get_term_by() should check object cache

[scribu]

A lot of queries could be saved if get_term_by() would first look in the term cache before calling $wpdb.

I was thinking of adding a get_group() method to WP_Object_Cache, but I don't want to needlessly clone all the objects in the cache.

Anyway, current patch saved around 25% of queries on a complex front page.

$wp_object_cache should never be directly referenced.

Maybe add a slug cache.

wp_cache_add($term->slug, $term->term_id, "{$taxonomy}_slugs");

introduce wp_cache_get_group()

In get_terms_by.2.diff: introduce and use wp_cache_get_group()

On multisite installs getting a group will return a colossal amount of data. You also can't assume the backend cache will support it. I think the only way we can do this is as we do users, adding a slug to id cache.

Slug -> id cache. Untested.

Yeah, but that wouldn't cover names.

I think I'll just use a wrapper function for get_term_by()

Sorry, I keep seeing your comments after I post...

IMO, what would really be needed here is a way to retrieve an object by more than one key.

In the case of the object cache, this would mean storing a reference to the same taxonomy object in multiple groups.

Related: #14983