SELECT 
  product_id, 
  parent_product_id 
FROM 
  cscart_product_variation_group_products 
WHERE 
  parent_product_id = 6840

Query time 0.00039

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "7.00"
    },
    "table": {
      "table_name": "cscart_product_variation_group_products",
      "access_type": "ref",
      "possible_keys": [
        "idx_parent_product_id"
      ],
      "key": "idx_parent_product_id",
      "used_key_parts": [
        "parent_product_id"
      ],
      "key_length": "3",
      "ref": [
        "const"
      ],
      "rows_examined_per_scan": 20,
      "rows_produced_per_join": 20,
      "filtered": "100.00",
      "cost_info": {
        "read_cost": "5.00",
        "eval_cost": "2.00",
        "prefix_cost": "7.00",
        "data_read_per_join": "320"
      },
      "used_columns": [
        "product_id",
        "parent_product_id"
      ]
    }
  }
}