ALTER TABLE message ADD COLUMN IF NOT EXISTS seq BIGINT NOT NULL DEFAULT 0;

WITH ranked AS (
    SELECT id, ROW_NUMBER() OVER (PARTITION BY channel_id ORDER BY created_at ASC, id ASC) AS rn
    FROM message
    WHERE seq = 0
)
UPDATE message m
SET seq = ranked.rn
FROM ranked
WHERE m.id = ranked.id;

CREATE UNIQUE INDEX IF NOT EXISTS idx_message_channel_seq ON message (channel_id, seq);