Convert results to distinct where needed in safe way

h2/src/main/org/h2/command/Parser.java

@@ -2535,7 +2535,7 @@ public class Parser {
         }
         currentSelect = temp;
         if (readIf(DISTINCT)) {
-            command.setDistinct(true);
+            command.setDistinct();
         } else {
             readIf(ALL);
         }

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

@@ -255,13 +255,17 @@ public abstract class Query extends Prepared {
 
     /**
      * Set the distinct flag.
-     *
-     * @param b the new value
      */
-    public void setDistinct(boolean b) {
-        distinct = b;
+    public void setDistinct() {
+        distinct = true;
     }
 
+    /**
+     * Set the distinct flag only if it is possible, may be used as a possible
+     * optimization only.
+     */
+    public abstract void setDistinctIfPossible();
+
     public boolean isDistinct() {
         return distinct;
     }

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

@@ -247,6 +247,13 @@ public class Select extends Query {
         return orderList != null || sort != null;
     }
 
+    @Override
+    public void setDistinctIfPossible() {
+        if (!distinct && offsetExpr == null && limitExpr == null) {
+            setDistinct();
+        }
+    }
+
     /**
      * Add a condition to the list of conditions.
      *
@@ -667,9 +674,6 @@ public class Select extends Query {
             result = createLocalResult(result);
             result.setDistinct();
         }
-        if (randomAccessResult) {
-            result = createLocalResult(result);
-        }
         if (isGroupQuery && !isGroupSortedQuery) {
             result = createLocalResult(result);
         }
@@ -724,8 +728,12 @@ public class Select extends Query {
             if (limitRows > 0) {
                 lazyResult.setLimit(limitRows);
             }
+            if (randomAccessResult) {
+                return convertToDistinct(lazyResult);
+            } else {
                 return lazyResult;
             }
+        }
         if (offsetExpr != null) {
             result.setOffset(offsetExpr.getValue(session).getInt());
         }
@@ -734,6 +742,9 @@ public class Select extends Query {
         }
         if (result != null) {
             result.done();
+            if (randomAccessResult && !distinct) {
+                result = convertToDistinct(result);
+            }
             if (target != null) {
                 while (result.next()) {
                     target.addRow(result.currentRow());
@@ -761,6 +772,18 @@ public class Select extends Query {
                 visibleColumnCount);
     }
 
+    private LocalResult convertToDistinct(ResultInterface result) {
+        LocalResult distinctResult = new LocalResult(session, expressionArray, visibleColumnCount);
+        distinctResult.setDistinct();
+        result.reset();
+        while (result.next()) {
+            distinctResult.addRow(result.currentRow());
+        }
+        result.close();
+        distinctResult.done();
+        return distinctResult;
+    }
+
     private void expandColumnList() {
         Database db = session.getDatabase();
 

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

@@ -125,6 +125,11 @@ public class SelectUnion extends Query {
         return orderList != null || sort != null;
     }
 
+    @Override
+    public void setDistinctIfPossible() {
+        setDistinct();
+    }
+
     private Value[] convert(Value[] values, int columnCount) {
         Value[] newValues;
         if (columnCount == values.length) {
@@ -210,8 +215,8 @@ public class SelectUnion extends Query {
             result.setSortOrder(sort);
         }
         if (distinct) {
-            left.setDistinct(true);
-            right.setDistinct(true);
+            left.setDistinctIfPossible();
+            right.setDistinctIfPossible();
             result.setDistinct();
         }
         if (randomAccessResult) {
@@ -220,15 +225,15 @@ public class SelectUnion extends Query {
         switch (unionType) {
         case UNION:
         case EXCEPT:
-            left.setDistinct(true);
-            right.setDistinct(true);
+            left.setDistinctIfPossible();
+            right.setDistinctIfPossible();
             result.setDistinct();
             break;
         case UNION_ALL:
             break;
         case INTERSECT:
-            left.setDistinct(true);
-            right.setDistinct(true);
+            left.setDistinctIfPossible();
+            right.setDistinctIfPossible();
             break;
         default:
             DbException.throwInternalError("type=" + unionType);

h2/src/main/org/h2/expression/ConditionInSelect.java

@@ -43,9 +43,7 @@ public class ConditionInSelect extends Condition {
     @Override
     public Value getValue(Session session) {
         query.setSession(session);
-        if (!query.hasOrder()) {
-            query.setDistinct(true);
-        }
+        query.setDistinctIfPossible();
         ResultInterface rows = query.query(0);
         Value l = left.getValue(session);
         if (!rows.hasNext()) {