[SPARK-7299][SQL] Set precision and scale for Decimal according to JDBC metadata instead of returned BigDecimal

JIRA: https://issues.apache.org/jira/browse/SPARK-7299

When connecting with oracle db through jdbc, the precision and scale of `BigDecimal` object returned by `ResultSet.getBigDecimal` is not correctly matched to the table schema reported by `ResultSetMetaData.getPrecision` and `ResultSetMetaData.getScale`.

So in case you insert a value like `19999` into a column with `NUMBER(12, 2)` type, you get through a `BigDecimal` object with scale as 0. But the dataframe schema has correct type as `DecimalType(12, 2)`. Thus, after you save the dataframe into parquet file and then retrieve it, you will get wrong result `199.99`.

Because it is reported to be problematic on jdbc connection with oracle db. It might be difficult to add test case for it. But according to the user's test on JIRA, it solves this problem.

Author: Liang-Chi Hsieh <[email protected]>

Closes apache#5833 from viirya/jdbc_decimal_precision and squashes the following commits:

69bc2b5 [Liang-Chi Hsieh] Merge remote-tracking branch 'upstream/master' into jdbc_decimal_precision
928f864 [Liang-Chi Hsieh] Add comments.
5f9da94 [Liang-Chi Hsieh] Set up Decimal's precision and scale according to table schema instead of returned BigDecimal.

Author: viirya

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

@@ -300,7 +300,7 @@ private[sql] class JDBCRDD(
   abstract class JDBCConversion
   case object BooleanConversion extends JDBCConversion
   case object DateConversion extends JDBCConversion
-  case object DecimalConversion extends JDBCConversion
+  case class  DecimalConversion(precisionInfo: Option[(Int, Int)]) extends JDBCConversion
   case object DoubleConversion extends JDBCConversion
   case object FloatConversion extends JDBCConversion
   case object IntegerConversion extends JDBCConversion
@@ -317,8 +317,8 @@ private[sql] class JDBCRDD(
     schema.fields.map(sf => sf.dataType match {
       case BooleanType           => BooleanConversion
       case DateType              => DateConversion
-      case DecimalType.Unlimited => DecimalConversion
-      case DecimalType.Fixed(d)  => DecimalConversion
+      case DecimalType.Unlimited => DecimalConversion(None)
+      case DecimalType.Fixed(d)  => DecimalConversion(Some(d))
       case DoubleType            => DoubleConversion
       case FloatType             => FloatConversion
       case IntegerType           => IntegerConversion
@@ -375,7 +375,22 @@ private[sql] class JDBCRDD(
               } else {
                 mutableRow.update(i, null)
               }
-            case DecimalConversion    =>
+            // When connecting with Oracle DB through JDBC, the precision and scale of BigDecimal
+            // object returned by ResultSet.getBigDecimal is not correctly matched to the table
+            // schema reported by ResultSetMetaData.getPrecision and ResultSetMetaData.getScale.
+            // If inserting values like 19999 into a column with NUMBER(12, 2) type, you get through
+            // a BigDecimal object with scale as 0. But the dataframe schema has correct type as
+            // DecimalType(12, 2). Thus, after saving the dataframe into parquet file and then
+            // retrieve it, you will get wrong result 199.99.
+            // So it is needed to set precision and scale for Decimal based on JDBC metadata.
+            case DecimalConversion(Some((p, s))) =>
+              val decimalVal = rs.getBigDecimal(pos)
+              if (decimalVal == null) {
+                mutableRow.update(i, null)
+              } else {
+                mutableRow.update(i, Decimal(decimalVal, p, s))
+              }
+            case DecimalConversion(None) =>
               val decimalVal = rs.getBigDecimal(pos)
               if (decimalVal == null) {
                 mutableRow.update(i, null)