block-unnecessary-image-queries.php

<?php
/*
Plugin Name: Block WordPress attachment queries
Description: Blocks MySQL queries for WordPress attachments that are on domains we know will never resolve to a local WordPress attachment ID.
Author: Mark Jaquith
*/

add_filter( 'query', function( $query ) {
	global $wpdb;

	// List the domains that are external and will never resolve to local WordPress attachment IDs.
	$domains = [
		'cdn.example.com',
	];

	// Quote each domain for use in a regular expression.
	$domains = array_map( function( $domain ) {
		return preg_quote( $domain, '#' );
	}, $domains );

	// Break the domains out.
	$domains = implode( '|', $domains );

	// Filter the query.
	return preg_replace( "#SELECT post_id FROM $wpdb->postmeta WHERE meta_key = '_wp_attached_file' AND meta_value = 'https?://($domains)[^']+'#", 'SELECT 0;', $query );
});