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 (
    339766, 339763, 339613, 339560, 339830, 
    339472, 339682, 339823, 339543, 339558, 
    339785, 339480, 339561, 339854, 339815, 
    339639, 339566, 339819, 339563, 339772, 
    339585, 339837, 339680, 339661, 339640, 
    339788, 381694, 381692, 339840, 339555, 
    339634, 339672, 339629, 339598, 339578, 
    339832, 339641, 339790, 339688, 220162, 
    339627, 339842, 381689, 381685, 339475, 
    339568, 339546, 339844, 339827, 339642, 
    339612, 265036, 339605, 381682, 220170, 
    339687, 339564, 339562, 430574, 271695, 
    339663, 339624, 339462, 271696, 339800, 
    431190, 339690, 392159, 381688, 381691, 
    339792, 381693, 339554, 339764, 431106, 
    339793, 435505, 220169, 431092, 339592, 
    381690, 339459, 339590, 220166, 431147, 
    220178, 339473, 339795, 339799, 339803, 
    339679, 339791, 339813, 339660, 339801, 
    339786, 430849, 339678, 339659, 339465
  ) 
GROUP BY 
  cscart_products_categories.product_id

Query time 0.00465

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "281.15"
    },
    "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": 100,
            "rows_produced_per_join": 100,
            "filtered": "100.00",
            "using_index": true,
            "cost_info": {
              "read_cost": "21.15",
              "eval_cost": "20.00",
              "prefix_cost": "41.15",
              "data_read_per_join": "1K"
            },
            "used_columns": [
              "product_id",
              "category_id",
              "link_type"
            ],
            "attached_condition": "(`webmarco`.`cscart_products_categories`.`product_id` in (339766,339763,339613,339560,339830,339472,339682,339823,339543,339558,339785,339480,339561,339854,339815,339639,339566,339819,339563,339772,339585,339837,339680,339661,339640,339788,381694,381692,339840,339555,339634,339672,339629,339598,339578,339832,339641,339790,339688,220162,339627,339842,381689,381685,339475,339568,339546,339844,339827,339642,339612,265036,339605,381682,220170,339687,339564,339562,430574,271695,339663,339624,339462,271696,339800,431190,339690,392159,381688,381691,339792,381693,339554,339764,431106,339793,435505,220169,431092,339592,381690,339459,339590,220166,431147,220178,339473,339795,339799,339803,339679,339791,339813,339660,339801,339786,430849,339678,339659,339465))"
          }
        },
        {
          "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": 100,
            "filtered": "100.00",
            "cost_info": {
              "read_cost": "100.00",
              "eval_cost": "20.00",
              "prefix_cost": "161.15",
              "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": "100.00",
              "eval_cost": "1.00",
              "prefix_cost": "281.15",
              "data_read_per_join": "29K"
            },
            "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')))"
          }
        }
      ]
    }
  }
}