Maintaining a counter up-to-date

I have an "ips" table with 100000+ records, each record having a
"catid" field representing its category. "catid" references a row in a
table called "categories".

For statistics purpose (generation of images with the evolution of the
number of rows by category), I am trying to reduce the load on the
database.

The request I was doing at the beginning was:
 SELECT catid, COUNT(*) FROM ips GROUP BY catid;

I then added a "nentries" field to the "categories" table with some
rules to maintain the counters up-to-date:
 CREATE RULE cat_ins AS   ON INSERT TO ips   DO     UPDATE categories       SET nentries = (categories.nentries + 1)
  WHERE (categories.catid = new.catid);
 
 CREATE RULE cat_del AS   ON DELETE TO ips   DO     UPDATE categories       SET nentries = (categories.nentries - 1)
  WHERE (categories.catid = old.catid);
 
 CREATE RULE cat_upd AS   ON UPDATE TO ips   WHERE old.catid <> new.catid   DO    (UPDATE categories       SET nentries
=(categories.nentries - 1)       WHERE (categories.catid = old.catid);     UPDATE categories       SET nentries =
(categories.nentries+ 1)       WHERE (categories.catid = new.catid); );
 

This works fine when inserting, deleting or updating one row in the
"ips" table. However, when i/d/u several rows at a time with the same
"catid", I only got an increment or decrement by one of the counter.

I have not found an easy way to maintain the counter up-to-date.
I have found a complex solution: I created a "counter" table with two
fields, "catid" and "value". The idea is to put 1 in "value" for every
insertion or new value for update, or -1 for every deletion or old
value for update.
 CREATE RULE counter_ins AS   ON INSERT TO ips   DO    (INSERT INTO counter (catid, value) VALUES (new.catid, 1);
UPDATEcategories       SET nentries = nentries +               (SELECT sum(*) FROM counter                WHERE
counter.catid= categories.catid)       WHERE (categories.catid = counter.catid);     DELETE FROM counter; );
 

(I do not show the equivalent "ON DELETE" and "ON UPDATE" rules)

I have two questions:
 1) Is this way of doing things correct? Do I have the guarantee that    all the commands in the "DO" part will be
executedin a    transaction even if the initial insertion into "ips" isn't?
 
 2) What is the simplest way of doing this? I guess doing stats in a    database is quite a pretty usual operation.

Thanks in advance.
 Sam

PS/ the real problem is more complex, as we need to do those   statistics on several fields, not only "catid"
-- 
Samuel Tardieu -- sam@rfc1149.net -- http://www.rfc1149.net/sam