wpdb->get_blog_prefix() doesn't allow wp_1_tablename structures

[MadtownLems]

Context: We have some OLD networks (10+ years, started as WPMU, etc)

In the process of dropping our Domain Mapping plugin for WordPress's native support, I encountered the following problem. Some (probably very few) old networks have the root site's tables as wp_1_options (instead of wp_options).

if ( defined( 'MULTISITE' ) && ( 0 === $blog_id || 1 === $blog_id ) ) {
     return $this->base_prefix;
}

This logic explicitly prevents wp_1_options and friends as being queried, with requests to the root/base site going to wp_options (which doesn't exist).

Perhaps there could be some kind of sanity check that checks for the existence of tables with the prefix in this kind of situation?

[ideag]

Hi, @MadtownLems. It has been a while since you opened this issue. I'm wondering if you still have any sites where a potential fix could be testable?