​ 接着上一篇文章，本章将介绍 第1步：sql 语句经过 SqlParser 解析成 Unresolved Logical Plan

​ 当我们执行：

val sqlDF = spark.sql("SELECT name FROM people order by name")

​ 看一下sql函数：

def sql(sqlText: String): DataFrame = {
    Dataset.ofRows(self, sessionState.sqlParser.parsePlan(sqlText))
  }

def parsePlan(sqlText: String): LogicalPlan

​ parsePlan 函数将 sql语句变成了 LogicalPlan

class SparkSqlParser(conf: SQLConf) extends AbstractSqlParser {
  val astBuilder = new SparkSqlAstBuilder(conf)

  private val substitutor = new VariableSubstitution(conf)

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

/** Creates LogicalPlan for a given SQL string. */
override def parsePlan(sqlText: String): LogicalPlan = parse(sqlText) { parser =>
  astBuilder.visitSingleStatement(parser.singleStatement()) match {
    case plan: LogicalPlan => plan
    case _ =>
      val position = Origin(None, None)
      throw new ParseException(Option(sqlText), "Unsupported SQL statement", position, position)
  }
}

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

    val lexer = new SqlBaseLexer(new UpperCaseCharStream(CharStreams.fromString(command)))
    lexer.removeErrorListeners()
    lexer.addErrorListener(ParseErrorListener)

    val tokenStream = new CommonTokenStream(lexer)
    val parser = new SqlBaseParser(tokenStream)
    parser.addParseListener(PostProcessor)
    parser.removeErrorListeners()
    parser.addErrorListener(ParseErrorListener)

    try {
      try {
        // first, try parsing with potentially faster SLL mode
        parser.getInterpreter.setPredictionMode(PredictionMode.SLL)
        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)
    }
  }

​ 仔细阅读 parse函数，可以发现其中主要的工作主力是SqlBaseLexer 和 SparkSqlAstBuilder,它们都是antlr4相关的代码。下一节会对antlr4进行详细介绍。