Postgres 12

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create table x(id int primary key);

CREATE TABLE

alter table x drop constraint x_pkey;

ALTER TABLE

alter table x add primary key (id) deferrable initially immediate;

ALTER TABLE

insert into x values (1), (2);

INSERT 0 2

set constraints x_pkey deferred;

SET CONSTRAINTS

update x set id = (case id when 1 then 2 when 2 then 1 else id end);

UPDATE 2

commit;

COMMIT