How to add "on delete cascade" constraints?

Question

In PostgreSQL 8 is it possible to add ON DELETE CASCADES to the both foreign keys in the following table without dropping the latter?

# \d scores         Table "public.scores"  Column  |         Type          | Modifiers ---------+-----------------------+-----------  id      | character varying(32) |  gid     | integer               |  money   | integer               | not null  quit    | boolean               |  last_ip | inet                  | Foreign-key constraints:    "scores_gid_fkey" FOREIGN KEY (gid) REFERENCES games(gid)    "scores_id_fkey" FOREIGN KEY (id) REFERENCES users(id) 

Both referenced tables are below - here:

# \d games                                      Table "public.games"   Column  |            Type             |                        Modifiers ----------+-----------------------------+----------------------------------------------------------  gid      | integer                     | not null default nextval('games_gid_seq'::regclass)  rounds   | integer                     | not null  finished | timestamp without time zone | default now() Indexes:     "games_pkey" PRIMARY KEY, btree (gid) Referenced by:     TABLE "scores" CONSTRAINT "scores_gid_fkey" FOREIGN KEY (gid) REFERENCES games(gid) 

And here:

# \d users                 Table "public.users"    Column   |            Type             |   Modifiers ------------+-----------------------------+---------------  id         | character varying(32)       | not null  first_name | character varying(64)       |  last_name  | character varying(64)       |  female     | boolean                     |  avatar     | character varying(128)      |  city       | character varying(64)       |  login      | timestamp without time zone | default now()  last_ip    | inet                        |  logout     | timestamp without time zone |  vip        | timestamp without time zone |  mail       | character varying(254)      | Indexes:     "users_pkey" PRIMARY KEY, btree (id) Referenced by:     TABLE "cards" CONSTRAINT "cards_id_fkey" FOREIGN KEY (id) REFERENCES users(id)     TABLE "catch" CONSTRAINT "catch_id_fkey" FOREIGN KEY (id) REFERENCES users(id)     TABLE "chat" CONSTRAINT "chat_id_fkey" FOREIGN KEY (id) REFERENCES users(id)     TABLE "game" CONSTRAINT "game_id_fkey" FOREIGN KEY (id) REFERENCES users(id)     TABLE "hand" CONSTRAINT "hand_id_fkey" FOREIGN KEY (id) REFERENCES users(id)     TABLE "luck" CONSTRAINT "luck_id_fkey" FOREIGN KEY (id) REFERENCES users(id)     TABLE "match" CONSTRAINT "match_id_fkey" FOREIGN KEY (id) REFERENCES users(id)     TABLE "misere" CONSTRAINT "misere_id_fkey" FOREIGN KEY (id) REFERENCES users(id)     TABLE "money" CONSTRAINT "money_id_fkey" FOREIGN KEY (id) REFERENCES users(id)     TABLE "pass" CONSTRAINT "pass_id_fkey" FOREIGN KEY (id) REFERENCES users(id)     TABLE "payment" CONSTRAINT "payment_id_fkey" FOREIGN KEY (id) REFERENCES users(id)     TABLE "rep" CONSTRAINT "rep_author_fkey" FOREIGN KEY (author) REFERENCES users(id)     TABLE "rep" CONSTRAINT "rep_id_fkey" FOREIGN KEY (id) REFERENCES users(id)     TABLE "scores" CONSTRAINT "scores_id_fkey" FOREIGN KEY (id) REFERENCES users(id)     TABLE "status" CONSTRAINT "status_id_fkey" FOREIGN KEY (id) REFERENCES users(id) 

And also I wonder if it makes sense to add 2 index'es to the former table?

UPDATE: Thank you, and also I've got the advice at the mailing list, that I could manage it in 1 statement and thus without explicitly starting a transaction:

ALTER TABLE public.scores DROP CONSTRAINT scores_gid_fkey, ADD CONSTRAINT scores_gid_fkey    FOREIGN KEY (gid)    REFERENCES games(gid)    ON DELETE CASCADE; 

Answer 1

I'm pretty sure you can't simply add on delete cascade to an existing foreign key constraint. You have to drop the constraint first, then add the correct version. In standard SQL, I believe the easiest way to do this is to

• start a transaction,
• drop the foreign key,
• add a foreign key with on delete cascade, and finally
• commit the transaction

Repeat for each foreign key you want to change.

But PostgreSQL has a non-standard extension that lets you use multiple constraint clauses in a single SQL statement. For example

alter table public.scores drop constraint scores_gid_fkey, add constraint scores_gid_fkey    foreign key (gid)    references games(gid)    on delete cascade; 

If you don't know the name of the foreign key constraint you want to drop, you can either look it up in pgAdminIII (just click the table name and look at the DDL, or expand the hierarchy until you see "Constraints"), or you can query the information schema.

select * from information_schema.key_column_usage where position_in_unique_constraint is not null 

Answer 2

Based off of @Mike Sherrill Cat Recall's answer, this is what worked for me:

ALTER TABLE "Children" DROP CONSTRAINT "Children_parentId_fkey", ADD CONSTRAINT "Children_parentId_fkey"   FOREIGN KEY ("parentId")   REFERENCES "Parent"(id)   ON DELETE CASCADE;