diff --git a/src/main/java/org/apache/sysds/runtime/compress/colgroup/mapping/MapToChar.java b/src/main/java/org/apache/sysds/runtime/compress/colgroup/mapping/MapToChar.java
index 50e327ba5a2..ba5f00d8cc0 100644
--- a/src/main/java/org/apache/sysds/runtime/compress/colgroup/mapping/MapToChar.java
+++ b/src/main/java/org/apache/sysds/runtime/compress/colgroup/mapping/MapToChar.java
@@ -168,7 +168,7 @@ protected static MapToChar readFields(DataInput in) throws IOException {
 		final int length = in.readInt();
 		final char[] data = new char[length];
 		for(int i = 0; i < length; i++)
-			data[i] = in.readChar();
+			data[i] = (char)in.readUnsignedShort();
 		return new MapToChar(unique, data);
 	}
 
diff --git a/src/main/java/org/apache/sysds/runtime/frame/data/columns/Array.java b/src/main/java/org/apache/sysds/runtime/frame/data/columns/Array.java
index fd77370f47d..00afcec68f0 100644
--- a/src/main/java/org/apache/sysds/runtime/frame/data/columns/Array.java
+++ b/src/main/java/org/apache/sysds/runtime/frame/data/columns/Array.java
@@ -20,20 +20,27 @@
 package org.apache.sysds.runtime.frame.data.columns;
 
 import java.lang.ref.SoftReference;
+import java.util.ArrayList;
+import java.util.Collections;
 import java.util.HashMap;
 import java.util.Iterator;
+import java.util.List;
 import java.util.Map;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Future;
 
 import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
 import org.apache.hadoop.io.Writable;
 import org.apache.sysds.common.Types.ValueType;
+import org.apache.sysds.hops.OptimizerUtils;
 import org.apache.sysds.runtime.DMLRuntimeException;
 import org.apache.sysds.runtime.compress.colgroup.mapping.AMapToData;
 import org.apache.sysds.runtime.compress.estim.sample.SampleEstimatorFactory;
 import org.apache.sysds.runtime.frame.data.columns.ArrayFactory.FrameArrayType;
 import org.apache.sysds.runtime.frame.data.compress.ArrayCompressionStatistics;
 import org.apache.sysds.runtime.matrix.data.Pair;
+import org.apache.sysds.runtime.util.CommonThreadPool;
 
 /**
  * Generic, resizable native arrays for the internal representation of the columns in the FrameBlock. We use this custom
@@ -119,12 +126,58 @@ public synchronized final Map<T, Long> getRecodeMap(int estimate) {
 	 * @return The recode map
 	 */
 	protected Map<T, Long> createRecodeMap(int estimate) {
-		final Map<T, Long> map = new HashMap<>((int)Math.min((long)estimate *2, size()));
+		// final Map<T, Long> map = new HashMap<>((int)Math.min((long)estimate *2, size()));
+		final int s = size();		
+		int k = OptimizerUtils.getTransformNumThreads();
+		if(k <= 1 || s < 10000)
+			return createRecodeMap(estimate, 0, s);
+		else
+			return parallelCreateRecodeMap(estimate, s, k);
+	}
+
+	private Map<T, Long> parallelCreateRecodeMap(int estimate, final int s, int k) {
+		final ExecutorService pool = CommonThreadPool.get(k);
+		try{
+
+			final int blk = Math.max(10000, (s + k) / k);
+			List<Future<Map<T,Long>>> tasks = new ArrayList<>();
+			for(int i = 0; i < s; i+= blk){
+				final int start = i;
+				final int end = Math.min(i + blk, s);
+				tasks.add(pool.submit(() -> createRecodeMap(estimate, start, end)));
+			}
+			final Map<T, Long> map = tasks.get(0).get();
+			for(int i = 1; i < tasks.size(); i++){
+				final Map<T, Long> map2 = tasks.get(i).get();
+				mergeRecodeMaps(map, map2);
+			}
+			return map;
+		}
+		catch(Exception e){
+			throw new RuntimeException(e);
+		}
+		finally{
+			pool.shutdown();
+		}
+	}
+
+	private void mergeRecodeMaps( Map<T, Long> target,  Map<T, Long> from){
+		List<T> fromEntriesOrdered = new ArrayList<>(Collections.nCopies(from.size(), null));
+		for(Map.Entry<T,Long> e : from.entrySet())
+			fromEntriesOrdered.set(e.getValue().intValue(), e.getKey());
+		long id = target.size();
+		for(T e : fromEntriesOrdered){
+			Long v = target.putIfAbsent(e, id );
+			if(v == null)
+				id++;
+		}
+	}
+
+	private Map<T, Long> createRecodeMap(final int estimate, final int s, final int e) {
+		final Map<T, Long> map = new HashMap<>((int)Math.min((long)estimate *2, e-s));
 		long id = 1;
-		final int s = size();
-		for(int i = 0; i < s; i++)
+		for(int i = s; i < e; i++)
 			id = addValRecodeMap(map, id, i);
-		
 		return map;
 	}
 
diff --git a/src/main/java/org/apache/sysds/runtime/frame/data/columns/OptionalArray.java b/src/main/java/org/apache/sysds/runtime/frame/data/columns/OptionalArray.java
index ba0197a6aa0..ebdf6327654 100644
--- a/src/main/java/org/apache/sysds/runtime/frame/data/columns/OptionalArray.java
+++ b/src/main/java/org/apache/sysds/runtime/frame/data/columns/OptionalArray.java
@@ -524,10 +524,8 @@ public void setM(Map<T, Long> map, int si, AMapToData m, int i) {
 
 	@Override
 	protected long addValRecodeMap(Map<T, Long> map, long id, int i) {
-
-		if(_n.get(i)) {
+		if(_n.get(i))
 			id = _a.addValRecodeMap(map, id, i);
-		}
 		return id;
 	}