2. timetravel.c - functions for implementing time travel feature.

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
I rewritten this, because:

on original version of postgresql 7.3.2-7.3.3:

the UPDATE not work on timetravel.example if I added
>create unique index tttest_idx on tttest (price_id,price_off);
>update tttest set price_val = 30 where price_id = 3;
ERROR:  Cannot insert a duplicate key into unique index tttest_idx

And UPDATE not work on table tttest after
>alter table tttest add column q1 text;
>alter table tttest add column q2 int;
>alter table tttest drop column q1;
>update tttest set price_val = 30 where price_id = 3;
ERROR:  Parameter '$5' is out of range

And I add a new optional feature: my new timetravel have +3 optional parameters:
inserter_user, updater_user, deleter_user.

And I add a new function: get_timetravel for get timetravel status
without change it.

A big difference: 
the old version on UPDATE changed oid on active ('infinity') record,
the new version UPDATE keep oid, and the overdued record have a new oid.
I sign with '!!!' my comment in this file.
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

   Old internally supported time-travel (TT) used insert/delete
transaction commit times. To get the same feature using triggers
you are to add to a table two columns of abstime type to store
date when a tuple was inserted (start_date) and changed/deleted 
(stop_date):

CREATE TABLE XXX (
	...		...
	date_on		abstime default currabstime(),
	date_off	abstime default 'infinity'
	...		...
/* !!! and (if have) */
	ins_user	text	/* user, who insert this record */
	upd_user	text	/* user, who updated this record */
	del_user	text	/* user, who deleted this record */
	...		...
);

!!! on INSERT my new version:
 ... and optionally set ins_user to current user, upd_user and del_user to null.

- so, tuples being inserted with NULLs in date_on/date_off will get
_current_date_ in date_on (name of start_date column in XXX) and INFINITY in
date_off (name of stop_date column in XXX).

   Tuples with stop_date equal INFINITY are "valid now": when trigger will
be fired for UPDATE/DELETE of a tuple with stop_date NOT equal INFINITY then
this tuple will not be changed/deleted!

   If stop_date equal INFINITY then on

UPDATE: 
original version was:
 only stop_date in tuple being updated will be changed to current
 date and new tuple with new data (coming from SET ... in UPDATE) will be
 inserted. Start_date in this new tuple will be setted to current date and
 stop_date - to INFINITY.
On my new version:
 insert a new tuple with old values, but stop_date changed to current date;
 and update original tuple with new data, and update start_date to current date
 and optionally set upd_user to current user and clear ins_user,del_user.

DELETE: new tuple will be inserted with stop_date setted to current date
(and with the same data in other columns as in tuple being deleted).
On my new version:
 ... and optionally set del_user to current user.

   NOTE:
1. To get tuples "valid now" you are to add _stop_date_ = 'infinity'
   to WHERE. Internally supported TT allowed to avoid this...
   Fixed rewriting RULEs could help here...
   As work arround you may use VIEWs...
2. You can't change start/stop date columns with UPDATE! 
   Use set_timetravel (below) if you need in this.

   FUNCTIONs:

timetravel() is general trigger function.

   You are to create trigger BEFORE UPDATE OR DELETE using this
function on a time-traveled table. You are to specify two arguments: name of
start_date column and name of stop_date column in triggered table.
Or add +3 arguments: 
  name of insert_user column, name of update_user column, name of delete_user column

currabstime() may be used in DEFAULT for start_date column to get
current date.
!!! I deleted this function, because I newer used this.

set_timetravel() allows you turn time-travel ON/OFF for a table:

   set_timetravel('XXX', 1) will turn TT ON for table XXX (and report
old status).
   set_timetravel('XXX', 0) will turn TT OFF for table XXX (-"-).

Turning TT OFF allows you do with a table ALL what you want.

get_timetravel() reports time-travel status ON(1)/OFF(0) for a table.
get_timetravel() and set_timetravel() not checking existing of table and
existing of timetravel trigger on specified table.

   There is example in timetravel.example.

   To CREATE FUNCTIONs use timetravel.sql (will be made by gmake from
timetravel.source).