On updating from EE 2.1.3 to EE 2.2.2. I ran into one issue that stopped the update cold. See, on the EE Requirements page, you list the minimum MySQL grant privileges for database tables.
EE 2.1.5, however, added INDEX. In /system/installer/updates/ud_215.php (lines 570-7), there’s this bit:
// We do a ton of template lookups based off the template name. How about indexing on it?
$this->EE->db->query("CREATE INDEX template_name on exp_templates(template_name)");
...Do please update the docs! I’ve filed a bug report: https://support.ellislab.com/bugs/detail/16523/