SELECT 
  category_id 
FROM 
  cscart_products_categories 
WHERE 
  product_id = 1135 
  AND link_type = 'M'

Query time 0.00017

Explain
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE cscart_products_categories ref link_type,pt pt 6 const,const 1 Using index condition

Result

category_id
734