Handy mysql query to get all posts with a specific custom field. We’ll query post_type=page for the purpose of this tutorial. You will have to replace CUSTOM_FIELD_NAME with the real custom field name.
SELECT wp_posts.ID, wp_postmeta.meta_key FROM wp_posts JOIN wp_postmeta ON wp_posts.ID = wp_postmeta.post_id AND wp_postmeta.meta_key = 'CUSTOM_FIELD_NAME' WHERE wp_posts.post_type = 'page' order by wp_posts.ID asc