How do I delete a fixed number of rows with sorting in PostgreSQL?

I'm trying to port some old MySQL queries to PostgreSQL, but I'm having trouble with this one:

DELETE FROM logtable ORDER BY timestamp LIMIT 10;

PostgreSQL doesn't allow ordering or limits in its delete syntax, and the table doesn't have a primary key so I can't use a subquery. Additionally, I want to preserve the behavior where the query deletes exactly the given number or records -- for example, if the table contains 30 rows but they all have the same timestamp, I still want to delete 10, although it doesn't matter which 10.

So; how do I delete a fixed number of rows with sorting in PostgreSQL?

Edit: No primary key means there's no log_id column or similar. Ah, the joys of legacy systems!

Solution 1:

You could try using the ctid:

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

The ctid is:

The physical location of the row version within its table. Note that although the ctid can be used to locate the row version very quickly, a row's ctid will change if it is updated or moved by VACUUM FULL. Therefore ctid is useless as a long-term row identifier.

There's also oid but that only exists if you specifically ask for it when you create the table.

Solution 2:

Postgres docs recommend to use array instead of IN and subquery. This should work much faster

DELETE FROM logtable 
WHERE id = any (array(SELECT id FROM logtable ORDER BY timestamp LIMIT 10));

This and some other tricks can be found here