SELECT 
  a.category_id 
FROM 
  cscart_categories as a 
  LEFT JOIN cscart_categories as b ON b.category_id IN (902) 
WHERE 
  a.id_path LIKE CONCAT(b.id_path, '/%')

Query time 0.00039

Explain
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE b const PRIMARY,p_category_id,idx_category_id PRIMARY 3 const 1
1 SIMPLE a ALL 415 Using where

Result

category_id
814
813
808
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009