Postgres 12

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CREATE TABLE transactions (id int, process_id int);
INSERT INTO transactions VALUES (1,2), (3,null);

DO
$do$
BEGIN
   EXECUTE format(
     'ALTER TABLE transactions
      ADD CONSTRAINT check_process_is_assigned CHECK (id <= %s OR process_id IS NOT NULL)'
    , (SELECT max(id) FROM transactions)
   );
END
$do$;

CREATE TABLE



INSERT 0 2



DO


-- now, this is still allowed:
INSERT INTO transactions VALUES (2,null);

INSERT 0 1


-- but this is not!
INSERT INTO transactions VALUES (4,null);

ERROR:  new row for relation "transactions" violates check constraint "check_process_is_assigned"
DETAIL:  Failing row contains (4, null).