Add some comments and reuse variable

h2/src/main/org/h2/expression/aggregate/AbstractAggregate.java

@@ -76,21 +76,26 @@ public abstract class AbstractAggregate extends DataAnalysisOperation {
             int rowIdColumn) {
         WindowFrame frame = over.getWindowFrame();
         if (frame == null || frame.isDefault()) {
+            // Aggregate all values before the current row (including)
             Object aggregateData = createAggregateData();
             for (Value[] row : ordered) {
+                // Collect values one by one
                 updateFromExpressions(session, aggregateData, row);
                 result.put(row[rowIdColumn].getInt(), getAggregatedValue(session, aggregateData));
             }
         } else if (frame.isFullPartition()) {
+            // Aggregate values from the whole partition
             Object aggregateData = createAggregateData();
             for (Value[] row : ordered) {
                 updateFromExpressions(session, aggregateData, row);
             }
+            // All rows have the same value
             Value value = getAggregatedValue(session, aggregateData);
             for (Value[] row : ordered) {
                 result.put(row[rowIdColumn].getInt(), value);
             }
         } else {
+            // All other types of frames (slow)
             int size = ordered.size();
             for (int i = 0; i < size; i++) {
                 Object aggregateData = createAggregateData();

h2/src/main/org/h2/expression/aggregate/DataAnalysisOperation.java

@@ -244,6 +244,16 @@ public abstract class DataAnalysisOperation extends Expression {
                 : getWindowResult(session, groupData);
     }
 
+    /**
+     * Returns result of this window function or window aggregate. This method
+     * is not used for plain aggregates.
+     *
+     * @param session
+     *            the session
+     * @param groupData
+     *            the group data
+     * @return result of this function
+     */
     private Value getWindowResult(Session session, SelectGroups groupData) {
         PartitionData partition;
         Object data;
@@ -251,15 +261,17 @@ public abstract class DataAnalysisOperation extends Expression {
         ValueArray key = over.getCurrentKey(session);
         partition = groupData.getWindowExprData(this, key);
         if (partition == null) {
+            // Window aggregates with FILTER clause may have no collected values
             data = forOrderBy ? new ArrayList<>() : createAggregateData();
             partition = new PartitionData(data);
             groupData.setWindowExprData(this, key, partition);
         } else {
             data = partition.getData();
         }
-        if (over.getOrderBy() != null || !isAggregate()) {
+        if (forOrderBy || !isAggregate()) {
             return getOrderedResult(session, groupData, partition, data);
         }
+        // Window aggregate without ORDER BY clause in window specification
         Value result = partition.getResult();
         if (result == null) {
             result = getAggregatedValue(session, data);