[SPARK-8359] [SQL] Fix incorrect decimal precision after multiplication

JIRA: https://issues.apache.org/jira/browse/SPARK-8359

Author: Liang-Chi Hsieh <viirya@gmail.com>

Closes #6814 from viirya/fix_decimal2 and squashes the following commits:

071a757 [Liang-Chi Hsieh] Remove maximum precision and use MathContext.UNLIMITED.
df217d4 [Liang-Chi Hsieh] Merge remote-tracking branch 'upstream/master' into fix_decimal2
a43bfc3 [Liang-Chi Hsieh] Add MathContext with maximum supported precision.
72eeb3f [Liang-Chi Hsieh] Merge remote-tracking branch 'upstream/master' into fix_decimal2
44c9348 [Liang-Chi Hsieh] Fix incorrect decimal precision after multiplication.

sql/catalyst/src/main/scala/org/apache/spark/sql/types/Decimal.scala

@@ -17,6 +17,8 @@
 
 package org.apache.spark.sql.types
 
+import java.math.{MathContext, RoundingMode}
+
 import org.apache.spark.annotation.DeveloperApi
 
 /**
@@ -137,9 +139,9 @@ final class Decimal extends Ordered[Decimal] with Serializable {
 
   def toBigDecimal: BigDecimal = {
     if (decimalVal.ne(null)) {
-      decimalVal
+      decimalVal(MathContext.UNLIMITED)
     } else {
-      BigDecimal(longVal, _scale)
+      BigDecimal(longVal, _scale)(MathContext.UNLIMITED)
     }
   }
 

sql/catalyst/src/test/scala/org/apache/spark/sql/types/decimal/DecimalSuite.scala

@@ -162,4 +162,9 @@ class DecimalSuite extends SparkFunSuite with PrivateMethodTester {
     assert(new Decimal().set(100L, 10, 0).toUnscaledLong === 100L)
     assert(Decimal(Long.MaxValue, 100, 0).toUnscaledLong === Long.MaxValue)
   }
+
+  test("accurate precision after multiplication") {
+    val decimal = (Decimal(Long.MaxValue, 38, 0) * Decimal(Long.MaxValue, 38, 0)).toJavaBigDecimal
+    assert(decimal.unscaledValue.toString === "85070591730234615847396907784232501249")
+  }
 }