Time to first response by group members

I’m assuming you want the average time to first response per group member, if you just want the time it took group members to respond to individual topics, use the fixed version of the first query in the OP:

-- [params]
-- date :start_date
-- date :end_date
-- string :group_name = staff

WITH group_response_times AS (
    SELECT
        t.category_id,
        t.id AS topic_id,
        EXTRACT(EPOCH FROM (p.created_at - t.created_at))/ 60 AS response_time_minutes,
        p.user_id AS staff_user_id,
        ROW_NUMBER() OVER (PARTITION BY t.id ORDER BY p.created_at) AS row_num
    FROM posts p
    JOIN topics t ON t.id = p.topic_id
    WHERE t.user_id NOT IN (SELECT user_id
                                FROM group_users gu JOIN groups g ON g.id = gu.group_id
                                WHERE gu.user_id > 0 AND g.name = :group_name)
        AND p.user_id IN (SELECT user_id
                              FROM group_users gu JOIN groups g ON g.id = gu.group_id
                              WHERE gu.user_id > 0 AND g.name = :group_name)
        AND t.archetype = 'regular'
        AND t.deleted_at IS NULL
        AND p.post_type = 1
        AND p.deleted_at IS NULL
        AND t.created_at BETWEEN :start_date AND :end_date
)

SELECT
    category_id,
    topic_id,
    staff_user_id,
    ROUND(response_time_minutes::numeric, 2) AS response_time_minutes
FROM group_response_times
WHERE row_num = 1
ORDER BY category_id, response_time_minutes

That will tell you the time it took group members to respond to individual topics. The results are organized by category, but the category can be ignored.

I’m not sure how meaningful data for average response times for individual group members will be. I’d be wary of using it for any kind of performance review because it might penalize group members who respond to difficult questions or topics that other group members have ignored. With that in mind, here’s a query that returns the average response times for group members and the number of topics they’ve been the first member of the group to respond to:

-- [params]
-- date :start_date
-- date :end_date
-- string :group_name = staff

WITH group_response_times AS (
    SELECT
        t.category_id,
        t.id AS topic_id,
        EXTRACT(EPOCH FROM (p.created_at - t.created_at))/ 60 AS response_time_minutes,
        p.user_id AS staff_user_id,
        ROW_NUMBER() OVER (PARTITION BY t.id ORDER BY p.created_at) AS row_num
    FROM posts p
    JOIN topics t ON t.id = p.topic_id
    WHERE t.user_id NOT IN (SELECT user_id
                                FROM group_users gu JOIN groups g ON g.id = gu.group_id
                                WHERE gu.user_id > 0 AND g.name = :group_name)
        AND p.user_id IN (SELECT user_id
                              FROM group_users gu JOIN groups g ON g.id = gu.group_id
                              WHERE gu.user_id > 0 AND g.name = :group_name)
        AND t.archetype = 'regular'
        AND t.deleted_at IS NULL
        AND p.post_type = 1
        AND p.deleted_at IS NULL
        AND t.created_at BETWEEN :start_date AND :end_date
)

SELECT
    staff_user_id,
    ROUND(AVG(response_time_minutes)::numeric, 2) AS average_response_time_minutes,
    COUNT(*) AS number_of_topics_responded_to
FROM group_response_times
WHERE row_num = 1
GROUP BY staff_user_id
ORDER BY average_response_time_minutes