WITH traffic_sources AS (
SELECT
EXTRACT(MONTH FROM user_visits.visited_at) AS month,
incoming_referers.incoming_domain_id,
COUNT(*) AS visit_count,
LAG(COUNT(*)) OVER (PARTITION BY incoming_referers.incoming_domain_id ORDER BY EXTRACT(MONTH FROM user_visits.visited_at)) AS prev_month_visit_count
FROM user_visits
JOIN incoming_referers
ON user_visits.user_id = incoming_referers.id
WHERE user_visits.visited_at IS NOT NULL
GROUP BY EXTRACT(MONTH FROM user_visits.visited_at), incoming_referers.incoming_domain_id
)
SELECT
month,
incoming_domain_id,
visit_count,
prev_month_visit_count,
visit_count - COALESCE(prev_month_visit_count, 0) AS change
FROM traffic_sources
ORDER BY month, incoming_domain_id
Hola, acabo de volver de cenar y actualicé la consulta; podrías probarla, si tienes algún avance, házmelo saber 
. Mi servidor de Discourse es uno recién creado, así que no hay datos suficientes para consultar. Creé algunos datos ficticios en mi SQL Server y luego los transferí a PGSQL.
