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SELECT 
  feature_id 
FROM 
  cscart_product_features 
WHERE 
  feature_id IN (550) 
  AND feature_type = 'E'

Query time 0.00007

Explain
id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	cscart_product_features	const	PRIMARY	PRIMARY	3	const	1

Result

feature_id
550