use postgresql "upsert" using writable CTE (psql 9.1 feature)

rss-update.lua

        -- use postgresql "upsert" using writable CTE (psql 9.1 feature)
        local sql = [[
WITH new_values (rss_feed, guid, title, url, pubDate, content) AS (
  VALUES
     ]]..sprintf('(%s, %s, %s, %s, %s::timestamp, %s)', rss_feed, quote(guid), quote(e.title), quote(e.link), quote(e.updated), quote(content))..[[
),
upsert as
(
    UPDATE rss_item m
        SET rss_feed = nv.rss_feed,
            guid = nv.guid,
            title = nv.title,
            url = nv.url,
            pubDate = nv.pubDate,
            content = nv.content
    FROM new_values nv
    WHERE m.rss_feed = nv.rss_feed
    AND m.guid = nv.guid
    RETURNING m.*
)
INSERT INTO rss_item (rss_feed, guid, title, url, pubDate, content)
SELECT rss_feed, guid, title, url, pubDate, content
FROM new_values
WHERE NOT EXISTS (SELECT 1
                  FROM upsert up
                  WHERE up.rss_feed = new_values.rss_feed
                  AND up.guid = new_values.guid)
]]
        local res = db.dbreq(sql)