How to replace subqueries containing aggregate functions in PostgreSQL with Windows functions?

Step 1: Identify the Subquery with Aggregate Function

Identify the subquery that contains the aggregate function and analyze its purpose in the overall query. Note the table(s) involved and the grouping criteria.

Step 2: Convert the Subquery into a CTE (Common Table Expression)

Transform the subquery into a CTE, which will allow us to use window functions to achieve the same result.

Step 3: Define the Window Function

Choose an appropriate window function that matches the aggregate function used in the subquery. Common window functions include 

SUM, COUNT, AVG, MIN, MAX, etc.

Step 4: Specify the Window Partition and Order

Specify the partitioning and ordering criteria for the window function. This determines how the window frame is defined for calculating the window function.

Step 5: Integrate the Window Function in the Main Query

Join the CTE (representing the transformed subquery) with the main query, using the appropriate join conditions based on the tables involved.

Here’s an example to illustrate the steps:

Original Query with Subquery:

SELECT a.category, (SELECT SUM(b.amount) FROM sales b WHERE b.category = a.category) AS total_sales
FROM categories a

Step 2: Convert Subquery into CTE:

WITH subquery_cte AS (
SELECT category, amount, SUM(amount) OVER (PARTITION BY category) AS total_sales
FROM sales)

Step 3: Define Window Function:

In this case, we’ll use the SUM window function to replace the SUM aggregate function in the subquery.

Step 4: Specify Window Partition and Order:

The PARTITION BY clause defines the grouping criteria, which in this case is the category column. The ordering is not necessary since we want the sum for each category.

Step 5: Integrate Window Function in the Main Query:

SELECT a.category, s.total_sales
FROM categories a
JOIN subquery_cte s ON s.category = a.category

By following these steps, you can replace subqueries with aggregate functions in PostgreSQL with window functions. This approach can improve query performance and simplify the overall query structure.

About Shiv Iyer 422 Articles
Open Source Database Systems Engineer with a deep understanding of Optimizer Internals, Performance Engineering, Scalability and Data SRE. Shiv currently is the Founder, Investor, Board Member and CEO of multiple Database Systems Infrastructure Operations companies in the Transaction Processing Computing and ColumnStores ecosystem. He is also a frequent speaker in open source software conferences globally.