Trigger or Rule ?

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Hi everybody,

I have tables like

create table xx (
	id int4,
	col1 whatever,
	col2 whatever,
	col3 whatever,
	valid_from date,
	valid_thru date);

where the primary key is always (id, valid_from).
Now my software has a routine that checks on update if
there is a currently valid row (valid_from <= current_date < valid_thru) with 
a valid_from matching the valid_from < today
If so, the currently valid row is updated with valid_thru = yesterday.
The new record then is inserted (instead of updated) with a valid_from of 
today and a valid_thru of some date far in the future ('9999-01-01')

If there is a currently valid record with  valid_from = today, then a normal 
update is performed.

In the end this produces a record history with a granularity of one day.
Since I perform a LOT of these calls (sometimes about 1000 such "update 
queries" at a time) I thought it might be a great idea to push this 
checking/updating/inserting into the database, instead of performing that 
outside from the application side.

Questions:

a) Which would be better (faster, less expensive): Trigger or Rule ?
b) If Trigger, how can I avoid that the trigger triggers itself (in the end it 
performs an update to the same table)
c) How can I avoid to create explicit updates/inserts for this, i.e. I don't 
want to to a "insert into xx (id,col1,col2,col3,valid_from,valid_thru) values 
(new.id,new.col1,new.col2 .........", instead I'd like to create a function 
that gets a tablename and rowset with the new data and creates the insert 
statement itself. Then it could be applied to any table matching the above 
schema.

My bet would be a trigger, since it calls a function directly, however then I 
obviously can't control the parameters given to the function.

Any help is greatly appreciated.

 UC

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