fix: dictionary decimal vector optimization

common/src/main/java/org/apache/comet/vector/CometDictionary.java

@@ -19,7 +19,9 @@
 
 package org.apache.comet.vector;
 
+import org.apache.arrow.vector.DecimalVector;
 import org.apache.arrow.vector.ValueVector;
+import org.apache.spark.sql.types.Decimal;
 import org.apache.spark.unsafe.types.UTF8String;
 
 /** A dictionary which maps indices (integers) to values. */
@@ -100,17 +102,21 @@ public void close() {
   }
 
   private void initialize() {
-    switch (values.getValueVector().getMinorType()) {
+    ValueVector vector = values.getValueVector();
+    switch (vector.getMinorType()) {
       case DECIMAL:
-        // We only need to copy values for decimal type as random access
-        // to the dictionary is not efficient for decimal (it needs to copy
-        // the value to a new byte array everytime).
-        binaries = new ByteArrayWrapper[numValues];
-        for (int i = 0; i < numValues; i++) {
-          // Need copying here since we re-use byte array for decimal
-          byte[] bytes = new byte[DECIMAL_BYTE_WIDTH];
-          bytes = values.copyBinaryDecimal(i, bytes);
-          binaries[i] = new ByteArrayWrapper(bytes);
+        if (values.useDecimal128
+            || ((DecimalVector) vector).getPrecision() > Decimal.MAX_INT_DIGITS()) {
+          // We only need to copy values for decimal 128 type as random access
+          // to the dictionary is not efficient for decimal (it needs to copy
+          // the value to a new byte array everytime).
+          binaries = new ByteArrayWrapper[numValues];
+          for (int i = 0; i < numValues; i++) {
+            // Need copying here since we re-use byte array for decimal
+            byte[] bytes = new byte[DECIMAL_BYTE_WIDTH];
+            bytes = values.copyBinaryDecimal(i, bytes);
+            binaries[i] = new ByteArrayWrapper(bytes);
+          }
         }
         break;
     }