Question: How do you limit the number of rows deleted in PostgreSQL?

Answer

PostgreSQL, unlike some other SQL databases, does not directly support the LIMIT clause in DELETE operations through its standard syntax. However, you can still limit the number of rows deleted in a PostgreSQL database by using a workaround involving subqueries or common table expressions (CTEs).

Using Subquery

A subquery with the IN operator can be used to specify which rows to delete. The subquery selects the primary key (or any unique column) of the rows that should be deleted, and the outer query deletes the rows that match these identifiers.

DELETE FROM your_table
WHERE id IN (
  SELECT id
  FROM your_table
  WHERE condition = true
  LIMIT 10
);

Replace your_table with the name of your table, id with the primary key or a unique identifying column, and adjust the condition = true part to fit your criteria.

Using Common Table Expressions (CTE)

With PostgreSQL 9.5 and later, you can use a CTE for deleting a limited number of rows in a more readable way.

WITH deleted AS (
  DELETE FROM your_table
  WHERE condition = true
  RETURNING *
)
SELECT * FROM deleted
LIMIT 10;

In this example, replace your_table and condition = true as necessary. This approach deletes the rows matching the condition and returns them, but the actual deletion isn't limited by the LIMIT clause directly—instead, it's the result set of the SELECT statement that's limited. Although this might not directly limit the deletions in all scenarios, it's a useful technique for returning information about the rows that were deleted.

Important Notes

• Use these methods with caution, especially in production environments, because deleting data is irreversible without proper backups.
• Always test your queries in a development environment before applying them to your production database.
• The choice between a subquery and CTE might depend on readability preferences, specific requirements of your application, or performance considerations.

Neither of these methods allows you to straightforwardly say 'delete exactly N rows matching these conditions,' but they provide the tools needed to achieve similar results with some creativity.