[SPARK-13790] Speed up ColumnVector's getDecimal

We should reuse an object similar to the other non-primitive type getters. For
a query that computes averages over decimal columns, this shows a 10% speedup
on overall query times.

TPCDS Snappy:                       Best/Avg Time(ms)    Rate(M/s)   Per Row(ns)
--------------------------------------------------------------------------------
q27-agg (master)                       10627 / 11057         10.8          92.3
q27-agg (this patch)                     9722 / 9832         11.8          84.4

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

@@ -68,6 +68,13 @@ final class Decimal extends Ordered[Decimal] with Serializable {
     this
   }
 
+  /**
+   * Just updates the underlying value to `v`, assuming precision and scale is unchanged.
+   */
+  def setInternal(v: Long): Unit = {
+    this.longVal = v
+  }
+
   /**
    * Set this Decimal to the given unscaled Long, with a given precision and scale.
    */

sql/core/src/main/java/org/apache/spark/sql/execution/vectorized/ColumnVector.java

@@ -536,9 +536,13 @@ private Array getByteArray(int rowId) {
    */
   public final Decimal getDecimal(int rowId, int precision, int scale) {
     if (precision <= Decimal.MAX_INT_DIGITS()) {
-      return Decimal.apply(getInt(rowId), precision, scale);
+      assert(resultDecimal != null);
+      resultDecimal.setInternal(getInt(rowId));
+      return resultDecimal;
     } else if (precision <= Decimal.MAX_LONG_DIGITS()) {
-      return Decimal.apply(getLong(rowId), precision, scale);
+      assert (resultDecimal != null);
+      resultDecimal.setInternal(getLong(rowId));
+      return resultDecimal;
     } else {
       // TODO: best perf?
       byte[] bytes = getBinary(rowId);
@@ -852,6 +856,11 @@ public final int appendStruct(boolean isNull) {
    */
   protected final ColumnarBatch.Row resultStruct;
 
+  /**
+   * Reusable object for getDecimal()
+   */
+  private Decimal resultDecimal;
+
   /**
    * The Dictionary for this column.
    *
@@ -927,5 +936,12 @@ protected ColumnVector(int capacity, DataType type, MemoryMode memMode) {
       this.resultArray = null;
       this.resultStruct = null;
     }
+
+    if (type instanceof DecimalType) {
+      DecimalType dt = (DecimalType)type;
+      if (dt.precision() <= Decimal.MAX_LONG_DIGITS()) {
+        resultDecimal = Decimal.apply(0, dt.precision(), dt.scale());
+      }
+    }
   }
 }