Spark 添加复用JDBC Schema功能

1) 修改原因

用户需要阅读大量相同的数据库表，比如相同schema的表有1000张（比如对mysql进行分表分库）需要全读，每次建立dataframe的时候需要通过jdbcrelation去查询每一张表的schema，需要消耗了大量时间。本文对提出一种修改办法，如果用户知道表的sechema相同，可以使用sechema复用功能。

2) 代码流程

val df = sqlContext.read().format("jdbc").options(dfOptions).load();

->resolved= ResolvedDataSource(

sqlContext,

userSpecifiedSchema =userSpecifiedSchema,

partitionColumns = Array.empty[String],

provider = source,

options = extraOptions.toMap) //解析数据源，获取jdbc、parquet、josn的schema参数

->dataSource.createRelation(sqlContext,new CaseInsensitiveMap(options)) //传入options

->JDBCRelation(url, table, parts, properties)(sqlContext) //获取jdbc的relation

->override val schema= JDBCRDD.resolveTable(url, table, properties) //获取schema

->conn.prepareStatement(s"SELECT * FROM $table WHERE1=0").executeQuery() //直接读取database，需要优化

3) 修改方法

在用户知道schema的情况下，没有必要重复获取schema；

用户定义是否需要重复使用schema，修改代码流程最小；

修改方法：

a) 用户通过Options传入需要复用schema的开关：

dfOptions.put("jdbcschemakey","sparkourtest");

b) 建立一个hashtable，保存已经获取的shema

val schemaHashTable= newjava.util.HashMap[String,StructType]()

c) schema获取流程：

4) 修改代码

29a30 > import org.apache.spark.Logging 40c41 < private[sql] object JDBCRelation { --- > private[sql] object JDBCRelation{ 48a50,55 > > > // add by Ricky for get same table schema > > val schemaHashTable= new java.util.HashMap[String,StructType]() > 117c124 < private[sql] case class JDBCRelation( --- > private[sql] case class JDBCRelation ( 124c131 < with InsertableRelation { --- > with InsertableRelation with Logging{ 128c135,160 < override val schema: StructType = JDBCRDD.resolveTable(url, table, properties) --- > > // add by Ricky for get same table schema > def getSchema():StructType={ > //val schemaKey = properties.getProperty("jdbcSchemaKey") > val schemaKey = properties.getProperty("jdbcschemakey") > if (schemaKey != null) { > val schemaStored = JDBCRelation.schemaHashTable.get(schemaKey) > if (schemaStored != null) { > schemaStored > } else { > val schemaStored = JDBCRDD.resolveTable(url, table, properties) > logInfo("schemaKey configed,schemaHashTable empty,now put "+schemaKey.toString) > JDBCRelation.schemaHashTable.put(schemaKey, schemaStored) > schemaStored > } > } > else > { > JDBCRDD.resolveTable(url, table, properties) > } > > } > > override val schema: StructType = getSchema() > // end by Ricky > // override val schema: StructType = JDBCRDD.resolveTable(url, table, properties)