SELECT 
  gp.*, 
  p.company_id 
FROM 
  cscart_product_variation_group_products AS gp 
  LEFT JOIN cscart_products AS p ON gp.product_id = p.product_id 
WHERE 
  gp.group_id IN (868)

Query time 0.00064

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "28.00"
    },
    "nested_loop": [
      {
        "table": {
          "table_name": "gp",
          "access_type": "ref",
          "possible_keys": [
            "idx_group_id"
          ],
          "key": "idx_group_id",
          "used_key_parts": [
            "group_id"
          ],
          "key_length": "3",
          "ref": [
            "const"
          ],
          "rows_examined_per_scan": 40,
          "rows_produced_per_join": 40,
          "filtered": "100.00",
          "cost_info": {
            "read_cost": "10.00",
            "eval_cost": "4.00",
            "prefix_cost": "14.00",
            "data_read_per_join": "640"
          },
          "used_columns": [
            "product_id",
            "parent_product_id",
            "group_id"
          ]
        }
      },
      {
        "table": {
          "table_name": "p",
          "access_type": "eq_ref",
          "possible_keys": [
            "PRIMARY"
          ],
          "key": "PRIMARY",
          "used_key_parts": [
            "product_id"
          ],
          "key_length": "3",
          "ref": [
            "test2_gaseus.gp.product_id"
          ],
          "rows_examined_per_scan": 1,
          "rows_produced_per_join": 40,
          "filtered": "100.00",
          "cost_info": {
            "read_cost": "10.00",
            "eval_cost": "4.00",
            "prefix_cost": "28.00",
            "data_read_per_join": "261K"
          },
          "used_columns": [
            "product_id",
            "company_id"
          ]
        }
      }
    ]
  }
}

Result

product_id parent_product_id group_id company_id
12575 12554 868 1
12576 12554 868 1
12577 12554 868 1
12578 12554 868 1
12579 12554 868 1
12580 12554 868 1
12581 12554 868 1
12582 12554 868 1
12583 12554 868 1
12584 12554 868 1
12585 12554 868 1
12586 12554 868 1
12587 12554 868 1
12588 12554 868 1
12589 12554 868 1
12590 12554 868 1
12591 12554 868 1
12592 12554 868 1
12593 12554 868 1
12564 12554 868 1
12565 12554 868 1
12566 12554 868 1
12567 12554 868 1
12568 12554 868 1
12569 12554 868 1
12570 12554 868 1
12571 12554 868 1
12572 12554 868 1
12573 12554 868 1
12574 12554 868 1
12554 0 868 1
12556 12554 868 1
12555 12554 868 1
12558 12554 868 1
12557 12554 868 1
12560 12554 868 1
12559 12554 868 1
12562 12554 868 1
12561 12554 868 1
12563 12554 868 1