Trigger-based equivalent of old PostgreSQL time travel module - see https://blog.myitcv.io/2014/02/25/row-level-version-control-with-postgresql.html for more details

time_travel_trigger.sql

/*
Copyright (c) 2015 Paul Jolly <[email protected])

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CREATE OR REPLACE FUNCTION process_timetravel_before() RETURNS TRIGGER AS $timetravel_before$
DECLARE
  temp_row RECORD; -- a temporary variable used on updates/deletes
  time_now TIMESTAMP; -- get the time now just once.... for consistency's sake
BEGIN
  time_now = now();

  IF (TG_OP = 'UPDATE' OR TG_OP = 'DELETE') THEN

    -- the user should not be able to update historic rows
    IF OLD.valid_to != 'infinity' THEN
      RAISE EXCEPTION 'Cannot % old row', TG_OP;
    END IF;

    -- use of TG_TABLE_NAME keeps this generic and non-table specific
    -- see http://www.postgresql.org/docs/9.3/static/plpgsql-statements.html#PLPGSQL-STATEMENTS-EXECUTING-DYN
    EXECUTE 'SELECT * FROM ' || TG_TABLE_NAME::regclass || ' WHERE ctid = $1 FOR UPDATE' USING OLD.ctid;

    -- not sure whether this is strictly required... could we modify OLD without side effects?
    temp_row := OLD;
    temp_row.valid_to := time_now;

    IF (TG_OP = 'UPDATE') THEN

      -- 'bump' the valid_from and ensure valid_to = 'infinity'
      NEW.valid_from := time_now;
      NEW.valid_to := 'infinity';

      -- allow the update to continue... so that the correct number of rows are reported
      -- as having been affected
      RETURN NEW;

    ELSIF (TG_OP = 'DELETE') THEN

      -- we want to allow the delete to continue... so that the correct number of rows are reported
      -- as having been affected
      RETURN OLD;

    END IF;

    RETURN NULL; -- shouldn't ever get here

  ELSIF (TG_OP = 'INSERT' AND NEW.valid_from is null OR NEW.valid_to is null) THEN

    -- this case could well be avoided by having a table definition with defaults:
    --
    --   valid_from = now()
    --   valid_to = 'infinity'
    --
    -- but we include this as a safety net
    IF NEW.valid_from is null THEN
      NEW.valid_from := time_now;
    END IF;
    IF NEW.valid_to is null THEN
      NEW.valid_to := 'infinity';
    END IF;

    -- allow the insert to continue... so that the correct number of rows are reported
    -- as having been affected
    RETURN NEW;

  ELSIF (TG_OP = 'INSERT') THEN

    -- allow the insert to continue... so that the correct number of rows are reported
    -- as having been affected
    RETURN NEW;

  END IF;

  RETURN NULL; -- won't get here if we only create the trigger for insert, update and delete

END;
$timetravel_before$ LANGUAGE plpgsql;

CREATE OR REPLACE FUNCTION process_timetravel_after() RETURNS TRIGGER AS $timetravel_after$
DECLARE
  temp_row RECORD; -- a temporary variable used on updates/deletes
  time_now TIMESTAMP; -- get the time now just once.... for consistency's sake
BEGIN
  time_now = now();

  IF (TG_OP = 'UPDATE' OR TG_OP = 'DELETE') THEN

    -- not sure whether this is strictly required... could we modify OLD without side effects?
    temp_row := OLD;
    IF (TG_OP = 'UPDATE') THEN
      temp_row.valid_to := NEW.valid_from;
    ELSIF (TG_OP = 'DELETE') THEN
      temp_row.valid_to := time_now;
    END IF;

    -- again, use of TG_TABLE_NAME keeps this generic and non-table specific
    EXECUTE 'INSERT INTO  ' || TG_TABLE_NAME::regclass || ' SELECT $1.*' USING temp_row;

  END IF;

  RETURN NULL; -- return value doesn't matter in after

END;
$timetravel_after$ LANGUAGE plpgsql;

This is pretty hot, thanks! I've made a minor tweak to this so that the id column doesn't change except on insert; this way, the id can be a stable primary key.

CREATE OR REPLACE FUNCTION process_timetravel_before() RETURNS TRIGGER AS $timetravel_before$
DECLARE
  temp_row RECORD; -- a temporary variable used on updates/deletes
  time_now TIMESTAMP; -- get the time now just once.... for consistency's sake
  id_next integer; -- <-- a variable to hold the next id
BEGIN
...
  ELSIF (TG_OP = 'INSERT') then
        -- user needs to create a sequence named {table_name_id_seq}
  	if new.id is null then
	  EXECUTE 'SELECT nextval(''' || TG_TABLE_NAME::regclass || '_id_seq' || ''')' into id_next;
      new.id := id_next;
    end if;
    RETURN NEW;

In addition, you can create a view on top of this that acts just like a normal table, with no additional code:
create view foobar as select * from _foobar_shadow where valid_to = 'infinity'

selects, updates, inserts, deletes, joins etc to the view foobar will behave exactly as if it was a normal table.

@nd2s did you have any luck with ON UPDATE CASCADE?

@micimize I am sorry i cannot remember what we tried to do back then. I can paste you what we ended up with (might be pretty much the same as the code from the initial post but I don't have time to go through it now). This works for us with PK over <id, valid_from> and unique constraint over <id, valid_to>.

CREATE OR REPLACE FUNCTION process_timetravel_before()
	RETURNS TRIGGER AS $timetravel_before$
DECLARE
	temp_row RECORD;    -- tmp var used on updates/deletes
	time_now TIMESTAMP; -- stores current time
BEGIN
	time_now = now();

	IF (TG_OP = 'UPDATE' OR TG_OP = 'DELETE') THEN
		-- the user should not be able to update historic rows
		IF OLD.valid_to != 'infinity' THEN
			RAISE EXCEPTION 'Cannot % old row', TG_OP;
		END IF;

		-- use of TG_TABLE_NAME keeps this generic and non-table specific
		-- see http://www.postgresql.org/docs/9.3/static/plpgsql-statements.html#PLPGSQL-STATEMENTS-EXECUTING-DYN
		EXECUTE 'SELECT * FROM ' || TG_TABLE_NAME::regclass ||
			' WHERE ctid = $1 FOR UPDATE' USING OLD.ctid;

		-- not sure whether this is strictly required... could we
		-- modify OLD without side effects?
		temp_row := OLD;
		temp_row.valid_to := time_now;

		IF (TG_OP = 'UPDATE') THEN
			-- 'bump' the valid_from and ensure valid_to = 'infinity'
			NEW.valid_from := time_now;
			NEW.valid_to := 'infinity';

			-- allow the update to continue... so that the correct
			-- number of rows are reported as having been affected
			RETURN NEW;

		ELSIF (TG_OP = 'DELETE') THEN
			-- we want to allow the delete to continue... so that
			-- the correct number of rows are reported as having
			-- been affected
			RETURN OLD;
		END IF;
	
		RETURN NULL; -- shouldn't ever get here

	ELSIF (TG_OP = 'INSERT') THEN
		IF NEW.valid_from IS NULL THEN
			NEW.valid_from := time_now;
		END IF;
		IF NEW.valid_to IS NULL THEN
			NEW.valid_to := 'infinity';
		END IF;

		-- allow the insert to continue... so that the correct number
		-- of rows are reported as having been affected
		RETURN NEW;
	END IF;

	-- won't get here if we only create the trigger for insert,
	-- update and delete
	RETURN NULL; 
END;
$timetravel_before$ LANGUAGE plpgsql;

CREATE OR REPLACE FUNCTION process_timetravel_after()
	RETURNS TRIGGER AS $timetravel_after$
DECLARE
	temp_row RECORD;    -- tmp var used on updates/deletes
	time_now TIMESTAMP; -- stores current time
BEGIN
	time_now = now();

	IF (TG_OP = 'UPDATE' OR TG_OP = 'DELETE') THEN
		-- not sure whether this is strictly required... could we modify
		-- OLD without side effects?
		temp_row := OLD;
		IF (TG_OP = 'UPDATE') THEN
			temp_row.valid_to := NEW.valid_from;
		ELSIF (TG_OP = 'DELETE') THEN
			temp_row.valid_to := time_now;
		END IF;

		-- again, use of TG_TABLE_NAME keeps this generic and non-table
		-- specific
		EXECUTE 'INSERT INTO  ' || TG_TABLE_NAME::regclass ||
			' SELECT $1.*' USING temp_row;
	END IF;

	-- return value doesn't matter in after
	RETURN NULL; 
END;
$timetravel_after$ LANGUAGE plpgsql;

@nd2s indeed it looks the same - I figured out the main trick is to only trigger after when (pg_trigger_depth() = 0).

I also EXECUTE 'UPDATE ...'; RETURN NULL on deletes - not sure what the consequences of the return null are though, or if it's really necessary once the trigger clause is modified.

Updated link for the related blog post: https://blog.myitcv.io/2014/02/25/row-level-version-control-with-postgresql.html

Thanks, @breml