SELECT 
  COUNT(*) 
FROM 
  cscart_product_options_inventory 
WHERE 
  product_id = 9851

Query time 0.00092

Explain
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE cscart_product_options_inventory ref pc pc 3 const 1100 Using index

Result

COUNT(*)
1696