[SPARK-10648] Oracle dialect to handle nonspecific numeric types #9495

Squashed commit of the following:

commit a1370a7
Author: Travis Hegner <[email protected]>
Date:   Thu Nov 5 11:10:35 2015 -0500

    style cleanup

commit 839bcb5
Author: Travis Hegner <[email protected]>
Date:   Thu Nov 5 10:06:19 2015 -0500

    more attribution

commit 3157ed5
Author: Travis Hegner <[email protected]>
Date:   Thu Nov 5 10:04:09 2015 -0500

    adding canHandle override, and registration of OracleDialect

commit cc764d4
Author: Travis Hegner <[email protected]>
Date:   Thu Nov 5 09:53:43 2015 -0500

    initial attempt

Conflicts:
	sql/core/src/main/scala/org/apache/spark/sql/jdbc/JdbcDialects.scala

Author: yhuai

sql/core/src/main/scala/org/apache/spark/sql/jdbc/JdbcDialects.scala

@@ -125,6 +125,7 @@ object JdbcDialects {
 
   registerDialect(MySQLDialect)
   registerDialect(PostgresDialect)
+  registerDialect(OracleDialect)
 
   /**
    * Fetch the JdbcDialect class corresponding to a given database url.
@@ -222,3 +223,25 @@ case object MySQLDialect extends JdbcDialect {
     s"`$colName`"
   }
 }
+
+/**
+ * :: DeveloperApi ::
+ * Default Oracle dialect, mapping a nonspecific
+ * numeric type to a general decimal type.
+ */
+@DeveloperApi
+case object OracleDialect extends JdbcDialect {
+  override def canHandle(url: String): Boolean = url.startsWith("jdbc:oracle")
+  override def getCatalystType(
+      sqlType: Int, typeName: String, size: Int, md: MetadataBuilder): Option[DataType] = {
+    // Handle NUMBER fields that have no precision/scale in special way
+    // because JDBC ResultSetMetaData converts this to 0 precision and -127 scale
+    if (sqlType == Types.NUMERIC && size == 0) {
+      // This is sub-optimal as we have to pick a precision/scale in advance whereas the data
+      // in Oracle is allowed to have different precision/scale for each value.
+      Some(DecimalType(38, 10))
+    } else {
+      None
+    }
+  }
+}