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SELECT 
  COUNT(*) 
FROM 
  cscart_discussion_posts 
  LEFT JOIN cscart_discussion_messages ON cscart_discussion_messages.post_id = cscart_discussion_posts.post_id 
WHERE 
  cscart_discussion_posts.thread_id = 8525 
  AND cscart_discussion_messages.message <> '' 
  AND cscart_discussion_posts.status = 'A'

Query time 0.00025

Explain
id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	cscart_discussion_posts	ref	PRIMARY,thread_id,thread_id_2	thread_id	3	const	1	Using where
1	SIMPLE	cscart_discussion_messages	eq_ref	PRIMARY	PRIMARY	3	mahm3t_cs443.cscart_discussion_posts.post_id	1	Using where

Result

COUNT(*)
0