How to Efficiently Count Events by Time Intervals in PostgreSQL?

Efficient Counting of Events by Time Intervals

Problem Statement

Consider an Events table containing time-stamped events. The objective is to report the count of events occurring within specific time intervals, such as days, hours, or user-defined intervals. The goal is to determine the most efficient approach for dynamically generating this information through a single SQL query in Postgres.

Solution

Using Date_bin() (Postgres 14 or Newer)

Postgres 14 introduces the 'date_bin()' function, providing a straightforward solution to this problem:

SELECT date_bin('15 min', e.ts, '2018-05-01') AS start_time,
       count(e.ts) AS events
FROM event e
GROUP BY 1
ORDER BY 1;

Generating a Full Set of Rows (Postgres 13 or Older)

For earlier versions of Postgres, the following query generates a full set of time slots and performs a LEFT JOIN to count events within each interval:

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;

Considerations

• Postgres 9.6 or later is required for 'generate_series()'.
• For large datasets, creating a separate 'Interval' field in the table and pre-storing the time interval values can improve performance, but requires additional storage space.
• When specifying the time interval, use the appropriate Postgres syntax, such as 'interval '17 min''.
• The 'BETWEEN' operator can be used to filter events within a specific time range, but ensure proper handling of upper and lower bounds.
• Use 'to_char()' to format the start_time column for display purposes without affecting the ORDER BY operation.

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