track revisions in postgresql

Question

I have to keep track of revisions of records in a table. What I've done is create a second table that inherits from the first and adds a revision counter.

CREATE TABLE A (
id SERIAL,
foo TEXT,
PRIMARY KEY (id));

CREATE TABLE B (
revision INTEGER NOT NULL) INHERITS (A);

Then I created a trigger that would update table B every time A is inserted/updated. What I can't figure out is how to make B.revision keep an individual "sequence" for each id.

Example: table A has 2 rows, i & j.
i has been updated 3 times and should have 3 revisions: (1, 2, 3).
j has been updated 2 times and should have two revisions: (1, 2).

Here is what I have so far, maybe I'm going down the wrong path and someone can help me!

CREATE OR REPLACE FUNCTION table_update() RETURNS TRIGGER AS $table_update$
    DECLARE
        last_revision INTEGER;
    BEGIN
        SELECT INTO last_revision MAX(revision) FROM B WHERE id = NEW.id;

        IF NOT FOUND THEN
            last_revision := 0;
        END IF;

        INSERT INTO B SELECT NEW.*;

        RETURN NEW;
    END;
$table_update$ LANGUAGE plpgsql;

CREATE TRIGGER table_update
AFTER INSERT OR UPDATE ON A
    FOR EACH ROW EXECUTE PROCEDURE table_update();

Answer 1

If you need the version numbers just for ordering, and don't specifically need them to be an integer that increase by one for each identifier, the easiest way to do it is to use a sequence for the revision and just let it do the tracking for you:

CREATE TABLE A (
    id SERIAL,
    foo TEXT,
    PRIMARY KEY (id)
);

CREATE TABLE B ( revision SERIAL NOT NULL) INHERITS (A);

CREATE OR REPLACE FUNCTION table_update() RETURNS TRIGGER AS $table_update$
    BEGIN
        INSERT INTO B SELECT NEW.*;
        RETURN NEW;
    END;
$table_update$ LANGUAGE plpgsql;

CREATE TRIGGER table_update
AFTER INSERT OR UPDATE ON A
    FOR EACH ROW EXECUTE PROCEDURE table_update();

Then do the inserts as usual:

    try=# insert into a (foo) values ('bar');
    INSERT 0 1
    try=# insert into a (foo) values ('bar');
    INSERT 0 1
    try=# update a set foo = 'you' where id = 1;
    UPDATE 2
    try=# select * from b;
     id | foo | revision 
    ----+-----+----------
      2 | bar |        2
      1 | you |        1
      1 | you |        3
    (3 rows)

So you can get all revisions for a given row like so:

    try=# select * from b where id = 1 order by revision;
     id | foo | revision 
    ----+-----+----------
      1 | you |        1
      1 | you |        3
    (2 rows)

Answer 2

Here's my suggestion:

CREATE OR REPLACE FUNCTION table_update() RETURNS TRIGGER AS $table_update$
DECLARE
    last_revision INTEGER;
BEGIN
    SELECT INTO last_revision coalesce(MAX(revision), 0) FROM B WHERE id = NEW.id;

    INSERT INTO B SELECT NEW.*, last_revision + 1;

    RETURN NEW;
END;
$table_update$ LANGUAGE plpgsql;

I changed the "if not found" into a coalesce, that will pick the "0" if there is no existing revision. Then, I insert in B the row, with the incremented revision.

Be careful with your inheritance: you will need to use the "only" keyword to limit yourself to the A table when selecting and updating, as such:

select * from only A
update only A set foo = ... where id = ...

Answer 3

Here is a feature rich Aduit package for postgres that I've used in the past: Audit Trigger. It tracks the type of update (insert, update, delete) as well as the before and after values for the update.

Answer 4

--THIS TABLE AUTOMATICALLY INCREMENT THE COLUMN VALUES USING TRIGGER
CREATE TABLE emp_table(
  emp_id int not null,
  emp_name varchar not null,
  emp_rollno int not null,
  primary key(emp_id)
);

--Now create table with three column and emp_id is primary key
--and emp_rollno both are automatically increment in trigger is fired
CREATE or REPLACE FUNCTION emp_fun() RETURNS TRIGGER AS $BODY$
--creating function emp_fun()
DECLARE
BEGIN
  IF(tg_op='INSERT') THEN
    NEW.emp_id=COALESCE((SELECT MAX(emp_id)+1 FROM emp_table), 1);
    NEW.emp_rollno=COALESCE((SELECT MAX(emp_rollno)+1 FROM emp_table), 1);
    --trigger is fired values is automatically increment
END IF;

IF tg_op='DELETE' THEN RETURN OLD; ELSE RETURN NEW; END IF;
END; $BODY$LANGUAGE PLPGSQL

CREATE TRIGGER emp_fun BEFORE INSERT ON
  emp_table FOR EACH ROW EXECUTE PROCEDURE emp_fun();

INSERT INTO emp_table(emp_name) VALUES('BBB');
--insert the value tanle emp_table
SELECT * FROM emp_table
-- Check the result