Postgres 13

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.
CREATE TABLE tbl(tbl_id serial PRIMARY KEY, column_name text);

INSERT INTO tbl (tbl_id, column_name) VALUES
  (1, '123')        -- pre-existing too short
, (2, '1234')       -- pre-existing too short
, (3, '12345')      -- pre-existing too short
, (11, '1234567')   -- ok
, (12, '12345678')  -- ok
;

CREATE TABLE



INSERT 0 5


-- trigger func
CREATE OR REPLACE FUNCTION trg_col_min_len6()
  RETURNS trigger
  LANGUAGE plpgsql AS
$func$
BEGIN
   IF TG_OP = 'UPDATE'
   AND OLD.column_name IS NOT DISTINCT FROM NEW.column_name THEN
      -- do nothing
   ELSE
      RAISE EXCEPTION 'New value for column "note" must have at least 6 characters.';
   END IF;
  
   RETURN NEW;
END
$func$;

-- trigger
CREATE TRIGGER tbl1_column_name_min_len6
BEFORE INSERT OR UPDATE ON tbl
FOR EACH ROW
WHEN (length(NEW.column_name) < 7)
EXECUTE FUNCTION trg_col_min_len6();

CREATE FUNCTION



CREATE TRIGGER


-- allowed
UPDATE tbl
SET column_name = '1234567'
WHERE tbl_id = 1;

UPDATE tbl
SET column_name = '12345678'
WHERE tbl_id = 11;

UPDATE tbl
SET column_name = NULL  -- !
WHERE tbl_id = 11;

INSERT INTO tbl (tbl_id, column_name) VALUES
  (23, '123456789')
, (24, NULL);  -- !

-- ALSO allowed
UPDATE tbl
SET    tbl_id = 12345
WHERE  tbl_id = 3;

UPDATE 1



UPDATE 1



UPDATE 1



INSERT 0 2



UPDATE 1


TABLE tbl;


tbl_id
column_name


2
1234

12
12345678

1
1234567

11
null

23
123456789

24
null

12345
12345



SELECT 7


-- not allowed
UPDATE tbl
SET column_name = '12345'
WHERE tbl_id = 2;

ERROR:  New value for column "note" must have at least 6 characters.
CONTEXT:  PL/pgSQL function trg_col_min_len6() line 7 at RAISE


-- not allowed
UPDATE tbl
SET column_name = '12345'
WHERE tbl_id = 12;

ERROR:  New value for column "note" must have at least 6 characters.
CONTEXT:  PL/pgSQL function trg_col_min_len6() line 7 at RAISE


-- not allowed
INSERT INTO tbl (tbl_id, column_name)
VALUES  (23, '123');

ERROR:  New value for column "note" must have at least 6 characters.
CONTEXT:  PL/pgSQL function trg_col_min_len6() line 7 at RAISE


TABLE tbl;


tbl_id
column_name


2
1234

12
12345678

1
1234567

11
null

23
123456789

24
null

12345
12345



SELECT 7
tbl_id	column_name
2	1234
12	12345678
1	1234567
11	null
23	123456789
24	null
12345	12345