Postgresql – Altering a parent table in Postgresql 8.4 breaks child table defaults

alter-tableinheritancepostgresqlpostgresql-8.4

The problem: In Postgresql, if table temp_person_two inherits fromtemp_person, default column values on the child table are ignored if the parent table is altered.

How to replicate:

First, create table and a child table. The child table should have one column that has a default value.

CREATE TEMPORARY TABLE temp_person (
    person_id SERIAL,
    name      VARCHAR
);

CREATE TEMPORARY TABLE temp_person_two (
    has_default character varying(4) DEFAULT 'en'::character varying NOT NULL
) INHERITS (temp_person);

Next, create a trigger on the parent table that copies its data to the child table (I know this appears like bad design, but this is a minimal test case to show the problem).

CREATE FUNCTION temp_person_insert() RETURNS trigger
LANGUAGE plpgsql
AS '
BEGIN
INSERT INTO temp_person_two VALUES ( NEW.* );
RETURN NULL;
END;
';

CREATE TRIGGER temp_person_insert_trigger
    BEFORE INSERT ON temp_person
    FOR EACH ROW
    EXECUTE PROCEDURE temp_person_insert();

Then insert data into parent and select data from child. The data should be correct.

INSERT INTO temp_person (name) VALUES ('ovid');
SELECT * FROM temp_person_two;
 person_id | name | has_default
-----------+------+-------------
         1 | ovid | en
(1 row )

Finally, alter parent table by adding a new, unrelated column. Attempt to insert data and watch a "not-null constraint" violation occur:

ALTER TABLE temp_person ADD column foo text;
INSERT INTO temp_person(name) VALUES ('Corinna');
ERROR:  null value in column "has_default" violates not-null constraint
CONTEXT:  SQL statement "INSERT INTO temp_person_two VALUES (  $1 .* )"
PL/pgSQL function "temp_person_insert" line 2 at SQL statement

My version:

testing=# select version();
                                                version
-------------------------------------------------------------------------------------------------------
 PostgreSQL 8.4.17 on x86_64-pc-linux-gnu, compiled by GCC gcc-4.4.real (Debian 4.4.5-8) 4.4.5, 64-bit
(1 row)

Best Answer

Your problem is that when you add a new column to the_person, its child, the_person_two will have this field appended at the end of columns list (4th position), so after has_default column. See:

db=> \d temp_person
  Column   |       Type        |                            Modifiers                            
-----------+-------------------+-----------------------------------------------------------------
 person_id | integer           | not null default nextval('temp_person_person_id_seq'::regclass)
 name      | character varying | 
 foo       | text              | 

db=> \d temp_person_two 
   Column    |         Type         |                            Modifiers                            
-------------+----------------------+-----------------------------------------------------------------
 person_id   | integer              | not null default nextval('temp_person_person_id_seq'::regclass)
 name        | character varying    | 
 has_default | character varying(4) | not null default 'en'::character varying
 foo         | text                 |

So, when you execute this:

INSERT INTO temp_person_two VALUES ( NEW.* );

PostgreSQL will actually understand that you want to insert on the first three columns of temp_person_two (as NEW.* will expand to three values), generating something similar to this:

INSERT INTO temp_person_two(person_id,name,has_default)
VALUES ( NEW.person_id, NEW.name, NEW.foo );

So, temp_person_two.has_default will get the value of NEW.foo, which is NULL in your case.

The solution is to simply expand the column names:

INSERT INTO temp_person_two(person_id,name,foo)
VALUES ( NEW.person_id, NEW.name, NEW.foo );

or, you could also use this:

INSERT INTO temp_person_two(person_id,name,foo)
VALUES ( NEW.* );

But this is weak, as any changes on column positions may break your statements, so I'd recommend the first one.

EDIT:

So the conclusion and the lesson learned here is:

Always explicitly type the names of the columns and the values when issuing an INSERT command, in fact, when issuing any SQL command at all... =D

This will save you a lot of time solving problems like that in future.

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