Queries with DISTINCT and ORDER BY will now use the index on ORDER BY if possible.

h2/src/docsrc/html/changelog.html

@@ -18,7 +18,9 @@ Change Log
 <h1>Change Log</h1>
 
 <h2>Next Version (unreleased)</h2>
-<ul><li>The new statement EXPLAIN ANALYZE executes the statement, 
+<ul><li>Queries with DISTINCT and ORDER BY will now use the index on ORDER BY if possible.
+    This is specially important for queries used inside IN(SELECT ...).
+</li><li>The new statement EXPLAIN ANALYZE executes the statement, 
     and displays the query plan with the actual row scan count for each table.
 </li><li>H2 Console: the auto-complete feature didn't quote column names
     that need quoting. Issue 186.

h2/src/main/org/h2/command/dml/Select.java

@@ -531,7 +531,7 @@ public class Select extends Query {
         }
         int columnCount = expressions.size();
         LocalResult result = new LocalResult(session, expressionArray, visibleColumnCount);
-        if (!sortUsingIndex) {
+        if (!sortUsingIndex || distinct) {
             result.setSortOrder(sort);
         }
         if (distinct && !isDistinctQuery) {
@@ -754,10 +754,8 @@ public class Select extends Query {
             Index current = topTableFilter.getIndex();
             if (index != null && (current.getIndexType().isScan() || current == index)) {
                 topTableFilter.setIndex(index);
-                if ((!distinct || isDistinctQuery) && (!topTableFilter.hasInComparisons())) {
-                    // - sort using index would not work correctly for distinct result sets
-                    //   because it would break too early when limit is used
-                    // - in(select ...) and in(1,2,3) indices are unsorted
+                if (!topTableFilter.hasInComparisons()) {
+                    // in(select ...) and in(1,2,3) my return the key is another order
                     sortUsingIndex = true;
                 }
             }