diff --git a/lib/ecto/query/api.ex b/lib/ecto/query/api.ex
index f161889a84..299144ee0a 100644
--- a/lib/ecto/query/api.ex
+++ b/lib/ecto/query/api.ex
@@ -278,7 +278,8 @@ defmodule Ecto.Query.API do
 
   @doc """
   Applies the given expression as a FILTER clause against an
-  aggregate. This is currently only supported by Postgres.
+  aggregate. Not all databases support this operation. Please
+  check your database documentation.
 
       from p in Payment, select: filter(avg(p.value), p.value > 0 and p.value < 100)
 
diff --git a/lib/ecto/query/window_api.ex b/lib/ecto/query/window_api.ex
index 6e6a900d23..d4fe813e42 100644
--- a/lib/ecto/query/window_api.ex
+++ b/lib/ecto/query/window_api.ex
@@ -160,7 +160,8 @@ defmodule Ecto.Query.WindowAPI do
 
   @doc """
   Applies the given expression as a FILTER clause against an
-  aggregate. This is currently only supported by Postgres.
+  aggregate. Not all databases support this operation. Please
+  check your database documentation.
 
       from p in Post,
            select: avg(p.value)