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diff --git a/storage/mozIStorageAggregateFunction.idl b/storage/mozIStorageAggregateFunction.idl
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+++ b/storage/mozIStorageAggregateFunction.idl
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+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsISupports.idl"
+
+interface mozIStorageConnection;
+interface mozIStorageValueArray;
+interface nsIArray;
+interface nsIVariant;
+
+/**
+ * mozIStorageAggregateFunction represents aggregate SQL function.
+ * Common examples of aggregate functions are SUM() and COUNT().
+ *
+ * An aggregate function calculates one result for a given set of data, where
+ * a set of data is a group of tuples. There can be one group
+ * per request or many of them, if GROUP BY clause is used or not.
+ */
+[scriptable, uuid(763217b7-3123-11da-918d-000347412e16)]
+interface mozIStorageAggregateFunction : nsISupports {
+  /**
+   * onStep is called when next value should be passed to
+   * a custom function.
+   * 
+   * @param aFunctionArguments    The arguments passed in to the function
+   */
+  void onStep(in mozIStorageValueArray aFunctionArguments);
+
+  /**
+   * Called when all tuples in a group have been processed and the engine
+   * needs the aggregate function's value.
+   *
+   * @returns aggregate result as Variant.
+   */
+  nsIVariant onFinal();
+};