_get_cat_children is very inefficient

See also #3614

Please forgive my ignorance, as your PHP skills would be way better than mine, but when I do hierachies in arrays, I generally use matrixes (matrices?):

array(
    parent => array(
        child => array(
            grandchild => 'value'
        ),
    ),
)

Is this what you meant, or is your method better performance-wise? Or should I just stay away from Trac? ;)

Acutally, we should probably do it with left and right pointers.

parent => (root, child), child => (parent, grandchild)

I agree.

If we use the hierarchy method mentioned above we would recursively have to search through the array whereas with the left and right pointer-method it wouldn't matter how "deep" the array in question was, the search is simple.

I have some free time tonight, I'd love to get some work done on this.

Just clarifying some things here -- feel free to correct me if I'm wrong in these

- Syncing the array should be done only when categories are created, edited, or deleted - A "sanity check" should be done upon access to the Manage Categories page. This is how it's done for plugins anyways, so I'm just trying to keep our methods consistent.

On second thought, perhaps instead of a sanity check this could be a good time to rebuild the array completely from the database.

I would also like this array to be storable using Wordpress' built-in caching functions.

For quickly and efficiently building the array I recommend we add a table to the schema: wp_cat_hierarchy

wp_cat_hierarchy would consist of 2 columns: parent_id and child_id. There would be a unique key on the pair. This would allow us to quickly and efficiently build our hierarchy array.

Actually, I'd like to try to get this into 2.2. WP sucks with a large number of categories because of this bug.

For 2.2, I think an array approach would suffice. I'd rather not muck with new tables right now.

First cut at caching category children

Patch creates array of categories and their children and sticks it in the options DB. Array is consulted to avoid descending through _get_cat_children() when there are no children. Next pass will remove the recursion.

(In [5178]) Cache category hierarchy to make cat listing faster. Phase 1. see #3985

That change avoids walking the category list for cats that have no children. With a categories table containing 3100 cats arranged in a flat hierarchy, time need to list categories went from minutes to seconds. Next I need to eliminate recursion altogether to sped up listing for those who use lots of hierarchy.

(In [5179]) Category listing speedups. see #3985

What about replace

delete_option('category_children'); 

in clean_category_cache() to

update_option('category_children', ""); 

clean_category_cache() will be called when publishing or editing posts. Category_children option will be deleted and added very often.

An empty string can work well, because _get_category_hierarchy() use is_array() to check category_children option. If it is not an array, it will be identified as invalid cache.

Or change it to update_option('category_children', array());

So it gets serialized and returns a blank array, so it's a valid cache.

(In [5295]) Fix variable collission in _get_cat_children. see #3985

(In [5296]) Fix variable collission in _get_cat_children. For 2.2. see #3985

Let's call this good enough for now.