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 = 394 
WHERE 
  cscart_products_categories.product_id IN (
    339776, 339474, 339552, 339649, 339743, 
    339756, 339647, 339665, 339470, 339773, 
    339577, 339848, 339703, 339749, 339669, 
    339455, 339666, 339789, 339651, 339565, 
    339544, 339626, 339670, 339543, 339646, 
    339824, 339833, 339834, 339575, 339784, 
    339698, 339830, 339631, 339628, 339662, 
    339767, 339557, 339567, 339549, 339595, 
    339823, 339673, 339853, 339658, 339717, 
    339763, 339852, 339542, 339818, 339653, 
    339487, 339822, 339472, 339613, 339682, 
    339480, 339555, 339639, 339560, 339579, 
    339680, 339559, 339788, 339850, 339854, 
    339815, 339772, 339561, 339785, 339766, 
    339475, 339688, 339566, 339661, 339558, 
    339819, 339585, 339837, 339563, 339640, 
    339568, 339790, 339578, 339634, 339598, 
    339629, 339672, 339546, 339844, 339641, 
    339832, 339840, 339624, 339627, 339687, 
    339842, 339612, 339827, 339642, 339800
  ) 
GROUP BY 
  cscart_products_categories.product_id

Query time 0.00366

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 (339776,339474,339552,339649,339743,339756,339647,339665,339470,339773,339577,339848,339703,339749,339669,339455,339666,339789,339651,339565,339544,339626,339670,339543,339646,339824,339833,339834,339575,339784,339698,339830,339631,339628,339662,339767,339557,339567,339549,339595,339823,339673,339853,339658,339717,339763,339852,339542,339818,339653,339487,339822,339472,339613,339682,339480,339555,339639,339560,339579,339680,339559,339788,339850,339854,339815,339772,339561,339785,339766,339475,339688,339566,339661,339558,339819,339585,339837,339563,339640,339568,339790,339578,339634,339598,339629,339672,339546,339844,339641,339832,339840,339624,339627,339687,339842,339612,339827,339642,339800))"
          }
        },
        {
          "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')))"
          }
        }
      ]
    }
  }
}