diff --git a/h/migrations/versions/78d3d6fe1d42_metadata_unique_constraint.py b/h/migrations/versions/78d3d6fe1d42_metadata_unique_constraint.py
new file mode 100644
index 00000000000..53cd44d9798
--- /dev/null
+++ b/h/migrations/versions/78d3d6fe1d42_metadata_unique_constraint.py
@@ -0,0 +1,39 @@
+"""Add missing unique constraint on annotation_medatada."""
+
+import sqlalchemy as sa
+from alembic import op
+
+revision = "78d3d6fe1d42"
+down_revision = "a122e276f8d1"
+
+
+def upgrade():
+    conn = op.get_bind()
+
+    # CONCURRENTLY can't be used inside a transaction. Finish the current one.
+    op.execute("COMMIT;")
+
+    # First create the index concurrently to avoid locking the table
+    conn.execute(
+        sa.text(
+            """
+        CREATE UNIQUE INDEX CONCURRENTLY ix__annotation_metadata__annotation_id ON annotation_metadata (annotation_id);
+    """
+        )
+    )
+    # Once the index is created, use it to create the unique constraint
+    conn.execute(
+        sa.text(
+            """
+            ALTER TABLE annotation_metadata ADD CONSTRAINT uq__annotation_metadata__annotation_id UNIQUE USING INDEX ix__annotation_metadata__annotation_id
+    """
+        )
+    )
+
+
+def downgrade():
+    op.drop_constraint(
+        op.f("uq__annotation_metadata__annotation_id"),
+        "annotation_metadata",
+        type_="unique",
+    )