Get featured images for multiple posts at one time (in one query)

featured_images.php

<?php
/**
 * Get attachment image src for multiple posts at one time
 *
 * @param array $post_ids - The post IDs you want the featured image retrieved
 * @param string $thumbnail_size - The thumbnail size name
 * @return array - An array with the urls for the image srcs if they exist
 */
function avenirer_get_attachment_image_src($post_ids = array(), $thumbnail_size = 'full') {
	if(sizeof($post_ids) == 0) {
		return [];
	}
	$post_ids_str = '(' . implode(',', $post_ids) .')';

	global $wpdb;

	$qstr = 'SELECT post_id, meta_key, meta_value FROM wp_postmeta WHERE post_id IN (SELECT meta_value FROM wp_postmeta WHERE post_id IN ' . $post_ids_str . ' AND meta_key = "_thumbnail_id") AND meta_key = "_wp_attachment_metadata"';

	$results = $wpdb->get_results( $qstr, ARRAY_N );

	$attachments = [];

	if($results) {
		$upload_dir = wp_upload_dir();
		foreach($results as $featured) {
			$metadata = unserialize($featured[2]);
			if(array_key_exists($thumbnail_size, $metadata['sizes'])) {
				$attachments[$featured[0]] = $upload_dir['baseurl'] . '/' . $metadata['sizes'][$thumbnail_size]['file'];
			}
			else {
				$attachments[$featured[0]] = $upload_dir['baseurl'] . '/' . $metadata['file'];
			}
		}
	}
	
	return $attachments;
}