Fix DISTINCT ON with ORDER BY

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

@@ -1825,7 +1825,7 @@ public class Select extends Query {
 
     @Override
     public boolean allowGlobalConditions() {
-        return offsetExpr == null && (limitExpr == null || sort == null);
+        return offsetExpr == null && (limitExpr == null && distinctExpressions == null || sort == null);
     }
 
     public SortOrder getSortOrder() {

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

@@ -56,7 +56,12 @@ class MVSortedTempResult extends MVTempResult {
      * {@link #contains(Value[])} method is invoked. Only the root result should
      * have an index if required.
      */
-    private MVMap<ValueRow, Boolean> index;
+    private MVMap<ValueRow, Object> index;
+
+    /**
+     * Used for DISTINCT ON in presence of ORDER BY.
+     */
+    private ValueDataType orderedDistinctOnType;
 
     /**
      * Cursor for the {@link #next()} method.
@@ -172,7 +177,11 @@ class MVSortedTempResult extends MVTempResult {
         if (distinct && length != visibleColumnCount || distinctIndexes != null) {
             int count = distinctIndexes != null ? distinctIndexes.length : visibleColumnCount;
             ValueDataType distinctType = new ValueDataType(database, new int[count]);
-            Builder<ValueRow, Boolean> indexBuilder = new MVMap.Builder<ValueRow, Boolean>().keyType(distinctType);
+            Builder<ValueRow, Object> indexBuilder = new MVMap.Builder<ValueRow, Object>().keyType(distinctType);
+            if (distinctIndexes != null && sort != null) {
+                indexBuilder.valueType(keyType);
+                orderedDistinctOnType = keyType;
+            }
             index = store.openMap("idx", indexBuilder);
         }
     }
@@ -189,8 +198,21 @@ class MVSortedTempResult extends MVTempResult {
                     newValues[i] = values[distinctIndexes[i]];
                 }
                 ValueRow distinctRow = ValueRow.get(newValues);
-                if (index.putIfAbsent(distinctRow, true) != null) {
-                    return rowCount;
+                if (orderedDistinctOnType == null) {
+                    if (index.putIfAbsent(distinctRow, true) != null) {
+                        return rowCount;
+                    }
+                } else {
+                    ValueRow previous = (ValueRow) index.get(distinctRow);
+                    if (previous == null) {
+                        index.put(distinctRow, key);
+                    } else if (orderedDistinctOnType.compare(previous, key) > 0) {
+                        map.remove(previous);
+                        rowCount--;
+                        index.put(distinctRow, key);
+                    } else {
+                        return rowCount;
+                    }
                 }
             } else if (expressions.length != visibleColumnCount) {
                 ValueRow distinctRow = ValueRow.get(Arrays.copyOf(values, visibleColumnCount));

h2/src/main/org/h2/result/LocalResultImpl.java

@@ -316,7 +316,8 @@ public class LocalResultImpl implements LocalResult {
         if (isAnyDistinct()) {
             if (distinctRows != null) {
                 ValueRow array = getDistinctRow(values);
-                if (!distinctRows.containsKey(array)) {
+                Value[] previous = distinctRows.get(array);
+                if (previous == null || sort != null && sort.compare(previous, values) > 0) {
                     distinctRows.put(array, values);
                 }
                 rowCount = distinctRows.size();

h2/src/test/org/h2/test/scripts/distinct.sql

@@ -144,6 +144,42 @@ SELECT DISTINCT ON(C1) C2 FROM TEST ORDER BY C1;
 > 4
 > rows (ordered): 3
 
+SELECT DISTINCT ON(C1) C1, C4, C5 FROM TEST ORDER BY C1, C5;
+> C1 C4 C5
+> -- -- --
+> 1  4  5
+> 2  8  9
+> 3  1  1
+> rows (ordered): 3
+
+SELECT DISTINCT ON(C1) C1, C4, C5 FROM TEST ORDER BY C1, C5 DESC;
+> C1 C4 C5
+> -- -- --
+> 1  6  7
+> 2  8  9
+> 3  1  1
+> rows (ordered): 3
+
+SELECT T1.C1, T2.C5 FROM TEST T1 JOIN (
+    SELECT DISTINCT ON(C1) C1, C4, C5 FROM TEST ORDER BY C1, C5
+) T2 ON T1.C4 = T2.C4 ORDER BY T1.C1;
+> C1 C5
+> -- --
+> 1  5
+> 2  9
+> 3  1
+> rows (ordered): 3
+
+SELECT T1.C1, T2.C5 FROM TEST T1 JOIN (
+    SELECT DISTINCT ON(C1) C1, C4, C5 FROM TEST ORDER BY C1, C5 DESC
+) T2 ON T1.C4 = T2.C4 ORDER BY T1.C1;
+> C1 C5
+> -- --
+> 1  7
+> 2  9
+> 3  1
+> rows (ordered): 3
+
 EXPLAIN SELECT DISTINCT ON(C1) C2 FROM TEST ORDER BY C1;
 >> SELECT DISTINCT ON(C1) C2 FROM PUBLIC.TEST /* PUBLIC.TEST.tableScan */ ORDER BY =C1