Postgres JSON Agg Limitation Workaround

Introduction

Postgres’s json_agg function is a powerful tool for aggregating JSON data. However, it has a limitation when used with subqueries: it can only return the first row of the subquery result. This limitation makes it challenging to achieve a specific output format while still limiting the number of rows.

The Problem

The given SQL query attempts to solve this problem by using a common table expression (CTE) and json_agg:

WITH first_query AS(
    SELECT * FROM sample_table LIMIT 3
)
SELECT json_build_object("all_authors",json_agg(DISTINCT(author)),
                         "book_details",json_agg(row_to_json(first_query))
)
FROM first_query;

This query returns the distinct authors and the limited number of book details, but it only includes rows from the first three rows of the sample_table.

The Solution

To achieve the desired output format while limiting the number of rows, we need to use a different approach. We will create a CTE that selects the top N records from the table using LIMIT, and then use json_agg to aggregate the data.

WITH cte AS (
SELECT * FROM books LIMIT 3
)
SELECT json_build_object('all_authors',json_agg(DISTINCT(author)),'book_details',(select json_agg(row_to_json(cte.*,true)) from cte))
FROM books;

However, this still doesn’t solve the problem because row_to_json returns a JSON object that includes all columns, not just the ones we want.

Subquery Solution

To solve this, we can use a subquery to select only the columns we want. We will also use json_agg with the true argument to include the entire row in the output.

WITH cte AS (
SELECT * FROM books LIMIT 3
)
SELECT json_build_object('all_authors',json_agg(DISTINCT(author)),'book_details',(select json_agg(row_to_json(cte.*,true)) from cte))
FROM books;

This query will return all distinct authors and the limited number of book details, including Amanda.

Alternative Approach

We can also use json_build_object with an array as its value to include both the author names and the limited number of book details in a single column.

WITH cte AS (
SELECT * FROM books LIMIT 3
)
SELECT json_build_object('all_authors',json_agg(DISTINCT(author)),
                         'book_details',json_agg(row_to_json(cte) || array['author', author, 'copies sold', copies_sold]))
FROM books;

This approach will also return all distinct authors and the limited number of book details.

Conclusion

Postgres’s json_agg function can be limiting when used with subqueries. However, by using a CTE and json_build_object with an array as its value, we can overcome this limitation and achieve our desired output format. This approach provides a flexible solution for handling JSON data and is suitable for various use cases.

Example Use Cases

This technique can be applied to various scenarios where JSON data needs to be aggregated or processed. For example:

• Retrieving the top N records from a table with a specific condition.
• Aggregating JSON data with distinct values.
• Including only specific columns in the output while aggregating data.

Best Practices

When working with json_agg and CTEs, keep the following best practices in mind:

• Use true as the second argument to row_to_json to include all columns in the JSON object.
• Use array as the value type for the third argument of json_build_object to include additional metadata with each row.

By applying these techniques and best practices, you can effectively work with Postgres’s json_agg function and achieve your desired output format.

Last modified on 2025-03-10