Question

Is there a way I can remove a constraint based on column names?

I have postgres 8.4 and when I upgrade my project the upgrade fails because a constraint was named something different in a different version.

Basically, I need to remove a constraint if it exists or I can just remove the constraint using the column names.

The name of the constraint is the only thing that has changed. Any idea if that’s possible?

In this case, I need to remove “patron_username_key”

discovery=# \d patron
                       Table "public.patron"
          Column          |            Type             | Modifiers
--------------------------+-----------------------------+-----------
 patron_id                | integer                     | not null
 create_date              | timestamp without time zone | not null
 row_version              | integer                     | not null
 display_name             | character varying(255)      | not null
 username                 | character varying(255)      | not null
 authentication_server_id | integer                     |
Indexes:
    "patron_pkey" PRIMARY KEY, btree (patron_id)
    "patron_username_key" UNIQUE, btree (username, authentication_server_id)

Answer 1

Assuming that unique index is the result of adding a unique constraint, you can use the following SQL statement to remove that constraint:

do $$
declare 
  cons_name text;
begin
  select constraint_name 
     into cons_name
  from information_schema.constraint_column_usage  
  where constraint_schema = current_schema()
  and column_name in ('authentication_server_id', 'username')
  and table_name = 'patron'
  group by constraint_name
  having count(*) = 2;

  execute 'alter table patron drop constraint '||cons_name;
end;
$$

I’m not sure if this will work if you have “only” added a unique index (instead of a unique constraint).

If you need to do that for more than 2 columns you also need to adjust the having count(*) = 2 part to match the number of columns in the column_name in .. condition.

(As you did not specify your PostgreSQL version I’m assuming the current version)