SELECT 
  cscart_ab__product_sets.set_id, 
  cscart_ab__product_sets.product_id, 
  cscart_ab__product_sets.status, 
  cscart_ab__product_sets.pos, 
  cscart_ab__product_sets_descriptions.set, 
  GROUP_CONCAT(
    cscart_ab__product_sets_products.product_id
  ) as products 
FROM 
  cscart_ab__product_sets 
  LEFT JOIN cscart_ab__product_sets_descriptions ON cscart_ab__product_sets_descriptions.set_id = cscart_ab__product_sets.set_id 
  AND cscart_ab__product_sets_descriptions.lang_code = 'en' 
  LEFT JOIN cscart_ab__product_sets_products ON cscart_ab__product_sets_products.set_id = cscart_ab__product_sets.set_id 
WHERE 
  cscart_ab__product_sets.product_id = 276428 
  AND cscart_ab__product_sets.status = "A" 
GROUP BY 
  cscart_ab__product_sets.set_id 
ORDER BY 
  cscart_ab__product_sets.pos ASC

Query time 0.00126

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "3.30"
    },
    "ordering_operation": {
      "using_temporary_table": true,
      "using_filesort": true,
      "grouping_operation": {
        "using_filesort": false,
        "nested_loop": [
          {
            "table": {
              "table_name": "cscart_ab__product_sets",
              "access_type": "ref",
              "possible_keys": [
                "PRIMARY",
                "status",
                "product_id"
              ],
              "key": "status",
              "used_key_parts": [
                "status"
              ],
              "key_length": "3",
              "ref": [
                "const"
              ],
              "rows_examined_per_scan": 1,
              "rows_produced_per_join": 1,
              "filtered": "100.00",
              "cost_info": {
                "read_cost": "1.00",
                "eval_cost": "0.10",
                "prefix_cost": "1.10",
                "data_read_per_join": "16"
              },
              "used_columns": [
                "set_id",
                "product_id",
                "pos",
                "status"
              ],
              "attached_condition": "(`s2cart`.`cscart_ab__product_sets`.`product_id` = 276428)"
            }
          },
          {
            "table": {
              "table_name": "cscart_ab__product_sets_descriptions",
              "access_type": "eq_ref",
              "possible_keys": [
                "PRIMARY"
              ],
              "key": "PRIMARY",
              "used_key_parts": [
                "set_id",
                "lang_code"
              ],
              "key_length": "10",
              "ref": [
                "s2cart.cscart_ab__product_sets.set_id",
                "const"
              ],
              "rows_examined_per_scan": 1,
              "rows_produced_per_join": 1,
              "filtered": "100.00",
              "cost_info": {
                "read_cost": "1.00",
                "eval_cost": "0.10",
                "prefix_cost": "2.20",
                "data_read_per_join": "784"
              },
              "used_columns": [
                "set_id",
                "set",
                "lang_code"
              ]
            }
          },
          {
            "table": {
              "table_name": "cscart_ab__product_sets_products",
              "access_type": "ref",
              "possible_keys": [
                "PRIMARY"
              ],
              "key": "PRIMARY",
              "used_key_parts": [
                "set_id"
              ],
              "key_length": "4",
              "ref": [
                "s2cart.cscart_ab__product_sets.set_id"
              ],
              "rows_examined_per_scan": 1,
              "rows_produced_per_join": 1,
              "filtered": "100.00",
              "using_index": true,
              "cost_info": {
                "read_cost": "1.00",
                "eval_cost": "0.10",
                "prefix_cost": "3.30",
                "data_read_per_join": "16"
              },
              "used_columns": [
                "set_id",
                "product_id"
              ]
            }
          }
        ]
      }
    }
  }
}