How to make PostgreSQL insert a row into a table when deleted from another table?

Question

We have an application, which will delete a row from a table based on user requests. I cannot change the application code. However, I want to insert a row into another table

Answer 1

Write a trigger function. Something like this:

CREATE OR REPLACE FUNCTION trg_backup_row()
  RETURNS trigger AS
$BODY$
BEGIN

INSERT INTO other_tbl
SELECT (OLD).*, t.other_col                -- all columns of from old table
-- SELECT OLD.col1, OLD.col2, t.other_col  -- alternative: some cols from old tbl
FROM   third_tbl t
WHERE  t.col = OLD.col  -- link to third table with info from deleted row
AND    ;

RETURN NULL;

END;
$BODY$
  LANGUAGE plpgsql VOLATILE;

And a trigger ON DELETE. Like this:

CREATE TRIGGER delaft
  AFTER DELETE
  ON tbl
  FOR EACH ROW
  EXECUTE PROCEDURE trg_backup_row();

Key elements

• Best make it a trigger AFTER DELETE and FOR EACH ROW.

• To return all columns from the old table use the syntax (OLD).*. See the manual about accessing composite types. Alternatively OLD.* is valid syntax, too, because OLD is added to the FROM clause implicitly. For a VALUES expression it would have to be (OLD).*, though. Like:

  INSERT INTO other_tbl
  VALUES((OLD).*, some_variable)
  

• You can include values from any other table like I demonstrate. Just make sure to get a single row, or you create multiple entries.

• As the trigger fires AFTER the event, the function can RETURN NULL.

---

About visibility

In response to @couling's watchful comment.

While foreign keys can be declared as DEFERRED, this will only defer the integrity check, not the deletion itself. Rows that are deleted in triggers executed before the one at hand or by ON DELETE CASCADE foreign keys will not be visible any more at the time this AFTER DELETE trigger is called. (It all happens in one transaction obviously. None of these details matter for other transactions, which will see all or none of the effects. Refer to the manual for more about the MVCC model and transaction isolation.)

Therefore, if you want to include values from rows depending in such a way in your INSERT, be sure to call this trigger before those rows get deleted.

You may have to you make this trigger BEFORE DELETE.

Or it can mean that you have to order your triggers accordingly, BEFORE triggers come before AFTER triggers, obviously. And triggers at the same level are executed in alphabetical order.

However, as long as I am super precise here, I might also add that changes made to the row (or depending rows) in other BEFORE triggers are also only visible if those are called before this one.

My advice to make it an AFTER trigger was because it is less prone to complications and cheaper if other trigger might cancel (roll back) the DELETE half way through the operation - as long as none of the above applies.