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 (
    66798, 66799, 66800, 66801, 66802, 66803, 
    66804, 66805, 66806, 66807, 66808, 
    66809, 66810, 66811, 66812, 66813, 
    66814, 66815, 66816, 66817, 66818, 
    66819, 66820, 66821, 66822, 66823, 
    66824, 66825, 66826, 66827, 66828, 
    66904, 66905
  ) 
GROUP BY 
  cscart_products_categories.product_id

Query time 0.00094

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "18.78"
    },
    "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": 33,
            "rows_produced_per_join": 33,
            "filtered": "100.00",
            "using_index": true,
            "cost_info": {
              "read_cost": "3.93",
              "eval_cost": "3.30",
              "prefix_cost": "7.23",
              "data_read_per_join": "528"
            },
            "used_columns": [
              "product_id",
              "category_id",
              "link_type"
            ],
            "attached_condition": "(`cscart_db`.`cscart_products_categories`.`product_id` in (66798,66799,66800,66801,66802,66803,66804,66805,66806,66807,66808,66809,66810,66811,66812,66813,66814,66815,66816,66817,66818,66819,66820,66821,66822,66823,66824,66825,66826,66827,66828,66904,66905))"
          }
        },
        {
          "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": [
              "cscart_db.cscart_products_categories.category_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 1,
            "filtered": "5.00",
            "cost_info": {
              "read_cost": "8.25",
              "eval_cost": "0.17",
              "prefix_cost": "18.78",
              "data_read_per_join": "4K"
            },
            "used_columns": [
              "category_id",
              "usergroup_ids",
              "status",
              "storefront_id"
            ],
            "attached_condition": "((`cscart_db`.`cscart_categories`.`storefront_id` in (0,1)) and ((`cscart_db`.`cscart_categories`.`usergroup_ids` = '') or (0 <> find_in_set(0,`cscart_db`.`cscart_categories`.`usergroup_ids`)) or (0 <> find_in_set(1,`cscart_db`.`cscart_categories`.`usergroup_ids`))) and (`cscart_db`.`cscart_categories`.`status` in ('A','H')))"
          }
        }
      ]
    }
  }
}