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