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 
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') 
WHERE 
  cscart_products_categories.product_id IN (
    355, 354, 353, 352, 351, 350, 349, 348, 
    347, 346, 345, 344, 342, 341, 340, 339
  ) 
GROUP BY 
  cscart_products_categories.product_id

Query time 0.00053

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "read_sorted_file": {
          "filesort": {
            "sort_key": "cscart_products_categories.product_id",
            "table": {
              "table_name": "cscart_products_categories",
              "access_type": "ALL",
              "possible_keys": ["PRIMARY", "pt"],
              "rows": 80,
              "filtered": 20,
              "attached_condition": "cscart_products_categories.product_id in (355,354,353,352,351,350,349,348,347,346,345,344,342,341,340,339)"
            }
          }
        }
      },
      {
        "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": ["demov2026.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')"
        }
      }
    ]
  }
}