SELECT 
  cscart_products_categories.product_id, 
  GROUP_CONCAT(
    IF(
      cscart_products_categories.link_type = "M", 
      CONCAT(
        cscart_products_categories.category_id, 
        "M"
      ), 
      cscart_products_categories.category_id
    )
  ) AS category_ids, 
  product_position_source.position AS position 
FROM 
  cscart_products_categories 
  INNER JOIN cscart_categories ON cscart_categories.category_id = cscart_products_categories.category_id 
  AND cscart_categories.storefront_id IN (0, 1) 
  AND (
    cscart_categories.usergroup_ids = '' 
    OR FIND_IN_SET(
      0, cscart_categories.usergroup_ids
    ) 
    OR FIND_IN_SET(
      1, cscart_categories.usergroup_ids
    )
  ) 
  AND cscart_categories.status IN ('A', 'H') 
  LEFT JOIN cscart_products_categories AS product_position_source ON cscart_products_categories.product_id = product_position_source.product_id 
  AND product_position_source.category_id = 266 
WHERE 
  cscart_products_categories.product_id IN (
    394196, 114756, 81595, 265003, 149755, 
    264991, 262367, 286468, 387085, 265175, 
    80451, 262401, 387100, 387101, 277549, 
    286472, 277552, 265004, 262376, 265174, 
    277550, 286478, 277548, 387099, 149820, 
    81523, 286465, 277547, 387097, 387102, 
    149750, 286467, 339527, 264986, 286475, 
    391790, 387103, 286479, 359819, 286473, 
    286469, 391787, 391791, 31800, 286466, 
    435216, 286470, 391792, 286471, 277551, 
    391739, 286474, 286, 391740, 262374, 
    381645, 359629, 264695, 262391, 535, 
    359628, 381653, 391793, 381646, 381643, 
    391795, 392213, 225059, 387091, 391794, 
    387043, 264694, 262397, 264699, 381647, 
    391788, 391797, 381644, 339513, 391796, 
    282476, 151903, 381642, 81654, 387038, 
    339519, 81637, 282477, 264697, 264700, 
    339510, 339508, 265002, 339518, 339713, 
    339505, 265235, 339525, 81590, 262396
  ) 
GROUP BY 
  cscart_products_categories.product_id

Query time 0.00283

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "297.96"
    },
    "grouping_operation": {
      "using_filesort": false,
      "nested_loop": [
        {
          "table": {
            "table_name": "cscart_products_categories",
            "access_type": "range",
            "possible_keys": [
              "PRIMARY",
              "link_type",
              "pt"
            ],
            "key": "pt",
            "used_key_parts": [
              "product_id"
            ],
            "key_length": "3",
            "rows_examined_per_scan": 106,
            "rows_produced_per_join": 106,
            "filtered": "100.00",
            "using_index": true,
            "cost_info": {
              "read_cost": "22.36",
              "eval_cost": "21.20",
              "prefix_cost": "43.56",
              "data_read_per_join": "1K"
            },
            "used_columns": [
              "product_id",
              "category_id",
              "link_type"
            ],
            "attached_condition": "(`webmarco`.`cscart_products_categories`.`product_id` in (394196,114756,81595,265003,149755,264991,262367,286468,387085,265175,80451,262401,387100,387101,277549,286472,277552,265004,262376,265174,277550,286478,277548,387099,149820,81523,286465,277547,387097,387102,149750,286467,339527,264986,286475,391790,387103,286479,359819,286473,286469,391787,391791,31800,286466,435216,286470,391792,286471,277551,391739,286474,286,391740,262374,381645,359629,264695,262391,535,359628,381653,391793,381646,381643,391795,392213,225059,387091,391794,387043,264694,262397,264699,381647,391788,391797,381644,339513,391796,282476,151903,381642,81654,387038,339519,81637,282477,264697,264700,339510,339508,265002,339518,339713,339505,265235,339525,81590,262396))"
          }
        },
        {
          "table": {
            "table_name": "product_position_source",
            "access_type": "eq_ref",
            "possible_keys": [
              "PRIMARY",
              "pt"
            ],
            "key": "PRIMARY",
            "used_key_parts": [
              "category_id",
              "product_id"
            ],
            "key_length": "6",
            "ref": [
              "const",
              "webmarco.cscart_products_categories.product_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 106,
            "filtered": "100.00",
            "cost_info": {
              "read_cost": "106.00",
              "eval_cost": "21.20",
              "prefix_cost": "170.76",
              "data_read_per_join": "1K"
            },
            "used_columns": [
              "product_id",
              "category_id",
              "position"
            ]
          }
        },
        {
          "table": {
            "table_name": "cscart_categories",
            "access_type": "eq_ref",
            "possible_keys": [
              "PRIMARY",
              "c_status",
              "p_category_id"
            ],
            "key": "PRIMARY",
            "used_key_parts": [
              "category_id"
            ],
            "key_length": "3",
            "ref": [
              "webmarco.cscart_products_categories.category_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 5,
            "filtered": "5.00",
            "cost_info": {
              "read_cost": "106.00",
              "eval_cost": "1.06",
              "prefix_cost": "297.96",
              "data_read_per_join": "31K"
            },
            "used_columns": [
              "category_id",
              "usergroup_ids",
              "status",
              "storefront_id"
            ],
            "attached_condition": "((`webmarco`.`cscart_categories`.`storefront_id` in (0,1)) and ((`webmarco`.`cscart_categories`.`usergroup_ids` = '') or find_in_set(0,`webmarco`.`cscart_categories`.`usergroup_ids`) or find_in_set(1,`webmarco`.`cscart_categories`.`usergroup_ids`)) and (`webmarco`.`cscart_categories`.`status` in ('A','H')))"
          }
        }
      ]
    }
  }
}