Source

File: wp-admin/includes/upgrade.php

function wp_should_upgrade_global_tables() {

	// Return false early if explicitly not upgrading.
	if ( defined( 'DO_NOT_UPGRADE_GLOBAL_TABLES' ) ) {
		return false;
	}

	// Assume global tables should be upgraded.
	$should_upgrade = true;

	// Set to false if not on main network (does not matter if not multi-network).
	if ( ! is_main_network() ) {
		$should_upgrade = false;
	}

	// Set to false if not on main site of current network (does not matter if not multi-site).
	if ( ! is_main_site() ) {
		$should_upgrade = false;
	}

	/**
	 * Filters if upgrade routines should be run on global tables.
	 *
	 * @param bool $should_upgrade Whether to run the upgrade routines on global tables.
	 */
	return apply_filters( 'wp_should_upgrade_global_tables', $should_upgrade );
}