How Can I Delete Rows in PostgreSQL in a Specific Order Without a Primary Key?

Preserving Row Deletion Order in PostgreSQL Without a Primary Key

In MySQL, the query DELETE FROM logtable ORDER BY timestamp LIMIT 10; effortlessly removes a specified number of rows from a table while maintaining sorting order. However, PostgreSQL poses a challenge by prohibiting ordering or limits in its delete syntax, especially when the table lacks a primary key.

Overcoming the Primary Key Constraint

To circumvent this obstacle, PostgreSQL offers a solution that utilizes the ctid. The ctid represents the physical location of each row within the table. Although it's not a permanent identifier, its uniqueness per table makes it a viable alternative for row deletion.

Utilizing the ctid

The ctid can be harnessed to delete a predetermined number of sorted rows with the following query:

DELETE FROM ONLY logtable
WHERE ctid IN (
    SELECT ctid
    FROM logtable
    ORDER BY timestamp
    LIMIT 10
);

DELETE FROM ONLY restricts the deletion operation to the specified table, preventing inadvertent deletions from inherited tables.

Handling Partitions and Security Settings

If the logtable is partitioned, you must include the tableoid (table ID) in the query to prevent deletions from occurring in multiple partitions.

Furthermore, if security policies are implemented, you may need to adjust the query to ensure appropriate access rights.

Additional Note

It's crucial to remember that the ctid can change when rows are updated or moved by VACUUM FULL. If you intend to use ctid as a long-term row identifier, keep this limitation in mind.

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