Loop through posts that are missing meta data, and add some default value

template.php

<?php
/**
 * Template Name: Full Width
 *
 * Gets all posts that are missing default meta value, and gives them a default meta value.
 * Add this file to your theme then add a page with this template.
 *
 * IMPORTANT:
 * Visit that page and keep refreshing until you see no more posts left.
 * Each page load adds the new post meta, so the loop count drops by the posts_per_page count each time.
 * Therefore regular pagination does not work as expected.
 */
while ( have_posts() ) : the_post();
	
	// Variables
	$post_type = array('resource');
	$posts_per_page = 500; // A very large number could crash. -1 to get all posts
	$meta_key = 'views';
	$default_meta_value = '';

	// Setup the query for posts where the meta_key doesn't exist
	$args = array(
		'post_type' => $post_type,
		'posts_per_page' => $posts_per_page,
		'paged' => $paged,
		'meta_key' => $meta_key,
		'meta_compare' => 'NOT EXISTS'
	);
	$query = new WP_Query( $args );

	if ( $query->have_posts() ) :
		echo '<h4>' . count($query->posts) . ' posts out of ' . $query->found_posts . ' have been updated.</h4>';
		echo '<p>To update the next batch, just refresh the page.</p>';

		// Update the meta key for all posts with the default initial value
		while ( $query->have_posts() ) : $query->the_post();
			$post_id = get_the_ID();
			update_post_meta( $post_id, $meta_key, $default_meta_value );
			// Just so we can see them
			echo '<p>' . get_the_title() . ' | ' . $meta_key . ' = ' .  get_post_meta( $post_id, $meta_key, true ). '</p>';
		endwhile;
		// Just in case
		wp_reset_postdata();
  
  else :
    echo '<p>Yay! No more missing data</p>';
	endif;
endwhile; ?>