diff --git a/cube-manager-data/src/main/java/org/gcube/data/analysis/tabulardata/cube/data/SQLDatabaseWrangler.java b/cube-manager-data/src/main/java/org/gcube/data/analysis/tabulardata/cube/data/SQLDatabaseWrangler.java
index 6fe3455..0554939 100644
--- a/cube-manager-data/src/main/java/org/gcube/data/analysis/tabulardata/cube/data/SQLDatabaseWrangler.java
+++ b/cube-manager-data/src/main/java/org/gcube/data/analysis/tabulardata/cube/data/SQLDatabaseWrangler.java
@@ -347,28 +347,32 @@ public class SQLDatabaseWrangler implements DatabaseWrangler {
 	public static class RandomString {
 
 		  private static final char[] symbols;
+		  private static final char[] startingSymbols;
 
 		  static {
 		    StringBuilder tmp = new StringBuilder();
-		    for (char ch = '0'; ch <= '9'; ++ch)
-		      tmp.append(ch);
 		    for (char ch = 'a'; ch <= 'z'; ++ch)
 		      tmp.append(ch);
+		    startingSymbols = tmp.toString().toCharArray();
+		    for (char ch = '0'; ch <= '9'; ++ch)
+			      tmp.append(ch);
 		    symbols = tmp.toString().toCharArray();
 		  }   
 
+		  private final int length;
+		  
 		  private final Random random = new Random();
 
-		  private final char[] buf;
-
 		  public RandomString(int length) {
 		    if (length < 1)
 		      throw new IllegalArgumentException("length < 1: " + length);
-		    buf = new char[length];
+		    this.length = length;
 		  }
 
-		  public String nextString() {
-		    for (int idx = 0; idx < buf.length; ++idx) 
+		  public synchronized String nextString() {
+			char[] buf = new char[this.length];
+			buf[0] = startingSymbols[random.nextInt(startingSymbols.length)];
+			for (int idx = 1; idx < buf.length; ++idx) 
 		      buf[idx] = symbols[random.nextInt(symbols.length)];
 		    return new String(buf);
 		  }