Postgresql时间片/窗口查询

Question

I'm using PostgreSQL 9.3 with a table of approx 7M rows.

I want to get a count of the number of records in a table on a per-device_group, per-day basis.

I'd like my result set to look like this:

device_group.id | date      | count
-------------------------------------
123             | 2013-11-28| 45
123             | 2013-11-29| 37
124             | 2013-11-28| 4
124             | 2013-11-29| 23
125             | 2013-11-28| 3

Where each device_group has a daily count available as a record in the result set.

Ideally, I can pass a start and end date to determine how many date records there should be per device_group in the result set.

I don't mind if the result set omits records with a count of zero.

A basic form of the tables looks like this:

CREATE TABLE session (
  id SERIAL PRIMARY KEY,
  acctstarttime TIMESTAMP,
  deviceid INTEGER (FOREIGN KEY)
);

CREATE TABLE device_group (
  id SERIAL PRIMARY KEY,
  name TEXT
);

CREATE TABLE device (
  id SERIAL PRIMARY KEY,
  device_group_id INTEGER (FOREIGN KEY)
  name TEXT
);

I've worked with windowing functions, and I can get a global number broken down by date, or a number by device_group_id, but not combined... and that's where i'm stuck.

What i've been working with so far:

SELECT 
  device_group_id,
  COUNT(s.id) 
OVER (PARTITION BY deviceid) 
FROM session s
LEFT JOIN device d ON s.deviceid = d.id 
WHERE acctstarttime > '2013-11-01' AND acctstarttime < '2013-11-28'
ORDER BY device_group_id;

and

SELECT
  TO_TIMESTAMP( EXTRACT ('epoch' FROM acctstarttime)::int / (86400) * 86400 ) AS timeslice,
  COUNT(username) 
FROM session 
WHERE acctstarttime > '2013-11-01' AND acctstarttime < '2013-11-28' 
GROUP BY timeslice
ORDER BY timeslice ASC;

Answer 1

For this task a simple GROUP BY query is sufficient:

select d.device_group_id, 
       date_trunc( 'day', s.acctstarttime ) date,
       count(*) 
from device d
join session s
on d.id = s.deviceid
group by d.device_group_id,
         date_trunc( 'day', s.acctstarttime )
order by d.device_group_id, date
;

and another version with the WHERE clause

select d.device_group_id, 
       date_trunc( 'day', s.acctstarttime ) date,
       count(*) 
from device d
join session s
on d.id = s.deviceid
where acctstarttime between 
          to_timestamp('2013-01-01', 'yyyy-mm-dd')
          and
          to_timestamp('2014-01-01', 'yyyy-mm-dd')
group by d.device_group_id,
         date_trunc( 'day', s.acctstarttime )
order by d.device_group_id, date
;

demo: --> http://www.sqlfiddle.com/#!15/4a3ef/7