## JDBC 调优

Spark 通过 JDBC 读取关系型数据库，默认查全表，只有一个task去执行查询操作，效率可想而知。

Property Name Meaning
url The JDBC URL to connect to. The source-specific connection properties may be specified in the URL. e.g., jdbc:postgresql://localhost/test?user=fred&password=secret
dbtable The JDBC table that should be read. Note that anything that is valid in a FROM clause of a SQL query can be used. For example, instead of a full table you could also use a subquery in parentheses.
driver The class name of the JDBC driver to use to connect to this URL
partitionColumn
lowerBound
upperBound
These options must all be specified if any of them is specified. In addition, numPartitions must be specified. They describe how to partition the table when reading in parallel from multiple workers. partitionColumn must be a numeric column from the table in question. Notice that lowerBound and upperBound are just used to decide the partition stride, not for filtering the rows in table. So all rows in the table will be partitioned and returned. This option applies only to reading.
numPartitions The maximum number of partitions that can be used for parallelism in table reading and writing. This also determines the maximum number of concurrent JDBC connections. If the number of partitions to write exceeds this limit, we decrease it to this limit by calling coalesce(numPartitions) before writing.

1. dbtable：写表名，就是查全表(全字段)。不想查全表，可以在括号里面写子查询。说白了，只要 SQL 语句里，FROM后面能跟的，都合法，因为他就是拼了个 SQL 语句，dbtable会填在FROM后面。
2. numPartitions：读、写的最大分区数，也决定了开启数据库连接的数目。注意最大两个字，也就是说你指定了32个分区，它也不一定就真的分32个分区了。比如：在读的时候，即便指定了numPartitions为任何大于1的值，如果没有指定分区规则，就只有一个task去执行查询。
3. partitionColumn, lowerBound, upperBound：指定读数据时的分区规则。要使用这三个参数，必须定义numPartitions，而且这三个参数不能单独出现，要用就必须全部指定。而且lowerBound, upperBound不是过滤条件，只是用于决定分区跨度。
val sparkSession = SparkSession.builder.appName("jdbc learn").getOrCreate()
.option("url", "jdbc:oracle:thin:@<host>:<port>:<SID> ")
// oracle 表自带整数列 rownum
.option("dbtable", "(SELECT a.*, rownum as rn FROM tablename a)")
.option("user", "user")
.option("fetchsize", 500)
.option("numPartitions", 32)
// 指定数字类型的列
.option("partitionColumn", "rn")
// 分区上下界
.option("lowerBound", "1")
.option("upperBound", "5000000")


## Spark 源码解读

Spark 版本：spark-sql_2.11-2.2.1

package org.apache.spark.sql.execution.datasources.jdbc

/**
* Instructions on how to partition the table among workers.
*/
private[sql] case class JDBCPartitioningInfo(
column: String,
lowerBound: Long,
upperBound: Long,
numPartitions: Int)

private[sql] object JDBCRelation extends Logging {
/**
* Given a partitioning schematic (a column of integral type, a number of
* partitions, and upper and lower bounds on the column's value), generate
* WHERE clauses for each partition so that each row in the table appears
* exactly once.  The parameters minValue and maxValue are advisory in that
* incorrect values may cause the partitioning to be poor, but no data
* will fail to be represented.
*
* Null value predicate is added to the first partition where clause to include
* the rows with null value for the partitions column.
*
* @param partitioning partition information to generate the where clause for each partition
* @return an array of partitions with where clause for each partition
*/
def columnPartition(partitioning: JDBCPartitioningInfo): Array[Partition] = {
if (partitioning == null || partitioning.numPartitions <= 1 ||
partitioning.lowerBound == partitioning.upperBound) {
return Array[Partition](JDBCPartition(null, 0))
}

val lowerBound = partitioning.lowerBound
val upperBound = partitioning.upperBound
require (lowerBound <= upperBound,
"Operation not allowed: the lower bound of partitioning column is larger than the upper " +
s"bound. Lower bound: $lowerBound; Upper bound:$upperBound")

/**
* 笔者注：
* 从这里可以看出upperBound和lowerBound的差如果小于你指定的numPartitions
* 那么实际的分区数其实是 upperBound-lowerBound
*/
val numPartitions =
if ((upperBound - lowerBound) >= partitioning.numPartitions || /* check for overflow */
(upperBound - lowerBound) < 0) {
partitioning.numPartitions
} else {
logWarning("The number of partitions is reduced because the specified number of " +
"partitions is less than the difference between upper bound and lower bound. " +
s"Updated number of partitions: ${upperBound - lowerBound}; Input number of " + s"partitions:${partitioning.numPartitions}; Lower bound: $lowerBound; " + s"Upper bound:$upperBound.")
upperBound - lowerBound
}
// Overflow and silliness can happen if you subtract then divide.
// Here we get a little roundoff, but that's (hopefully) OK.
// 笔者注：分区的跨度
val stride: Long = upperBound / numPartitions - lowerBound / numPartitions
val column = partitioning.column
var i: Int = 0
var currentValue: Long = lowerBound
var ans = new ArrayBuffer[Partition]()
/**
* 笔者注：
* 假设指定partitionColumn = 32，且partitionColumn - lowerBound >= partitionColumn。
* 对于 partition_i，构造条件语句如下：
* partition_0 : whereClause = $partitionColumn <$lowerBound + $stride or$partitionColumn is null
* partition_1~30 : whereClause = $partitionColumn >=$lowerBound + $stride * i AND$partitionColumn < $lowerBound +$stride * (i + 1)
* partition_31 : whereClause = $partitionColumn >=$lowerBound + $stride * i * 注意：$lowerBound + $stride * 31 并不一定等于$upperBound - $stride */ while (i < numPartitions) { // 笔者注：构造分区下界条件语句，若是第一个分区(partition0)，下界条件为null val lBound = if (i != 0) s"$column >= $currentValue" else null currentValue += stride // 笔者注：构造分区上界条件语句，若是最后一个分区，上界条件为null val uBound = if (i != numPartitions - 1) s"$column < $currentValue" else null val whereClause = if (uBound == null) { lBound } else if (lBound == null) { s"$uBound or $column is null" } else { s"$lBound AND $uBound" } ans += JDBCPartition(whereClause, i) i = i + 1 } ans.toArray } }  这下清楚了，如果你指定的partitionColumn不是连续的数(分布不均匀)，那么每个task中的数据量就会分配不均匀。 如果不用numPartitionspartitionColumn, lowerBound, upperBound，就不能提高task并发量了吗？其实不然。我们可以通过dbtable构造自己的子查询，并行执行多个查询得到多个结果RDD，最后通过reduce合并成一个RDD，这样查询的速度也是很快的。大概思路如下： // 为了不丢失数据，向上取整，将数据分成32份 val stride = Math.ceil(1384288 / 32) val tableName = "TABLE" // 创建32个task val registerDF = Range(0, 32) .map { index => sparkSession .read .format("jdbc") .option("url", jdbcProps.getProperty("url")) .option("dbtable", s"(SELECT * FROM (SELECT a.*, rownum as rn FROM$tableName a) b WHERE b.rn > ${stride * index} AND b.rn <=${stride * (index + 1)})")
.option("user", jdbcProps.getProperty("user"))