[SPARK-26038][BRANCH-2.4] Decimal toScalaBigInt/toJavaBigInteger for decimals not fitting in long

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

@@ -185,9 +185,21 @@ final class Decimal extends Ordered[Decimal] with Serializable {
     }
   }
 
-  def toScalaBigInt: BigInt = BigInt(toLong)
+  def toScalaBigInt: BigInt = {
+    if (decimalVal.ne(null)) {
+      decimalVal.toBigInt()
+    } else {
+      BigInt(toLong)
+    }
+  }
 
-  def toJavaBigInteger: java.math.BigInteger = java.math.BigInteger.valueOf(toLong)
+  def toJavaBigInteger: java.math.BigInteger = {
+    if (decimalVal.ne(null)) {
+      decimalVal.underlying().toBigInteger()
+    } else {
+      java.math.BigInteger.valueOf(toLong)
+    }
+  }
 
   def toUnscaledLong: Long = {
     if (decimalVal.ne(null)) {

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

@@ -228,4 +228,15 @@ class DecimalSuite extends SparkFunSuite with PrivateMethodTester {
     val decimal = Decimal.apply(bigInt)
     assert(decimal.toJavaBigDecimal.unscaledValue.toString === "9223372036854775808")
   }
+
+  test("SPARK-26038: toScalaBigInt/toJavaBigInteger") {
+    // not fitting long
+    val decimal = Decimal("1234568790123456789012348790.1234879012345678901234568790")
+    assert(decimal.toScalaBigInt == scala.math.BigInt("1234568790123456789012348790"))
+    assert(decimal.toJavaBigInteger == new java.math.BigInteger("1234568790123456789012348790"))
+    // fitting long
+    val decimalLong = Decimal(123456789123456789L, 18, 9)
+    assert(decimalLong.toScalaBigInt == scala.math.BigInt("123456789"))
+    assert(decimalLong.toJavaBigInteger == new java.math.BigInteger("123456789"))
+  }
 }