Best way to count records by arbitrary time intervals in Rails+Postgres

Question

My app has a Events table with time-stamped events.

I need to report the count of events during each of the most recent N time intervals. For different reports, the interval could be "each week" or "each day" or "each hour" or "each 15-minute interval".

For example, a user can display how many orders they received each week, day, or hour, or quarter-hour.

1) My preference is to dynamically do a single SQL query (I'm using Postgres) that groups by an arbitrary time interval. Is there a way to do that?

2) An easy but ugly brute force way is to do a single query for all records within the start/end timeframe sorted by timestamp, then have a method manually build a tally by whatever interval.

3) Another approach would be add separate fields to the event table for each interval and statically store an the_week the_day, the_hour, and the_quarter_hour field so I take the 'hit' at the time the record is created (once) instead of every time I report on that field.

What's best practice here, given I could modify the model and pre-store interval data if required (although at the modest expense of doubling the table width)?

Answer 1

Luckily, you are using PostgreSQL. The window function generate_series() is your friend.

Test case

Given the following test table (which you should have provided):

CREATE TABLE event(event_id serial, ts timestamp);
INSERT INTO event (ts)
SELECT generate_series(timestamp '2018-05-01'
                     , timestamp '2018-05-08'
                     , interval '7 min') + random() * interval '7 min';

One event for every 7 minutes (plus 0 to 7 minutes, randomly).

Basic solution

This query counts events for any arbitrary time interval. 17 minutes in the example:

WITH grid AS (
   SELECT start_time
        , lead(start_time, 1, 'infinity') OVER (ORDER BY start_time) AS end_time
   FROM  (
      SELECT generate_series(min(ts), max(ts), interval '17 min') AS start_time
      FROM   event
      ) sub
   )
SELECT start_time, count(e.ts) AS events
FROM   grid       g
LEFT   JOIN event e ON e.ts >= g.start_time
                   AND e.ts <  g.end_time
GROUP  BY start_time
ORDER  BY start_time;

The query retrieves minimum and maximum ts from the base table to cover the complete time range. You can use an arbitrary time range instead.

Provide any time interval as needed.

Produces one row for every time slot. If no event happened during that interval, the count is 0.

Be sure to handle upper and lower bound correctly. See:

• Unexpected results from SQL query with BETWEEN timestamps

The window function lead() has an often overlooked feature: it can provide a default for when no leading row exists. Providing 'infinity' in the example. Else the last interval would be cut off with an upper bound NULL.

Minimal equivalent

The above query uses a CTE and lead() and verbose syntax. Elegant and maybe easier to understand, but a bit more expensive. Here is a shorter, faster, minimal version:

SELECT start_time, count(e.ts) AS events
FROM  (SELECT generate_series(min(ts), max(ts), interval '17 min') FROM event) g(start_time)
LEFT   JOIN event e ON e.ts >= g.start_time
                   AND e.ts <  g.start_time + interval '17 min'
GROUP  BY 1
ORDER  BY 1;

Example for "every 15 minutes in the past week"`

Formatted with to_char().

SELECT to_char(start_time, 'YYYY-MM-DD HH24:MI'), count(e.ts) AS events
FROM   generate_series(date_trunc('day', localtimestamp - interval '7 days')
                     , localtimestamp
                     , interval '15 min') g(start_time)
LEFT   JOIN event e ON e.ts >= g.start_time
                   AND e.ts <  g.start_time + interval '15 min'
GROUP  BY start_time
ORDER  BY start_time;

Still ORDER BY and GROUP BY on the underlying timestamp value, not on the formatted string. That's faster and more reliable.

db<>fiddle here

Related answer producing a running count over the time frame:

• PostgreSQL: running count of rows for a query 'by minute'