Github user JustinPihony commented on a diff in the pull request:

    https://github.com/apache/spark/pull/12601#discussion_r77947902
  
    --- Diff: 
sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/jdbc/JdbcRelationProvider.scala
 ---
    @@ -17,39 +17,113 @@
     
     package org.apache.spark.sql.execution.datasources.jdbc
     
    +import java.sql.SQLException
     import java.util.Properties
     
    -import org.apache.spark.sql.SQLContext
    -import org.apache.spark.sql.sources.{BaseRelation, DataSourceRegister, 
RelationProvider}
    +import org.apache.spark.sql.{DataFrame, SaveMode, SQLContext}
    +import org.apache.spark.sql.sources.{BaseRelation, 
CreatableRelationProvider, DataSourceRegister, RelationProvider, 
SchemaRelationProvider}
    +import org.apache.spark.sql.types.StructType
     
    -class JdbcRelationProvider extends RelationProvider with 
DataSourceRegister {
    +class JdbcRelationProvider extends CreatableRelationProvider
    +  with SchemaRelationProvider with RelationProvider with 
DataSourceRegister {
     
       override def shortName(): String = "jdbc"
     
    -  /** Returns a new base relation with the given parameters. */
       override def createRelation(
           sqlContext: SQLContext,
           parameters: Map[String, String]): BaseRelation = {
    +    createRelation(sqlContext, parameters, null)
    +  }
    +
    +  /** Returns a new base relation with the given parameters. */
    +  override def createRelation(
    +      sqlContext: SQLContext,
    +      parameters: Map[String, String],
    +      schema: StructType): BaseRelation = {
         val jdbcOptions = new JDBCOptions(parameters)
    -    if (jdbcOptions.partitionColumn != null
    -      && (jdbcOptions.lowerBound == null
    -        || jdbcOptions.upperBound == null
    -        || jdbcOptions.numPartitions == null)) {
    -      sys.error("Partitioning incompletely specified")
    -    }
    +    val partitionColumn = jdbcOptions.partitionColumn
    +    val lowerBound = jdbcOptions.lowerBound
    +    val upperBound = jdbcOptions.upperBound
    +    val numPartitions = jdbcOptions.numPartitions
     
    -    val partitionInfo = if (jdbcOptions.partitionColumn == null) {
    -      null
    -    } else {
    +    val partitionInfo = if (partitionColumn == null) null
    +    else {
           JDBCPartitioningInfo(
    -        jdbcOptions.partitionColumn,
    -        jdbcOptions.lowerBound.toLong,
    -        jdbcOptions.upperBound.toLong,
    -        jdbcOptions.numPartitions.toInt)
    +        partitionColumn, lowerBound.toLong, upperBound.toLong, 
numPartitions.toInt)
         }
         val parts = JDBCRelation.columnPartition(partitionInfo)
         val properties = new Properties() // Additional properties that we 
will pass to getConnection
         parameters.foreach(kv => properties.setProperty(kv._1, kv._2))
    -    JDBCRelation(jdbcOptions.url, jdbcOptions.table, parts, 
properties)(sqlContext.sparkSession)
    +    JDBCRelation(jdbcOptions.url, jdbcOptions.table, parts, properties,
    +      Option(schema))(sqlContext.sparkSession)
    +  }
    +
    +  /*
    +   * The following structure applies to this code:
    +   *                 |    tableExists            |          !tableExists
    +   
*------------------------------------------------------------------------------------
    +   * Ignore          | BaseRelation              | CreateTable, saveTable, 
BaseRelation
    +   * ErrorIfExists   | ERROR                     | CreateTable, saveTable, 
BaseRelation
    +   * Overwrite*      | (DropTable, CreateTable,) | CreateTable, saveTable, 
BaseRelation
    +   *                 | saveTable, BaseRelation   |
    +   * Append          | saveTable, BaseRelation   | CreateTable, saveTable, 
BaseRelation
    +   *
    +   * *Overwrite & tableExists with truncate, will not drop & create, but 
instead truncate
    +   */
    +  override def createRelation(
    +      sqlContext: SQLContext,
    +      mode: SaveMode,
    +      parameters: Map[String, String],
    +      data: DataFrame): BaseRelation = {
    +    val jdbcOptions = new JDBCOptions(parameters)
    +    val url = jdbcOptions.url
    +    val table = jdbcOptions.table
    +
    +    import collection.JavaConverters._
    +    val props = new Properties()
    +    props.putAll(parameters.asJava)
    +    val conn = JdbcUtils.createConnectionFactory(url, props)()
    +
    +    try {
    +      val tableExists = JdbcUtils.tableExists(conn, url, table)
    +
    +      val (doCreate, doSave) = (mode, tableExists) match {
    --- End diff --
    
    I did add a comment in the method signature. That and the variable naming 
conventions should cover this.


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