SELECT 
  product_id 
FROM 
  cscart_product_related_products 
WHERE 
  related_id = 198

Query time 0.00023

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_product_related_products",
      "access_type": "index",
      "key": "PRIMARY",
      "key_length": "6",
      "used_key_parts": ["product_id", "related_id"],
      "rows": 1,
      "filtered": 100,
      "attached_condition": "cscart_product_related_products.related_id = 198",
      "using_index": true
    }
  }
}