = sqlContext.optimizer.execute(withCachedData)  // TODO: Don't just pick the first one...  lazy val sparkPlan: SparkPlan = {    SparkPlan.currentContext.set(sqlContext)    sqlContext.planner.plan(optimizedPlan).next()  }  // executedPlan should not be used to initialize any SparkPlan. It should be  // only used for execution.  lazy val executedPlan: SparkPlan = sqlContext.prepareForExecution.execute(sparkPlan)  /** Internal version of the RDD. Avoids copies and has no schema */  //调用toRDD方法运行任务将结果转换为RDD  lazy val toRdd: RDD[InternalRow] = executedPlan.execute()  protected def stringOrError[A](f: => A): String =    try f.toString catch { case e: Throwable => e.toString }  def simpleString: String = {    s"""== Physical Plan ==       |${stringOrError(executedPlan)}      """.stripMargin.trim  }  override def toString: String = {    def output =      analyzed.output.map(o => s"${o.name}: ${o.dataType.simpleString}").mkString(", ")    s"""== Parsed Logical Plan ==       |${stringOrError(logical)}       |== Analyzed Logical Plan ==       |${stringOrError(output)}       |${stringOrError(analyzed)}       |== Optimized Logical Plan ==       |${stringOrError(optimizedPlan)}       |== Physical Plan ==       |${stringOrError(executedPlan)}       |Code Generation: ${stringOrError(executedPlan.codegenEnabled)}    """.stripMargin.trim  }}

能够看到，SQL的运行流程为

1.Parsed Logical Plan：LogicalPlan

2.Analyzed Logical Plan：

lazy val analyzed: LogicalPlan = sqlContext.analyzer.execute(logical)

3.Optimized Logical Plan：lazy val optimizedPlan: LogicalPlan = sqlContext.optimizer.execute(withCachedData)

4. Physical Plan：lazy val executedPlan: SparkPlan = sqlContext.prepareForExecution.execute(sparkPlan)

能够调用results.queryExecution方法查看，代码例如以下：

scala> results.queryExecutionres1: org.apache.spark.sql.SQLContext#QueryExecution === Parsed Logical Plan =='Project [unresolvedalias('name)] 'UnresolvedRelation [people], None== Analyzed Logical Plan ==name: stringProject [name#0] Subquery people  LogicalRDD [name#0,age#1], MapPartitionsRDD[4] at createDataFrame at 
      
       :47== Optimized Logical Plan ==Project [name#0] LogicalRDD [name#0,age#1], MapPartitionsRDD[4] at createDataFrame at 
       
        :47== Physical Plan ==TungstenProject [name#0] Scan PhysicalRDD[name#0,age#1]Code Generation: true
       
      

（4） 然后调用DataFrame的主构造器完毕DataFrame的构造

class DataFrame private[sql](    @transient val sqlContext: SQLContext,    @DeveloperApi @transient val queryExecution: QueryExecution) extends Serializable

（5）

当调用DataFrame的collect等方法时，便会触发运行executedPlan

def collect(): Array[Row] = withNewExecutionId {    queryExecution.executedPlan.executeCollect()  }

比如：

scala> results.collectres6: Array[org.apache.spark.sql.Row] = Array([Michael], [Andy], [Justin])

总体流程图例如以下：