Recursive MySQL trigger which calls the same table and the same trigger - mysql

Question

I'm writing a simple forum for a php site. I'm trying to calculate the post counts for each category. Now a category can belong to another category with root categories being defined as having a NULL parent_category_id. With this architecture a category can have an unlimited number of sub-categories and keeps the table structure fairly simple.
To keep things simple lets say the categories table has 3 fields: category_id, parent_category_id, post_count. I don't think the remaining database structure is relevant so I'll leave it out for now.
Another trigger is calling the categories table causing this trigger to run. What I want is it to update the post count and then recursively go through each parent category increasing that post count.
DELIMITER $$
CREATE TRIGGER trg_update_category_category_post_count BEFORE UPDATE ON categories FOR EACH ROW
BEGIN
IF OLD.post_count != NEW.post_count THEN
IF OLD.post_count < NEW.post_count THEN
UPDATE categories SET post_count = post_count + 1 WHERE categories.category_id = NEW.parent_category_id;
ELSEIF OLD.post_count > NEW.post_count THEN
UPDATE categories SET post_count = post_count - 1 WHERE categories.category_id = NEW.parent_category_id;
END IF;
END IF;
END $$
DELIMITER ;
The error I'm getting is:
#1442 - Can't update table 'categories' in stored function/trigger because it is already used by statement which invoked this stored function/trigger.
I figure you can do a count() on each page load to calculate the total posts but on large forums this will slow things down as discussed many times on here (e.g. Count posts with php or store in database). Therefore for future proofing i'm storing the post count in the table. To go one step further I thought i'd use triggers to update these counts rather than PHP.
I understand there are limitations in MySQL for running triggers on the same table that's being updated which is what is causing this error (i.e. to stop an infinite loop) but in this case surely the loop would stop once it reaches a category with a NULL parent_category_id? There must be some kind of solution whether it's adjusting this trigger or something different entirely. Thanks.
EDIT I appreciate this might not be the best way of doing things but it is the best thing I can think of. I suppose if you changed a parents category to another it would mess things up, but this could be fixed by another trigger which re-syncs everything. I'm open to other suggestions on how to solve this problem.

Answer 1

I usually recommend against using triggers unless you really, really need to; recursive triggers are a great way of introducing bugs that are really hard to reproduce, and require developers to understand the side effects of an apparently simple action - "all I did was insert a record into the categories table, and now the whole database has locked up". I've seen this happen several times - nobody did anything wrong or stupid, it's just a risk you run with side effects.
So, I would only resort to triggers once you can prove you need to; rather than relying on the opinion of strangers based on generalities, I'd rig up a test environment, drop in a few million test records, and try to optimize the "calculate posts on page load" solution so it works.
A database design that might help with that is Joe Celko's "nested set" schema - this takes a while to get your head round, but can be very fast for querying.
Only once you know you have a problem that you really can't solve other than by pre-computing the post count would I consider a trigger-based approach. I'd separate out the "post counts" into a separate table; that keeps your design a little cleaner, and should get round the recursive trigger issue.

Answer 2

The easiest solution is to fetch all the posts per category and afterwards link them together using a script/programming language:
for instance in php:
<?php
// category: id, parent, name
// posts: id, title, message
$sql = "select *, count(posts.id) From category left join posts ON posts.cat = category.id Group by category.id";
$query = mysql_query($sql);
$result = array();
while($row = mysql_fetch_assoc($query)){
$parent = $row['parent'] == null ? 0 : $row['parent'];
$result[$parent][] = $row;
}
recur_count(0);
var_dump($result);
function recur_count($depth){
global $result;
var_dump($result[$depth],$depth);
foreach($result[$depth] as $id => $o){
$count = $o['count'];
if(isset($result[$o['id']])){
$result[$depth][$id]['count'] += recur_count($o['id']);
}
}
return $count;
}

Answer 3

Ok so for anyone wondering how I solved this I used a mixture of both triggers and PHP.
Instead of getting each category to update it's parent, I've left it to the following structure: a post updates it's thread and then a thread updates it's category with the post count.
I've then used PHP to pull all categories from the database and loop through adding up each post count value using something like this:
function recursiveCategoryCount($categories)
{
$count = $categories['category']->post_count;
if(!is_null($categories['children']))
foreach($categories['children'] as $child)
$count += recursiveCategoryCount($child);
return $count;
}
At worst instead of PHP adding up every post on every page load, it only adds up the total category posts (depending at what node in the tree you are in). This should be very efficient as you're reducing the total calculations from 1000s to 10s or 100s depending on your number of categories. I would also recommend running a script every week to recalculate the post counts in case they become out of sync, much like phpBB. If I run into issues using triggers then I'll move that functionality into the code. Thanks for everyones suggestions.