SELECT 
  product_id, 
  parent_product_id 
FROM 
  cscart_product_variation_group_products 
WHERE 
  parent_product_id = 5476

Query time 0.00041

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_product_variation_group_products",
      "access_type": "ref",
      "possible_keys": ["idx_parent_product_id"],
      "key": "idx_parent_product_id",
      "key_length": "3",
      "used_key_parts": ["parent_product_id"],
      "ref": ["const"],
      "rows": 8,
      "filtered": 100
    }
  }
}