How Can I Correctly Use PostgreSQL's DISTINCT ON with Different ORDER BY Clauses?

Understanding PostgreSQL's DISTINCT ON and ORDER BY Interactions

PostgreSQL's DISTINCT ON clause is designed to select the first row from each group of rows that have the same values in the specified expression(s). The crucial point is that the selection of the "first" row depends entirely on the ORDER BY clause. They must align.

A common mistake is using a DISTINCT ON clause with an ORDER BY clause that doesn't include the DISTINCT ON expression(s). This leads to unpredictable results because the database's choice of the "first" row becomes arbitrary.

Correcting Order Issues with DISTINCT ON

The error arises when the fields in DISTINCT ON don't match the leading fields in ORDER BY. To fix this, ensure the ORDER BY clause starts with the same expressions as DISTINCT ON. This guarantees a consistent and predictable selection of the first row within each group.

Alternative Approaches for "Greatest N Per Group" Problems

If the objective is to find the latest purchase for each address_id, ordered by purchase date, this is a classic "greatest N per group" query. Here are two efficient solutions:

General SQL Solution:

This approach uses a subquery to find the maximum purchased_at for each address_id and then joins it back to the original table to retrieve the complete row.

<code class="language-sql">SELECT t1.* 
FROM purchases t1
JOIN (
    SELECT address_id, max(purchased_at) max_purchased_at
    FROM purchases
    WHERE product_id = 1
    GROUP BY address_id
) t2
ON t1.address_id = t2.address_id AND t1.purchased_at = t2.max_purchased_at
ORDER BY t1.purchased_at DESC</code>

PostgreSQL-Specific Optimization:

PostgreSQL offers a more concise and potentially faster solution using a nested DISTINCT ON query:

<code class="language-sql">SELECT * FROM (
  SELECT DISTINCT ON (address_id) *
  FROM purchases 
  WHERE product_id = 1
  ORDER BY address_id, purchased_at DESC
) t
ORDER BY purchased_at DESC</code>

These alternatives provide cleaner and more efficient solutions compared to relying solely on DISTINCT ON when dealing with "greatest N per group" scenarios. They avoid unnecessary sorting and improve query performance.

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