Postgres UPSERT (INSERT or UPDATE) only if value is different

Question

I'm updating a Postgres 8.4 database (from C# code) and the basic task is simple enough: either UPDATE an existing row or INSERT a new one if one doesn't exist yet. Normally I would do this:

UPDATE my_table
SET value1 = :newvalue1, ..., updated_time = now(), updated_username = 'evgeny'
WHERE criteria1 = :criteria1 AND criteria2 = :criteria2

and if 0 rows were affected then do an INSERT:

INSERT INTO my_table(criteria1, criteria2, value1, ...)
VALUES (:criteria1, :criteria2, :newvalue1, ...)

There is a slight twist, though. I don't want to change the updated_time and updated_username columns unless any of the new values are actually different from the existing values to avoid misleading users about when the data was updated.

If I was only doing an UPDATE then I could add WHERE conditions for the values as well, but that won't work here, because if the DB is already up to date the UPDATE will affect 0 rows and then I would try to INSERT.

Can anyone think of an elegant way to do this, other than SELECT, then either UPDATE or INSERT?

Answer 1

Take a look at a BEFORE UPDATE trigger to check and set the correct values:

CREATE OR REPLACE FUNCTION my_trigger() RETURNS TRIGGER LANGUAGE plpgsql AS
$$
BEGIN
    IF OLD.content = NEW.content THEN
        NEW.updated_time= OLD.updated_time; -- use the old value, not a new one.
    ELSE
        NEW.updated_time= NOW();
    END IF;
    RETURN NEW;
END;
$$;

Now you don't even have to mention the field updated_time in your UPDATE query, it will be handled by the trigger.

http://www.postgresql.org/docs/current/interactive/plpgsql-trigger.html

Answer 2

Two things here. Firstly depending on activity levels in your database you may hit a race condition between checking for a record and inserting it where another process may create that record in the interim. The manual contains an example of how to do this link example

To avoid doing an update there is the suppress_redundant_updates_trigger() procedure. To use this as you wish you wold have to have two before update triggers the first will call the suppress_redundant_updates_trigger() to abort the update if no change made and the second to set the timestamp and username if the update is made. Triggers are fired in alphabetical order. Doing this would also mean changing the code in the example above to try the insert first before the update.

Example of how suppress update works:

    DROP TABLE sru_test;

    CREATE TABLE sru_test(id integer not null primary key,
    data text,
    updated timestamp(3));

    CREATE TRIGGER z_min_update
    BEFORE UPDATE ON sru_test
    FOR EACH ROW EXECUTE PROCEDURE suppress_redundant_updates_trigger();

    DROP FUNCTION set_updated();

    CREATE FUNCTION set_updated()
    RETURNS TRIGGER
    AS $$
    DECLARE
    BEGIN
        NEW.updated := now();
        RETURN NEW;
    END;
    $$ LANGUAGE plpgsql;

    CREATE TRIGGER zz_set_updated
    BEFORE INSERT OR UPDATE ON sru_test
    FOR EACH ROW EXECUTE PROCEDURE  set_updated();

insert into sru_test(id,data) VALUES (1,'Data 1');
insert into sru_test(id,data) VALUES (2,'Data 2');

select * from sru_test;

update sru_test set data = 'NEW';

select * from sru_test;

update sru_test set data = 'NEW';

select * from sru_test;

update sru_test set data = 'ALTERED'  where id = 1;

select * from sru_test;

update sru_test set data = 'NEW' where id = 2;

select * from sru_test;