SELECT 
  product_id, 
  parent_product_id 
FROM 
  cscart_product_variation_group_products 
WHERE 
  parent_product_id = 290

Query time 0.00024

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "message": "Impossible WHERE noticed after reading const tables"
    }
  }
}