Rules and Triggers

Hi:
 Are there cases when triggers are faster than rules?
It appears that on the database that I used; rules
used sequential scan while triggers used index scan.   I had a table "sc_redeem". There is a rule which I
place on that table. The body of rule is "  CREATE RULE rule_update_redeem as  ON insert TO sc_redeem  DO     UPDATE
sc_customer_card    SET   redeemed = redeemed + new.points,           balance  = balance - new.points     WHERE
customer_id= new.customer_id AND           card_type_cd = new.card_type_cd;"
 

The index of sc_customer_card is on the field
"customer_id". I noticed when I use the SQL statement. EXPLAIN INSERT INTO sc_redeem(....) VALUES (....);
It reported that it performed a sequential scan on the
table sc_customer_card (around 100,000+ records w/
unique customer id for each record).
 I tried converting the rule to function & trigger
and it reported that it used an Index scan given the
same INSERT statement.
  I was wondering why the rule used sequentail scan
and the equivalent function trigger used index scan.

Thank you,
ludwig lim 
 

__________________________________________________
Do You Yahoo!?
LAUNCH - Your Yahoo! Music Experience
http://launch.yahoo.com