如何查看SparkSQL 生成的抽象语法树？

前言

    在《Spark SQL内核剖析》书中4.3章节，谈到Catalyst体系中生成的抽象语法树的节点都是以Context来结尾，在ANLTR4以及生成的SqlBaseParser解析SQL生成，其源码部分就是语法解析，其生成的抽象语法树的节点都是ParserRuleContext的子类。

提出问题

    ANLTR4解析SQL生成抽象语法树，最终这颗树长成什么样子，如何查看？

源码分析

测试示例

spark.sql("select id, count(name) from student group by id").show()

源码入口

SparkSession的sql 方法如下：

def sql(sqlText: String): DataFrame = {
    // TODO 1. 生成LogicalPlan
    // sqlParser 为 SparkSqlParser
    val logicalPlan: LogicalPlan = sessionState.sqlParser.parsePlan(sqlText)
    // 根据 LogicalPlan
    val frame: DataFrame = Dataset.ofRows(self, logicalPlan)
    frame // sqlParser
  }

定位SparkSqlParser

入口源码涉及到SessionState这个关键类，其初始化代码如下：

lazy val sessionState: SessionState = {
    parentSessionState
      .map(_.clone(this))
      .getOrElse {
        // 构建 org.apache.spark.sql.internal.SessionStateBuilder
        val state = SparkSession.instantiateSessionState(
          SparkSession.sessionStateClassName(sparkContext.conf),
          self)
        initialSessionOptions.foreach { case (k, v) => state.conf.setConfString(k, v) }
        state
      }
  }

org.apache.spark.sql.SparkSession$#sessionStateClassName 方法具体如下：

private def sessionStateClassName(conf: SparkConf): String = {
    // spark.sql.catalogImplementation, 分为 hive 和 in-memory模式，默认为 in-memory 模式
    conf.get(CATALOG_IMPLEMENTATION) match {
      case "hive" => HIVE_SESSION_STATE_BUILDER_CLASS_NAME // hive 实现 org.apache.spark.sql.hive.HiveSessionStateBuilder
      case "in-memory" => classOf[SessionStateBuilder].getCanonicalName // org.apache.spark.sql.internal.SessionStateBuilder
    }
  }

其中，这里用到了builder模式，org.apache.spark.sql.internal.SessionStateBuilder就是用来构建 SessionState的。在 SparkSession.instantiateSessionState 中有具体说明，如下：

/**
   * Helper method to create an instance of `SessionState` based on `className` from conf.
   * The result is either `SessionState` or a Hive based `SessionState`.
   */
  private def instantiateSessionState(
      className: String,
      sparkSession: SparkSession): SessionState = {
    try {
      // org.apache.spark.sql.internal.SessionStateBuilder
      // invoke `new [Hive]SessionStateBuilder(SparkSession, Option[SessionState])`
      val clazz = Utils.classForName(className)
      val ctor = clazz.getConstructors.head
      ctor.newInstance(sparkSession, None).asInstanceOf[BaseSessionStateBuilder].build()
    } catch {
      case NonFatal(e) =>
        throw new IllegalArgumentException(s"Error while instantiating '$className':", e)
    }
  }

其中，BaseSessionStateBuilder下面有两个主要实现，分别为 org.apache.spark.sql.hive.HiveSessionStateBuilder（hive模式） 和 org.apache.spark.sql.internal.SessionStateBuilder（in-memory模式，默认）

org.apache.spark.sql.internal.BaseSessionStateBuilder#build 方法，源码如下：

/**
   * Build the [[SessionState]].
   */
  def build(): SessionState = {
    new SessionState(
      session.sharedState,
      conf,
      experimentalMethods,
      functionRegistry,
      udfRegistration,
      () => catalog,
      sqlParser,
      () => analyzer,
      () => optimizer,
      planner,
      streamingQueryManager,
      listenerManager,
      () => resourceLoader,
      createQueryExecution,
      createClone)
  }

SessionState中，包含了很多的参数，关键参数介绍如下：

conf：SparkConf对象，对SparkSession的配置

functionRegistry：FunctionRegistry对象，负责函数的注册，其内部维护了一个map对象用于维护注册的函数。

UDFRegistration：UDFRegistration对象，用于注册UDF函数，其依赖于FunctionRegistry

catalogBuilder: () => SessionCatalog：返回SessionCatalog对象，其主要用于管理SparkSession的Catalog

sqlParser: ParserInterface, 实际为 SparkSqlParser 实例，其内部调用ASTBuilder将SQL解析为抽象语法树

analyzerBuilder: () => Analyzer, org.apache.spark.sql.internal.BaseSessionStateBuilder.analyzer 自定义 org.apache.spark.sql.catalyst.analysis.Analyzer.Analyzer

optimizerBuilder: () => Optimizer, // org.apache.spark.sql.internal.BaseSessionStateBuilder.optimizer --> 自定义 org.apache.spark.sql.execution.SparkOptimizer.SparkOptimizer

planner: SparkPlanner, // org.apache.spark.sql.internal.BaseSessionStateBuilder.planner --> 自定义 org.apache.spark.sql.execution.SparkPlanner.SparkPlanner

resourceLoaderBuilder: () => SessionResourceLoader，返回资源加载器，主要用于加载函数的jar或资源

createQueryExecution: LogicalPlan => QueryExecution：根据LogicalPlan生成QueryExecution对象

parsePlan方法

SparkSqlParser没有该方法的实现，具体是现在其父类 AbstractSqlParser中，如下：

/** Creates LogicalPlan for a given SQL string. */
    // TODO 根据 sql语句生成 逻辑计划 LogicalPlan
  override def parsePlan(sqlText: String): LogicalPlan = parse(sqlText) { parser =>
      val singleStatementContext: SqlBaseParser.SingleStatementContext = parser.singleStatement()
    astBuilder.visitSingleStatement(singleStatementContext) match {
      case plan: LogicalPlan => plan
      case _ =>
        val position = Origin(None, None)
        throw new ParseException(Option(sqlText), "Unsupported SQL statement", position, position)
    }
  }

其中 parse 方法后面的方法是一个回调函数，它在parse 方法中被调用，如下：

org.apache.spark.sql.execution.SparkSqlParser#parse源码如下：

private val substitutor = new VariableSubstitution(conf) // 参数替换器

  protected override def parse[T](command: String)(toResult: SqlBaseParser => T): T = {
    super.parse(substitutor.substitute(command))(toResult)
  }

其中，substitutor是一个参数替换器，用于把SQL中的参数都替换掉，继续看其父类AbstractSqlParser的parse 方法：

protected def parse[T](command: String)(toResult: SqlBaseParser => T): T = {
    logDebug(s"Parsing command: $command")

    // 词法分析
    val lexer = new SqlBaseLexer(new UpperCaseCharStream(CharStreams.fromString(command)))
    lexer.removeErrorListeners()
    lexer.addErrorListener(ParseErrorListener)
    lexer.legacy_setops_precedence_enbled = SQLConf.get.setOpsPrecedenceEnforced

    // 语法分析
    val tokenStream = new CommonTokenStream(lexer)
    val parser = new SqlBaseParser(tokenStream)
    parser.addParseListener(PostProcessor)
    parser.removeErrorListeners()
    parser.addErrorListener(ParseErrorListener)
    parser.legacy_setops_precedence_enbled = SQLConf.get.setOpsPrecedenceEnforced

    try {
      try {
        // first, try parsing with potentially faster SLL mode
        parser.getInterpreter.setPredictionMode(PredictionMode.SLL)
        // 使用 AstBuilder 生成 Unresolved LogicalPlan
        toResult(parser)
      }
      catch {
        case e: ParseCancellationException =>
          // if we fail, parse with LL mode
          tokenStream.seek(0) // rewind input stream
          parser.reset()

          // Try Again.
          parser.getInterpreter.setPredictionMode(PredictionMode.LL)
          toResult(parser)
      }
    }
    catch {
      case e: ParseException if e.command.isDefined =>
        throw e
      case e: ParseException =>
        throw e.withCommand(command)
      case e: AnalysisException =>
        val position = Origin(e.line, e.startPosition)
        throw new ParseException(Option(command), e.message, position, position)
    }
  }

在这个方法中调用ANLTR4的API将SQL转换为AST抽象语法树，然后调用 toResult(parser) 方法，这个 toResult 方法就是parsePlan 方法的回调方法。

截止到调用astBuilder.visitSingleStatement 方法之前， AST抽象语法树已经生成。

打印生成的AST

修改源码

下面，看 astBuilder.visitSingleStatement  方法：

override def visitSingleStatement(ctx: SingleStatementContext): LogicalPlan = withOrigin(ctx) {
    val statement: StatementContext = ctx.statement
    printRuleContextInTreeStyle(statement, 1)
    // 调用accept 生成 逻辑算子树AST
    visit(statement).asInstanceOf[LogicalPlan]
  }

在使用访问者模式访问AST节点生成UnResolved LogicalPlan之前，我定义了一个方法用来打印刚解析生成的抽象语法树， printRuleContextInTreeStyle 代码如下：

/**
   * 树形打印抽象语法树
   */
  private def printRuleContextInTreeStyle(ctx: ParserRuleContext, level:Int): Unit = {
    val prefix:String = "|"
    val curLevelStr: String = "-" * level
    val childLevelStr: String = "-" * (level + 1)
    println(s"${prefix}${curLevelStr} ${ctx.getClass.getCanonicalName}")
    val children: util.List[ParseTree] = ctx.children
    if( children == null || children.size() == 0) {
      return
    }
    children.iterator().foreach {
      case context: ParserRuleContext => printRuleContextInTreeStyle(context, level + 1)
      case _ => println(s"${prefix}${childLevelStr} ${ctx.getClass.getCanonicalName}")
    }
  }

三种SQL打印示例

SQL示例1（带where）

select name from student where age > 18

其生成的AST如下：

|- org.apache.spark.sql.catalyst.parser.SqlBaseParser.StatementDefaultContext
|-- org.apache.spark.sql.catalyst.parser.SqlBaseParser.QueryContext
|--- org.apache.spark.sql.catalyst.parser.SqlBaseParser.SingleInsertQueryContext
|---- org.apache.spark.sql.catalyst.parser.SqlBaseParser.QueryTermDefaultContext
|----- org.apache.spark.sql.catalyst.parser.SqlBaseParser.QueryPrimaryDefaultContext
|------ org.apache.spark.sql.catalyst.parser.SqlBaseParser.QuerySpecificationContext
|------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.QuerySpecificationContext
|------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.NamedExpressionSeqContext
|-------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.NamedExpressionContext
|--------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.ExpressionContext
|---------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.PredicatedContext
|----------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.ValueExpressionDefaultContext
|------------ org.apache.spark.sql.catalyst.parser.SqlBaseParser.ColumnReferenceContext
|------------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.IdentifierContext
|-------------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.UnquotedIdentifierContext
|--------------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.UnquotedIdentifierContext
|------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.FromClauseContext
|-------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.FromClauseContext
|-------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.RelationContext
|--------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.TableNameContext
|---------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.TableIdentifierContext
|----------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.IdentifierContext
|------------ org.apache.spark.sql.catalyst.parser.SqlBaseParser.UnquotedIdentifierContext
|------------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.UnquotedIdentifierContext
|---------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.TableAliasContext
|------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.QuerySpecificationContext
|------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.PredicatedContext
|-------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.ComparisonContext
|--------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.ValueExpressionDefaultContext
|---------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.ColumnReferenceContext
|----------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.IdentifierContext
|------------ org.apache.spark.sql.catalyst.parser.SqlBaseParser.UnquotedIdentifierContext
|------------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.UnquotedIdentifierContext
|--------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.ComparisonOperatorContext
|---------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.ComparisonOperatorContext
|--------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.ValueExpressionDefaultContext
|---------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.ConstantDefaultContext
|----------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.NumericLiteralContext
|------------ org.apache.spark.sql.catalyst.parser.SqlBaseParser.IntegerLiteralContext
|------------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.IntegerLiteralContext
|---- org.apache.spark.sql.catalyst.parser.SqlBaseParser.QueryOrganizationContext

SQL示例2（带排序）

select name from student where age > 18 order by id desc

其生成的AST如下：

|- org.apache.spark.sql.catalyst.parser.SqlBaseParser.StatementDefaultContext
|-- org.apache.spark.sql.catalyst.parser.SqlBaseParser.QueryContext
|--- org.apache.spark.sql.catalyst.parser.SqlBaseParser.SingleInsertQueryContext
|---- org.apache.spark.sql.catalyst.parser.SqlBaseParser.QueryTermDefaultContext
|----- org.apache.spark.sql.catalyst.parser.SqlBaseParser.QueryPrimaryDefaultContext
|------ org.apache.spark.sql.catalyst.parser.SqlBaseParser.QuerySpecificationContext
|------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.QuerySpecificationContext
|------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.NamedExpressionSeqContext
|-------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.NamedExpressionContext
|--------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.ExpressionContext
|---------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.PredicatedContext
|----------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.ValueExpressionDefaultContext
|------------ org.apache.spark.sql.catalyst.parser.SqlBaseParser.ColumnReferenceContext
|------------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.IdentifierContext
|-------------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.UnquotedIdentifierContext
|--------------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.UnquotedIdentifierContext
|------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.FromClauseContext
|-------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.FromClauseContext
|-------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.RelationContext
|--------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.TableNameContext
|---------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.TableIdentifierContext
|----------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.IdentifierContext
|------------ org.apache.spark.sql.catalyst.parser.SqlBaseParser.UnquotedIdentifierContext
|------------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.UnquotedIdentifierContext
|---------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.TableAliasContext
|------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.QuerySpecificationContext
|------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.PredicatedContext
|-------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.ComparisonContext
|--------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.ValueExpressionDefaultContext
|---------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.ColumnReferenceContext
|----------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.IdentifierContext
|------------ org.apache.spark.sql.catalyst.parser.SqlBaseParser.UnquotedIdentifierContext
|------------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.UnquotedIdentifierContext
|--------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.ComparisonOperatorContext
|---------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.ComparisonOperatorContext
|--------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.ValueExpressionDefaultContext
|---------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.ConstantDefaultContext
|----------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.NumericLiteralContext
|------------ org.apache.spark.sql.catalyst.parser.SqlBaseParser.IntegerLiteralContext
|------------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.IntegerLiteralContext
|---- org.apache.spark.sql.catalyst.parser.SqlBaseParser.QueryOrganizationContext
|----- org.apache.spark.sql.catalyst.parser.SqlBaseParser.QueryOrganizationContext
|----- org.apache.spark.sql.catalyst.parser.SqlBaseParser.QueryOrganizationContext
|----- org.apache.spark.sql.catalyst.parser.SqlBaseParser.SortItemContext
|------ org.apache.spark.sql.catalyst.parser.SqlBaseParser.ExpressionContext
|------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.PredicatedContext
|-------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.ValueExpressionDefaultContext
|--------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.ColumnReferenceContext
|---------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.IdentifierContext
|----------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.UnquotedIdentifierContext
|------------ org.apache.spark.sql.catalyst.parser.SqlBaseParser.UnquotedIdentifierContext
|------ org.apache.spark.sql.catalyst.parser.SqlBaseParser.SortItemContext

SQL示例2（带分组）

select id, count(name) from student group by id

其生成的AST如下：

|- org.apache.spark.sql.catalyst.parser.SqlBaseParser.StatementDefaultContext
|-- org.apache.spark.sql.catalyst.parser.SqlBaseParser.QueryContext
|--- org.apache.spark.sql.catalyst.parser.SqlBaseParser.SingleInsertQueryContext
|---- org.apache.spark.sql.catalyst.parser.SqlBaseParser.QueryTermDefaultContext
|----- org.apache.spark.sql.catalyst.parser.SqlBaseParser.QueryPrimaryDefaultContext
|------ org.apache.spark.sql.catalyst.parser.SqlBaseParser.QuerySpecificationContext
|------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.QuerySpecificationContext
|------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.NamedExpressionSeqContext
|-------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.NamedExpressionContext
|--------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.ExpressionContext
|---------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.PredicatedContext
|----------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.ValueExpressionDefaultContext
|------------ org.apache.spark.sql.catalyst.parser.SqlBaseParser.ColumnReferenceContext
|------------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.IdentifierContext
|-------------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.UnquotedIdentifierContext
|--------------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.UnquotedIdentifierContext
|-------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.NamedExpressionSeqContext
|-------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.NamedExpressionContext
|--------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.ExpressionContext
|---------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.PredicatedContext
|----------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.ValueExpressionDefaultContext
|------------ org.apache.spark.sql.catalyst.parser.SqlBaseParser.FunctionCallContext
|------------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.QualifiedNameContext
|-------------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.IdentifierContext
|--------------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.UnquotedIdentifierContext
|---------------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.UnquotedIdentifierContext
|------------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.FunctionCallContext
|------------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.ExpressionContext
|-------------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.PredicatedContext
|--------------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.ValueExpressionDefaultContext
|---------------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.ColumnReferenceContext
|----------------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.IdentifierContext
|------------------ org.apache.spark.sql.catalyst.parser.SqlBaseParser.UnquotedIdentifierContext
|------------------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.UnquotedIdentifierContext
|------------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.FunctionCallContext
|------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.FromClauseContext
|-------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.FromClauseContext
|-------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.RelationContext
|--------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.TableNameContext
|---------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.TableIdentifierContext
|----------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.IdentifierContext
|------------ org.apache.spark.sql.catalyst.parser.SqlBaseParser.UnquotedIdentifierContext
|------------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.UnquotedIdentifierContext
|---------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.TableAliasContext
|------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.AggregationContext
|-------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.AggregationContext
|-------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.AggregationContext
|-------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.ExpressionContext
|--------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.PredicatedContext
|---------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.ValueExpressionDefaultContext
|----------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.ColumnReferenceContext
|------------ org.apache.spark.sql.catalyst.parser.SqlBaseParser.IdentifierContext
|------------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.UnquotedIdentifierContext
|-------------- org.apache.spark.sql.catalyst.parser.SqlBaseParser.UnquotedIdentifierContext
|---- org.apache.spark.sql.catalyst.parser.SqlBaseParser.QueryOrganizationContext

总结

在本篇文章中，主要从测试代码出发，到如何调用ANTLR4解析SQL得到生成AST，并且修改了源码来打印这个AST树。尽管现在看来，使用ANTLR解析SQL生成AST是一个black box，但对于Spark SQL来说，其后续流程的输入已经得到。