Question

We’re trying to add a unique constraint to our postgres table in a way that deletes the duplicates rather than throwing an error. The unique constraint spans two columns, and there is no primary key. For example:

i_id | term   | date_created
1    | 'mako' | 123456789
1    | 'mako' | 123451234
1    | 'tele' | 213456852
2    | 'rake' | 598521542

So, in this example, we would need to remove that second row before we could safely add the unique constraint. Normally we would do a delete command with a select distinct thrown in, but we don’t have any distinguishing key for the rows. Specifically, the unique key would be over the columns [i_id, term].

(WTF didn’t we have a unique constraint from the start? Go figure.)

I’m thinking a delete statement would be the best, but I can’t simply write

delete from table where row_id not in (select row_id ... distinct something ... )

because there is no primary key for the row. I’d rather avoid a temporary table, if possible. Any suggestions?

EDIT: Sorry. We’re using postgres 8.4.

EDIT 2: The solution we’re using is:

delete from table where ctid not in (
  select 
    distinct on (i_id, term)
    ctid
    from table
    order by i_id, term
);

Thanks guys!

Answer 1

DELETE FROM the_table
WHERE ctid NOT IN (SELECT min(b.ctid)
                   FROM the_table b
                   GROUP BY b.i_id, b.term)