Unknown Post Meta Records

you can achieve this in 2 step

First Step : Remove ORPHANED UNUSED post meta data
(SQL Query)

DELETE pm FROM wp_postmeta pm
LEFT JOIN wp_posts wp ON wp.ID = pm.post_id
WHERE wp.ID IS NULL

Second Step INFORMATION : Identify all post meta which is currently set to that post

$meta_values = get_post_meta( get_the_ID() );
echo '<pre>'. print_r($meta_values, 1). '</pre>';

if you found any extra meta, which is not used by your system you can delete it.

delete_post_meta( $post->ID, 'custom_key' );

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