Descending indexes on MVStore tables did not work properly.

h2/src/main/org/h2/mvstore/db/ValueDataType.java

@@ -119,8 +119,17 @@ public class ValueDataType implements DataType {
         if (a == b) {
             return 0;
         }
-        boolean aNull = a == null || a == ValueNull.INSTANCE;
-        boolean bNull = b == null || b == ValueNull.INSTANCE;
+        // null is never stored;
+        // comparison with null is used to retrieve all entries
+        // in which case null is always lower than all entries
+        // (even for descending ordered indexes)
+        if (a == null) {
+            return -1;
+        } else if (b == null) {
+            return 1;
+        }
+        boolean aNull = a == ValueNull.INSTANCE;
+        boolean bNull = b == ValueNull.INSTANCE;
         if (aNull || bNull) {
             return SortOrder.compareNull(aNull, sortType);
         }

h2/src/test/org/h2/test/testScript.sql

@@ -3,6 +3,24 @@
 -- Initial Developer: H2 Group
 --
 --- special grammar and test cases ---------------------------------------------------------------------------------------------
+create table test(a int, b int);
+> ok
+
+insert into test values(1, 1);
+> update count: 1
+
+create index on test(a, b desc);
+> ok
+
+select * from test where a = 1;
+> A B
+> - -
+> 1 1
+> rows: 1
+
+drop table test;
+> ok
+
 create table test(id int, name varchar) as select 1, 'a';
 > ok