Add primary key to PostgreSQL table only if it does not exist

Question

I have simple table creating script in Postgres 9.1. I need it to create the table with 2-attributes PK only if it does not exist.

CREATE TABLE IF NOT EXISTS "mail_app_recipients"
(
    "id_draft" Integer NOT NULL,
    "id_person" Integer NOT NULL
) WITH (OIDS=FALSE); -- this is OK

ALTER TABLE "mail_app_recipients" ADD PRIMARY KEY IF NOT EXISTS ("id_draft","id_person");
-- this is problem since "IF NOT EXISTS" is not allowed.

Any solution how to solve this problem? Thanks in advance.

Answer 1

Why not include the PK definition inside the CREATE TABLE:

CREATE TABLE IF NOT EXISTS mail_app_recipients
(
    id_draft Integer NOT NULL,
    id_person Integer NOT NULL,
    constraint pk_mail_app_recipients primary key (id_draft, id_person)
)

Answer 2

You could do something like the following, however it is better to include it in the create table as a_horse_with_no_name suggests.

if NOT exists (select constraint_name from information_schema.table_constraints where table_name = 'table_name' and constraint_type = 'PRIMARY KEY') then

ALTER TABLE table_name
  ADD PRIMARY KEY (id);

end if;

Answer 3

You can try to DROP it before creating it ( DROP has the IF EXISTS clause):

ALTER TABLE mail_app_recipients DROP CONSTRAINT IF EXISTS mail_app_recipients_pkey;
ALTER TABLE mail_app_recipients ADD CONSTRAINT mail_app_recipients_pkey PRIMARY KEY ("id_draft","id_person");

Note that this require that you give a name to the primary key constraint - in this example mail_app_recipients_pkey .