SELECT COUNT( DISTINCT( wpp.id ) )
FROM `wpdgls_posts` wpp JOIN `wpdgls_postmeta` wpm ON wpp.id = wpm.post_id
WHERE wpm.meta_key = '_form_id' AND wpm.meta_value = '14' ;

SELECT COUNT( DISTINCT( wpp.id ) )
FROM `wpdgls_posts` wpp JOIN `wpdgls_postmeta` wpm ON   wpp.id = wpm.post_id
WHERE wpm.meta_key = '_form_id' AND wpm.meta_value = '14' ;

KEY `post_id` (`post_id`),
KEY `meta_key` (`meta_key`(191)),
KEY `meta_value` (`meta_value`(255)),
KEY `wp_meta` (`post_id`,`meta_key`,`meta_value`(8))