diff --git a/kinto/core/storage/postgresql/__init__.py b/kinto/core/storage/postgresql/__init__.py
index 283c4c867..275238007 100644
--- a/kinto/core/storage/postgresql/__init__.py
+++ b/kinto/core/storage/postgresql/__init__.py
@@ -77,7 +77,7 @@ class Storage(StorageBase, MigratorMixin):
 
     # MigratorMixin attributes.
     name = "storage"
-    schema_version = 21
+    schema_version = 22
     schema_file = os.path.join(HERE, "schema.sql")
     migrations_directory = os.path.join(HERE, "migrations")
 
@@ -201,7 +201,7 @@ def resource_timestamp(self, resource_name, parent_id):
             FROM objects
             WHERE parent_id = :parent_id
               AND resource_name = :resource_name
-            ORDER BY last_modified DESC
+            ORDER BY as_epoch(last_modified) DESC
             LIMIT 1
           )
           -- Timestamp of empty resource.
diff --git a/kinto/core/storage/postgresql/migrations/migration_021_022.sql b/kinto/core/storage/postgresql/migrations/migration_021_022.sql
new file mode 100644
index 000000000..5eed3f06b
--- /dev/null
+++ b/kinto/core/storage/postgresql/migrations/migration_021_022.sql
@@ -0,0 +1,62 @@
+DROP TRIGGER IF EXISTS tgr_objects_last_modified ON objects;
+
+CREATE OR REPLACE FUNCTION bump_timestamp()
+RETURNS trigger AS $$
+DECLARE
+    previous BIGINT;
+    current BIGINT;
+BEGIN
+    previous := NULL;
+    WITH existing_timestamps AS (
+      -- Timestamp of latest record.
+      (
+        SELECT last_modified
+        FROM objects
+        WHERE parent_id = NEW.parent_id
+          AND resource_name = NEW.resource_name
+        ORDER BY as_epoch(last_modified) DESC
+        LIMIT 1
+      )
+      -- Timestamp when resource was empty.
+      UNION
+      (
+        SELECT last_modified
+        FROM timestamps
+        WHERE parent_id = NEW.parent_id
+          AND resource_name = NEW.resource_name
+      )
+    )
+    SELECT as_epoch(MAX(last_modified)) INTO previous
+      FROM existing_timestamps;
+
+    --
+    -- This bumps the current timestamp to 1 msec in the future if the previous
+    -- timestamp is equal to the current one (or higher if was bumped already).
+    --
+    -- If a bunch of requests from the same user on the same resource
+    -- arrive in the same millisecond, the unicity constraint can raise
+    -- an error (operation is cancelled).
+    -- See https://github.com/mozilla-services/cliquet/issues/25
+    --
+    current := as_epoch(clock_timestamp()::TIMESTAMP);
+    IF previous IS NOT NULL AND previous >= current THEN
+        current := previous + 1;
+    END IF;
+
+    IF NEW.last_modified IS NULL OR
+       (previous IS NOT NULL AND as_epoch(NEW.last_modified) = previous) THEN
+        -- If record does not carry last-modified, or if the one specified
+        -- is equal to previous, assign it to current (i.e. bump it).
+        NEW.last_modified := from_epoch(current);
+    END IF;
+
+    RETURN NEW;
+END;
+$$ LANGUAGE plpgsql;
+
+CREATE TRIGGER tgr_objects_last_modified
+BEFORE INSERT OR UPDATE OF data ON objects
+FOR EACH ROW EXECUTE PROCEDURE bump_timestamp();
+
+-- Bump storage schema version.
+INSERT INTO metadata (name, value) VALUES ('storage_schema_version', '22');
diff --git a/kinto/core/storage/postgresql/schema.sql b/kinto/core/storage/postgresql/schema.sql
index 47030838d..d0a9a4413 100644
--- a/kinto/core/storage/postgresql/schema.sql
+++ b/kinto/core/storage/postgresql/schema.sql
@@ -64,8 +64,8 @@ DROP TRIGGER IF EXISTS tgr_objects_last_modified ON objects;
 CREATE OR REPLACE FUNCTION bump_timestamp()
 RETURNS trigger AS $$
 DECLARE
-    previous TIMESTAMP;
-    current TIMESTAMP;
+    previous BIGINT;
+    current BIGINT;
 BEGIN
     previous := NULL;
     WITH existing_timestamps AS (
@@ -75,7 +75,7 @@ BEGIN
         FROM objects
         WHERE parent_id = NEW.parent_id
           AND resource_name = NEW.resource_name
-        ORDER BY last_modified DESC
+        ORDER BY as_epoch(last_modified) DESC
         LIMIT 1
       )
       -- Timestamp when resource was empty.
@@ -87,7 +87,7 @@ BEGIN
           AND resource_name = NEW.resource_name
       )
     )
-    SELECT MAX(last_modified) INTO previous
+    SELECT as_epoch(MAX(last_modified)) INTO previous
       FROM existing_timestamps;
 
     --
@@ -99,16 +99,16 @@ BEGIN
     -- an error (operation is cancelled).
     -- See https://github.com/mozilla-services/cliquet/issues/25
     --
-    current := clock_timestamp();
+    current := as_epoch(clock_timestamp()::TIMESTAMP);
     IF previous IS NOT NULL AND previous >= current THEN
-        current := previous + INTERVAL '1 milliseconds';
+        current := previous + 1;
     END IF;
 
     IF NEW.last_modified IS NULL OR
-       (previous IS NOT NULL AND as_epoch(NEW.last_modified) = as_epoch(previous)) THEN
+       (previous IS NOT NULL AND as_epoch(NEW.last_modified) = previous) THEN
         -- If record does not carry last-modified, or if the one specified
         -- is equal to previous, assign it to current (i.e. bump it).
-        NEW.last_modified := current;
+        NEW.last_modified := from_epoch(current);
     END IF;
 
     RETURN NEW;
@@ -131,4 +131,4 @@ INSERT INTO metadata (name, value) VALUES ('created_at', NOW()::TEXT);
 
 -- Set storage schema version.
 -- Should match ``kinto.core.storage.postgresql.PostgreSQL.schema_version``
-INSERT INTO metadata (name, value) VALUES ('storage_schema_version', '21');
+INSERT INTO metadata (name, value) VALUES ('storage_schema_version', '22');