How to get the main blog's id and db prefix from a mu plugin?

Question

How can one get the blog ID as well as the database prefix of the main blog in a wordpress multisite installation? Is the main blog's ID always 1? Also how can the main blog's table prefix be acquired from within the mu-plugin's code?

Answer 1

Use the constant BLOG_ID_CURRENT_SITE. And to get the database table prefix, use;

$main_blog_prefix = $wpdb->get_blog_prefix(BLOG_ID_CURRENT_SITE);

Answer 2

This is how we do it to be 100% as we have multiple main sites in the network installation:

function name_ly_get_main_blog_id () {
  global $current_site;
  global $wpdb;

  return $wpdb->get_var ( $wpdb->prepare ( "SELECT `blog_id` FROM `$wpdb->blogs` WHERE `domain` = '%s' AND `path` = '%s' ORDER BY `blog_id` ASC LIMIT 1", $current_site->domain, $current_site->path ) );
}