Re: [GENERAL] Best way to alter a foreign constraint

2017-03-18 20:40 GMT+01:00 Adrian Klaver <adrian.klaver@aklaver.com>:

On 03/18/2017 12:05 PM, Sylvain Marechal wrote:

Hello all,

Some of my tables were badly designed and have 2 indexes, like the
following example (lots of tables have same problem):

<<<
postgres=# \d test1
     Table "public.test1"
 Column |  Type   | Modifiers
--------+---------+-----------
 t1     | integer | not null
Indexes:
    "test1_pkey" PRIMARY KEY, btree (t1)
    "test1_t1_key" UNIQUE CONSTRAINT, btree (t1)
Referenced by:
    TABLE "test2" CONSTRAINT "test2_t1_fkey" FOREIGN KEY (t1) REFERENCES
test1(t1)

postgres=# \d test2
     Table "public.test2"
 Column |  Type   | Modifiers
--------+---------+-----------
 t2     | integer | not null
 t1     | integer |
Indexes:
    "test2_pkey" PRIMARY KEY, btree (t2)
Foreign-key constraints:
    "test2_t1_fkey" FOREIGN KEY (t1) REFERENCES test1(t1)

It is not possible to remove the "test1_t1_key" constraint because the
"test2_t1_fkey"  internally references it:
<<<
postgres=# ALTER TABLE test1 DROP CONSTRAINT test1_t1_key;
ERROR:  cannot drop constraint test1_t1_key on table test1 because other
objects depend on it
DETAIL:  constraint test2_t1_fkey on table test2 depends on index
test1_t1_key
HINT:  Use DROP ... CASCADE to drop the dependent objects too.

Why not CASCADE?:

In fact, CASCADE is not enough, because I don't know how the test2_t1_fkey is built : does it use the test1_pkey primary key or the test1_t1_key unique key?
I am sure this information can be found in system catalogs, but I find it safer to explicitely delete then recreate the foreign constraint.

Sylvain