Let's start with a small table, where the answer is fairly obvious:

postgres=# select * from ord;

We assume that the output is as follows:

 orderid | customerid |  amt   
---------+------------+--------
   10677 |          2 |   5.50
    5019 |          3 | 277.44
    9748 |          3 |  77.17
(3 rows)

First of all, there's no need to do this through a brute-force approach. Checking all the permutations of columns to see which is unique might take you a long time.

Let's start by using PostgreSQL's own optimizer statistics. Run the following command on our table to get a fresh sample of statistics:

postgres=# analyze ord;
ANALYZE

This runs quickly, so we don't have to wait too much. Now, we can examine the relevant columns of the statistics:

postgres=# SELECT attname, n_distinct FROM pg_stats WHERE schemaname = 'public' AND tablename =...