Sometimes it can be difficult to find a unique set of key columns that describe the data.
Let's start with a small table, where the answer is fairly obvious.
postgres=# select * from ord; 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 of 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 not long to wait.
Now we ...