SELECT    DATE_TRUNC ('minute', GENERATE_SERIES (MIN(activitytime), MAX(activitytime))  - (CAST(EXTRACT(MINUTE FROM activitytime) AS integer) % 5) * interval '1 minute' AS trunc_5_minute
FROM      activities
LEFT JOIN (
SELECT    date_trunc('minute', activitytime) - (CAST(EXTRACT(MINUTE FROM activitytime) AS integer) % 5) * interval '1 minute' AS trunc_5_minute, count(*)
FROM      activities
GROUP BY trunc_5_minute
ORDER BY trunc_5_minute