diff --git a/sql/upgrades/021.sql b/sql/upgrades/021.sql
new file mode 100644
index 00000000..a0bf5f19
--- /dev/null
+++ b/sql/upgrades/021.sql
@@ -0,0 +1,40 @@
+-- SWH DB schema upgrade
+-- from_version: 20
+-- to_version: 21
+-- description: Add function swh_occurrence_history_add
+
+insert into dbversion(version, release, description)
+      values(21, now(), 'Work In Progress');
+
+CREATE OR REPLACE FUNCTION swh_occurrence_history_add() RETURNS void
+    LANGUAGE plpgsql
+    AS $$
+begin
+    -- Update intervals we have the data to update
+    with new_intervals as (
+        select t.origin, t.branch, t.authority, t.validity,
+	       o.validity - t.validity as new_validity
+	from tmp_occurrence_history t
+        left join occurrence_history o
+        using (origin, branch, authority)
+	where o.origin is not null),
+    -- do not update intervals if they would become empty (perfect overlap)
+    to_update as (
+        select * from new_intervals
+	where not isempty(new_validity))
+    update occurrence_history o set validity = t.new_validity
+    from to_update t
+    where o.origin = t.origin and o.branch = t.branch and o.authority = t.authority;
+
+    -- Now only insert intervals that aren't already present
+    insert into occurrence_history (origin, branch, revision, authority, validity)
+	select distinct origin, branch, revision, authority, validity
+	from tmp_occurrence_history t
+	where not exists (
+	    select 1 from occurrence_history o
+	    where o.origin = t.origin and o.branch = t.branch and
+	          o.authority = t.authority and o.revision = t.revision and
+		  o.validity = t.validity);
+    return;
+end
+$$;