It usually does not happen. Most of the time, not all of the time but most of the time, a query has been performed and a post has been retrieved. This post retrieval is supposed to cause all meta data for the post to be retrieved in a single query, though you can force it not to happen. In any case, with this there are not extra queries for getting meta data because it is in the WP cache.
Extra queries will also be run if you use get_post_meta($post_id, ‘meta_key’), only in this case it is only one extra query instead of 2. Forcing WP to get all meta values with get_post_pets($post_id) [no field name] when you are getting meta values without getting a post is a long established performance optimization.
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