Proper way to bulk-delete variables on module uninstall

I am attempting to make the best hook_uninstall ever :D Sorry in advance for the long post.

As is the case with a lot of modules, mine makes many variables per node type it is enabled for. It would be tiresome and inefficient to delete every one of these variables with variable_del.

A common solution that appears to have its roots in Drupal 6 is db_query("DELETE FROM {variable} WHERE name LIKE 'mymodule_%'");
Or the Drupal 7 converted version, which is technically incorrect, being db_delete('variable')->condition('name', 'mymodule_%', 'LIKE')->execute();
Or the even better and correct D7 version db_delete('variable')->condition('name', db_like('mymodule_') . '%', 'LIKE')->execute();

This doesn't fully replicate variable_del though, as variable_del also flushes the cache and removes the variables from the global $conf array. Clearing the cache is simple, just add the same bit of code cache_clear_all('variables', 'cache_bootstrap');

Clearing the variables from the $conf array is a bit trickier, and there are a few methods that I can see.

1. We could do a db_select before the db_delete to obtain a list of keys to remove from the $conf array. This involves 2 db queries, so probably not the most efficient (unless there is a way to combine the select and delete into 1 that I am not aware of?)
2. Search through the $conf array for keys that start with 'mymodule_', much the same as the db like query. I tested a few different methods for this, which gave varying results (timed with microtime() and displayed as difference*1000).

   1.  $conf = array_intersect_key($conf, array_flip(preg_grep("/^mymodule_.*/", array_keys($conf), PREG_GREP_INVERT)));
      // Result 0.41699409484863 

   2. foreach($conf as $key => $value) {
        if (substr($key, 0, strlen('mymodule_')) === 'mymodule_') {
          unset($conf[$key]);
        }
      } // Result 1.2187957763672
      

   3. foreach($conf as $key => $value) {
        if (0 === strpos($key, 'mymodule_')) {
          unset($conf[$key]);
        }
      } // Result 0.69212913513184
      

   4. foreach($conf as $key => $value) {
        if (preg_match("/^mymodule_.*/", $key) === 1) {
          unset($conf[$key]);
        }
      } // Result 1.2669563293457
      

   So 2.1 is the fastest, but it does a lot. 2.3 is 50% slower, but is a lot easier to understand.

So, here is my final code (using 2.1). My questions to those smarter than me are: will it correctly modify the global $conf (or does it need to be @$conf of something?), is it the best method, and do you have any other comments?

global $conf;
$module_string = "mymodule_";

db_delete('variable')->condition('name', db_like($module_string) . '%', 'LIKE')->execute();
cache_clear_all('variables', 'cache_bootstrap');

$conf = array_intersect_key($conf, array_flip(preg_grep("/^{$module_string}.*/", array_keys($conf), PREG_GREP_INVERT)));