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SELECT 
  id_path 
FROM 
  cscart_categories 
WHERE 
  category_id IN (808, 813, 814, 1004, 998, 999, 1000)

Explain
id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	cscart_categories	range	PRIMARY,p_category_id,idx_category_id	PRIMARY	3		7	Using index condition