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

Question

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.

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

delete from logtable where log_id in (
    select log_id from logtable order by timestamp limit 10);

Assuming you want to delete ANY 10 records (without the ordering) you could do this:

DELETE FROM logtable as t1 WHERE t1.ctid < (select t2.ctid from logtable as t2  where (Select count(*) from logtable t3  where t3.ctid < t2.ctid ) = 10 LIMIT 1);

For my use case, deleting 10M records, this turned out to be faster.