How to restrict only 1 row to get updated in postgres?

Question

I am trying to convert the below Oracle sql to postgres sql:

Update Table_name set flag='U' WHERE id= 1 and ROWNUM < 2.

if the data in the table is some thing like

Then the query should update only one record as Unique like flag='U'.. and other records should not be effected.

Note:There is no unique key nor primary key on the table. I don't have access rights to create a constraint.

Since ROWNUM is not available in postgres, I am not able to do convert this query into postgres.

Thanks in advance..

Answer 1

You can use the pseudo column CTID here -

Update Table_name
   SET flag='U'
 WHERE CTID IN (SELECT CTID FROM Table_name WHERE id = 1 LIMIT 1);