Postgres 17

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CREATE TABLE test (
  id INT,
  val VARCHAR,
  total TEXT
);

CREATE TABLE

CREATE FUNCTION alter_input() RETURNS trigger AS $alter_input$
    BEGIN
        NEW.total = row_to_json(NEW);
        RETURN NEW;
    END;
$alter_input$ LANGUAGE plpgsql;

CREATE TRIGGER alter_input BEFORE INSERT ON test
    FOR EACH ROW EXECUTE FUNCTION alter_input();

CREATE FUNCTION

CREATE TRIGGER

INSERT INTO test VALUES (123, 'Some value.', 'Initial Total');

INSERT 0 1

SELECT * FROM test;

id
val
total

123
Some value.
{"id":123,"val":"Some value.","total":"Initial Total"}

SELECT 1