postgresql trigger for filling new column on insert - postgresql

Question

I have a table Table_A:
\d "Table_A";
Table "public.Table_A"
Column | Type | Modifiers
----------+---------+-------------------------------------------------------------
id | integer | not null default nextval('"Table_A_id_seq"'::regclass)
field1 | bigint |
field2 | bigint |
and now I want to add a new column. So I run:
ALTER TABLE "Table_A" ADD COLUMN "newId" BIGINT DEFAULT NULL;
now I have:
\d "Table_A";
Table "public.Table_A"
Column | Type | Modifiers
----------+---------+-------------------------------------------------------------
id | integer | not null default nextval('"Table_A_id_seq"'::regclass)
field1 | bigint |
field2 | bigint |
newId | bigint |
And I want newId to be filled with the same value as id for new/updated rows.
I created the following function and trigger:
CREATE OR REPLACE FUNCTION autoFillNewId() RETURNS TRIGGER AS $$
BEGIN
NEW."newId" := NEW."id";
RETURN NEW;
END $$ LANGUAGE plpgsql;
CREATE TRIGGER "newIdAutoFill" AFTER INSERT OR UPDATE ON "Table_A" EXECUTE PROCEDURE autoFillNewId();
Now if I insert something with:
INSERT INTO "Table_A" values (97, 1, 97);
newId is not filled:
select * from "Table_A" where id = 97;
id | field1 | field2 | newId
----+----------+----------+-------
97 | 1 | 97 |
Note: I also tried with FOR EACH ROW from some answer here in SO
What's missing me?

Answer 1

You need a BEFORE INSERT OR UPDATE ... FOR EACH ROW trigger to make this work:
CREATE TRIGGER "newIdAutoFill"
BEFORE INSERT OR UPDATE ON "Table_A"
FOR EACH ROW EXECUTE PROCEDURE autoFillNewId();
A BEFORE trigger takes place before the new row is inserted or updated, so you can still makes changes to the field values. An AFTER trigger is useful to implement some side effect, like auditing of changes or cascading changes to other tables.
By default, triggers are FOR EACH STATEMENT and then the NEW parameter is not defined (because the trigger does not operate on a row). So you have to specify FOR EACH ROW.