ALTER TABLE posts ADD COLUMN IF NOT EXISTS description text NOT NULL DEFAULT '';
UPDATE posts p SET description=left(COALESCE((
 SELECT a.body FROM post_assets pa JOIN assets a ON a.id=pa.asset_id
 WHERE pa.post_id=p.id AND a.kind='text' AND a.body IS NOT NULL
 ORDER BY pa.position LIMIT 1
),p.title),200) WHERE p.description='';