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 = 271 
WHERE 
  cscart_products_categories.product_id IN (
    15943, 15951, 15952, 15956, 15958, 15959, 
    15944, 15960, 15961, 15962, 15966, 
    15957, 15965, 15945, 15968, 15946, 
    15947, 15949, 15950, 12520, 12497, 
    12528, 12512, 12511, 12527, 10580, 
    10579, 9936, 9931, 12521, 12504, 12502, 
    12505, 13099, 12514, 12513, 9759, 10496, 
    10560, 10561, 10581, 10102, 9940, 10578, 
    10143, 9947, 10567, 10568, 10565, 9933, 
    12499, 12500, 10200, 10201, 10566, 
    10545, 10227, 10552, 12506, 10693, 
    12508, 9953, 12522, 12507
  ) 
GROUP BY 
  cscart_products_categories.product_id

Query time 0.00136

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_products_categories",
      "access_type": "range",
      "possible_keys": ["PRIMARY", "pt"],
      "key": "pt",
      "key_length": "3",
      "used_key_parts": ["product_id"],
      "rows": 121,
      "filtered": 100,
      "index_condition": "cscart_products_categories.product_id in (15943,15951,15952,15956,15958,15959,15944,15960,15961,15962,15966,15957,15965,15945,15968,15946,15947,15949,15950,12520,12497,12528,12512,12511,12527,10580,10579,9936,9931,12521,12504,12502,12505,13099,12514,12513,9759,10496,10560,10561,10581,10102,9940,10578,10143,9947,10567,10568,10565,9933,12499,12500,10200,10201,10566,10545,10227,10552,12506,10693,12508,9953,12522,12507)"
    },
    "table": {
      "table_name": "product_position_source",
      "access_type": "eq_ref",
      "possible_keys": ["PRIMARY", "pt"],
      "key": "PRIMARY",
      "key_length": "6",
      "used_key_parts": ["category_id", "product_id"],
      "ref": [
        "const",
        "noframes_angrybeaver.cscart_products_categories.product_id"
      ],
      "rows": 1,
      "filtered": 100
    },
    "table": {
      "table_name": "cscart_categories",
      "access_type": "eq_ref",
      "possible_keys": ["PRIMARY", "c_status", "p_category_id"],
      "key": "PRIMARY",
      "key_length": "3",
      "used_key_parts": ["category_id"],
      "ref": ["noframes_angrybeaver.cscart_products_categories.category_id"],
      "rows": 1,
      "filtered": 100,
      "attached_condition": "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')"
    }
  }
}