How to Get a Running Row Count by Minute in PostgreSQL?

PostgreSQL: Calculating a Running Row Count by Minute

This article demonstrates how to obtain a running row count for a query, grouped by minute in PostgreSQL. The challenge is to accurately count rows for every minute, even those with no activity.

Leveraging Window Functions

The most efficient solution utilizes PostgreSQL's powerful window functions. These functions operate on sets of rows, enabling calculations across multiple rows. Here, we use the COUNT function within a window partitioned by minute.

Efficient Query using SELECT DISTINCT and ORDER BY

This query uses SELECT DISTINCT to ensure unique minute entries and COUNT with a window function to generate the running count. ORDER BY guarantees the correct cumulative sum for each minute interval.

SELECT DISTINCT
       date_trunc('minute', "when") AS minute,
       count(*) OVER (ORDER BY date_trunc('minute', "when")) AS running_ct
FROM   mytable
ORDER  BY 1;

Understanding the Window Function Syntax

Let's break down the window function:

• COUNT(*): Counts rows within each minute partition.
• OVER (ORDER BY date_trunc('minute', "when")): Defines the window as a sequence of rows, ordered by minute, accumulating the count from the beginning.

Handling Minutes with Zero Activity

To include minutes with no activity (zero rows), we use generate_series to create a series of minutes and then perform a LEFT JOIN to include those minutes even if they lack corresponding entries in the table:

SELECT DISTINCT
       minute, count(c.minute) OVER (ORDER BY minute) AS running_ct
FROM  (
   SELECT generate_series(date_trunc('minute', min("when")), max("when"), interval '1 min')
   FROM   mytable
   ) m(minute)
LEFT   JOIN (SELECT date_trunc('minute', "when") AS minute FROM mytable) c USING (minute)
ORDER  BY 1;

Performance Considerations

While window functions are generally optimal for large datasets, for smaller datasets, an alternative using SUM() might offer better performance. This approach first groups and counts rows per minute, then uses SUM() to calculate the running total:

SELECT minute, sum(minute_ct) OVER (ORDER BY minute) AS running_ct
FROM  (
   SELECT date_trunc('minute', "when") AS minute,
        count(*) AS minute_ct
   FROM   tbl
   GROUP  BY 1
   ) sub
ORDER  BY 1;

The best approach depends on your dataset size and performance requirements. Testing both methods is recommended to determine the most efficient solution for your specific use case.

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