[SPARK-9060] [SQL] Revert SPARK-8359, SPARK-8800, and SPARK-8677

JIRA: https://issues.apache.org/jira/browse/SPARK-9060

This PR reverts:
* apache@31bd306 (SPARK-8359)
* apache@24fda73 (SPARK-8677)
* apache@4b5cfc9 (SPARK-8800)

Author: Yin Huai <[email protected]>

Closes apache#7426 from yhuai/SPARK-9060 and squashes the following commits:

651264d [Yin Huai] Revert "[SPARK-8359] [SQL] Fix incorrect decimal precision after multiplication"
cfda7e4 [Yin Huai] Revert "[SPARK-8677] [SQL] Fix non-terminating decimal expansion for decimal divide operation"
2de9afe [Yin Huai] Revert "[SPARK-8800] [SQL] Fix inaccurate precision/scale of Decimal division operation"

Author: yhuai

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

@@ -17,8 +17,6 @@
 
 package org.apache.spark.sql.types
 
-import java.math.{MathContext, RoundingMode}
-
 import org.apache.spark.annotation.DeveloperApi
 
 /**
@@ -138,14 +136,6 @@ final class Decimal extends Ordered[Decimal] with Serializable {
   }
 
   def toBigDecimal: BigDecimal = {
-    if (decimalVal.ne(null)) {
-      decimalVal(MathContext.UNLIMITED)
-    } else {
-      BigDecimal(longVal, _scale)(MathContext.UNLIMITED)
-    }
-  }
-
-  def toLimitedBigDecimal: BigDecimal = {
     if (decimalVal.ne(null)) {
       decimalVal
     } else {
@@ -273,15 +263,8 @@ final class Decimal extends Ordered[Decimal] with Serializable {
 
   def * (that: Decimal): Decimal = Decimal(toBigDecimal * that.toBigDecimal)
 
-  def / (that: Decimal): Decimal = {
-    if (that.isZero) {
-      null
-    } else {
-      // To avoid non-terminating decimal expansion problem, we get scala's BigDecimal with limited
-      // precision and scala.
-      Decimal(toLimitedBigDecimal / that.toLimitedBigDecimal)
-    }
-  }
+  def / (that: Decimal): Decimal =
+    if (that.isZero) null else Decimal(toBigDecimal / that.toBigDecimal)
 
   def % (that: Decimal): Decimal =
     if (that.isZero) null else Decimal(toBigDecimal % that.toBigDecimal)

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

@@ -171,22 +171,4 @@ 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")
-  }
-
-  test("fix non-terminating decimal expansion problem") {
-    val decimal = Decimal(1.0, 10, 3) / Decimal(3.0, 10, 3)
-    // The difference between decimal should not be more than 0.001.
-    assert(decimal.toDouble - 0.333 < 0.001)
-  }
-
-  test("fix loss of precision/scale when doing division operation") {
-    val a = Decimal(2) / Decimal(3)
-    assert(a.toDouble < 1.0 && a.toDouble > 0.6)
-    val b = Decimal(1) / Decimal(8)
-    assert(b.toDouble === 0.125)
-  }
 }