Can't Register Shortcode w/Database Call

The following circumstances are predicated by this being a client site that I have to deal with and can’t just do things in a more sensible way. So I’m trying to make this as sensible as possible.

THE PROBLEM: I want to register a shortcode. I can set up a simple one that works, but if I try to make a database call in the function that returns the shortcode value, something is not happy. From what I can see/test, things should work. But it’s not.

EXPLANATION:
I’m calling the shortcode from a page that uses a custom template. The template dumps the content out through the_content(). A simple shortcode function works (static string value type of stuff) but the other one does not.

THE NON-WORKING CODE, IN functions.php:

function idxtoday() {
  // Air Quality Index Info
  global $index_today;
  $index_today = $wpdb->get_var( "SELECT aqi FROM $wpdb->datapoints WHERE date = '" . date( 'Y-m-d' ) . "';" );
  $index_today = ( is_null( $index_today ) ) ? '--' : $index_today;

  return "<b>$index_today</b>";
}

function register_shortcodes () {
	add_shortcode('idxtoday','idxtoday');
}

add_action( 'init', 'register_shortcodes');

Now, if I remove all the database stuff and just make $index_today some string like “Hi there, sucker!” then the shortcode works.

So… I’m guessing this is either a database thing, or a database thing combined with a functions.php thing combined with a shortcode thing. Or something.

The page I need to implement this on is in draft, as I can’t publish an unfinished page and they have no staging site. But the site is: https://littlestepsbigimpact.com/

Any ideas?