spark load data from mysql

code first

本机通过spark-shell.cmd启动一个spark进程

    SparkSession spark = SparkSession.builder().appName("Simple Application").master("local[2]").getOrCreate();

        Map<String, String> map = new HashMap<>();
        map.put("url","jdbc:mysql:xxx");
        map.put("user", "user");
        map.put("password", "pass");
        String tableName = "table";
        map.put("dbtable", tableName);
        map.put("driver", "com.mysql.jdbc.Driver");
        String lowerBound = 1 + "";   //低界限
        String upperBound = 10000 + "";  //高界限

        map.put("fetchsize", "100000");  //实例和mysql服务端单次拉取行数，拉取后才能执行rs.next()
        map.put("numPartitions", "50");  //50个分区区间，将以范围[lowerBound,upperBound]划分成50个分区，每个分区执行一次查询
        map.put("partitionColumn", "id");  //分区条件列
        System.out.println("tableName:" + tableName + ", lowerBound:"+lowerBound+", upperBound:"+upperBound);
        map.put("lowerBound", lowerBound);
        map.put("upperBound", upperBound);

        Dataset dataset = spark.read().format("jdbc").options(map).load(); //transform操作
        dataset.registerTempTable("tmp__");
        Dataset<Row> ds = spark.sql("select * from tmp__"); //transform操作
        ds.cache().show();  //action，触发sql真正执行

执行到show时，任务开始真正执行，此时，我们单机debug，来跟踪partitionColumn的最终实现方式

debug类

org.apache.spark.sql.execution.datasources.jdbc.JDBCRelation.buildScan

此时parts为size=50的分区列表

  override def buildScan(requiredColumns: Array[String], filters: Array[Filter]): RDD[Row] = {
    // Rely on a type erasure hack to pass RDD[InternalRow] back as RDD[Row]
    JDBCRDD.scanTable(
      sparkSession.sparkContext,
      schema,
      requiredColumns,
      filters,
      parts,
      jdbcOptions).asInstanceOf[RDD[Row]]
  }

单个分区内的whereClause值

whereCluase="id < 21 or id is null"

继续往下断点,到单个part的执行逻辑,此时代码应该是在Executor中的某个task线程中
org.apache.spark.sql.execution.datasources.jdbc.JDBCRDD.compute

    val myWhereClause = getWhereClause(part)

    val sqlText = s"SELECT $columnList FROM ${options.table} $myWhereClause"
    stmt = conn.prepareStatement(sqlText,
        ResultSet.TYPE_FORWARD_ONLY, ResultSet.CONCUR_READ_ONLY)
    stmt.setFetchSize(options.fetchSize)
    rs = stmt.executeQuery()
    val rowsIterator = JdbcUtils.resultSetToSparkInternalRows(rs, schema, inputMetrics)

    CompletionIterator[InternalRow, Iterator[InternalRow]](
      new InterruptibleIterator(context, rowsIterator), close())

此时
myWhereClause=WHERE id < 21 or id is null

最终的sql语句
sqlText=SELECT id,xx FROM tablea WHERE id < 21 or id is null

所有part都会经过compute
Executor执行完任务后，将信息发送回Driver
Executor: Finished task 7.0 in stage 2.0 (TID 12). 1836 bytes result sent to driver

总结

numPartitions、partitionColumn、lowerBound、upperBound结合后，spark将生成很多个parts,每个part对应一个查询whereClause，最终查询数据将分成numPartitions个任务来拉取数据，因此，partitionColumn必须是索引列，否则，效率将大大降低
自动获取table schema,程序会执行类型select * from tablea where 1=0 来获取字段及类型