How to Retrieve the Earliest Entry for Each Group in PostgreSQL?

Efficiently Selecting the First Row of Each Group in PostgreSQL

This guide demonstrates how to efficiently retrieve the earliest entry for each group in PostgreSQL, a common task when working with grouped data. The most effective method utilizes the DISTINCT ON clause.

Leveraging the DISTINCT ON Clause

PostgreSQL's DISTINCT ON clause provides a concise and efficient way to select the first row from each group defined by specified columns. Its syntax is straightforward:

SELECT DISTINCT ON (column_list)
FROM table_name
ORDER BY column_list, ...;

Example: Identifying the First Purchase for Each Customer

Consider a purchases table with columns id, customer, and total. To find the earliest purchase (based on the id column) for each customer with the highest total, use this query:

SELECT DISTINCT ON (customer)
       id, customer, total
FROM   purchases
ORDER  BY customer, total DESC, id;

Important Considerations:

• NULL Handling: If the total column allows NULL values, include NULLS LAST in the ORDER BY clause to ensure non-NULL values are prioritized.
• SELECT List Flexibility: The SELECT list isn't limited to the columns in DISTINCT ON or ORDER BY.
• PostgreSQL Specificity: DISTINCT ON is a PostgreSQL-specific extension, not part of standard SQL.
• Alternative Ordering: To order the final results differently from the grouping order, nest the query and apply a secondary ORDER BY clause.

Optimizing Performance with Indexing:

Creating a multi-column index on (customer, total DESC, id) significantly improves query performance, particularly when the number of rows per customer is relatively small.

Alternative Approaches for Large Datasets:

For datasets with numerous rows per customer, alternative strategies might be more efficient:

• Subqueries: Employ a subquery or correlated subquery to find the maximum total for each customer, then select the row with the minimum id for those customers.
• Temporary Tables/CTEs: Create a temporary table or Common Table Expression (CTE) containing unique customers, and join it with the original table to retrieve the desired rows. This approach can be beneficial for complex scenarios.

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