(Side notes too big for a Comment)
There is no need for an AUTO_INCREMENT
id in a mapping table; get rid of it.
Change the PRIMARY KEY
to (role_id, role_group_id)
(in either order). This will make accesses faster.
Since you probably want to map both directions, also add an INDEX
with those two columns in the opposite order. (There is no need to make it UNIQUE
.)
More tips: http://mysql.rjweb.org/doc.php/index_cookbook_mysql#speeding_up_wp_postmeta