id | time
1 | 2010-12-30 12:00:00
2 | 2010-12-30 12:05:00
3 | 2010-12-31 12:01:00
4 | 2010-12-31 12:02:00
5 | 2010-12-31 15:03:00
SQL запрос был бы таким:
SELECT count(id), date_trunc('days', time) FROM table GROUP BY date_trunc('days', time)
count | date_trunc
-------+------------------------
2 | 2010-12-30 00:00:00+03
3 | 2010-12-3100:00:00+03