How to Efficiently Calculate Cumulative Sum in PostgreSQL Using Window Functions?

Calculate cumulative sum in PostgreSQL

Use window functions in PostgreSQL to efficiently calculate the cumulative sum of fields to update data from the staging table to the target table. The given problem requires finding the cumulative amount of the "amount" field based on "circle_id" while retaining other attributes.

To do this, follow these steps:

1. Partition the data: Use the PARTITION BY clause to create partitions based on "circle_id". This ensures that each circle is calculated individually.
2. Order the data: Specify an ORDER BY clause to sort the rows based on "ea_year" and "ea_month" (or the derived "the_date" column). This determines the order in which the cumulative sum is calculated.
3. Calculate the cumulative sum: Apply the SUM() aggregate function using the OVER clause. This will calculate the cumulative sum of the "amount" field within each partition and for the rows preceding the current row.

Generated query:

SELECT ea_month, id, amount, ea_year, circle_id,
     sum(amount) OVER (PARTITION BY circle_id ORDER BY ea_year, ea_month) AS cum_amt
FROM tbl
ORDER BY circle_id, ea_year, ea_month;

This query will generate the required target table where the cumulative amount ("cum_amt") is calculated for each row according to the specified sort order.

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