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 (
    3820, 3821, 3822, 3823, 3824, 3825, 3826, 
    3827, 3828, 3829, 3830, 3831, 3832, 
    3778, 2103, 2104, 2105, 2106, 2107, 
    2108, 2109, 2110, 2111, 2112, 2113, 
    2114, 2115, 2116, 2117, 2118, 2119, 
    2120, 2121, 2122, 2123, 2124, 2125, 
    2126, 2127, 2128, 2129, 3752, 3753, 
    3754, 3755, 3756, 3757, 3758, 3759, 
    3760, 3761, 3762, 3763, 3764, 3765, 
    3766, 3767, 3768, 3769, 3770, 3771, 
    3772, 3773, 3774, 3775, 3776, 3777, 
    1143, 1139, 1140, 1141, 1142, 3681, 
    3682, 3683, 3684, 3685, 2207, 2208, 
    2209, 2210, 2211, 2212, 2213, 2214, 
    2215, 2216, 5808, 5809, 5810, 5811, 
    5812, 5813, 5814, 5815, 5816
  ) 
GROUP BY 
  cscart_products_categories.product_id

Query time 0.00401

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "61.18"
    },
    "grouping_operation": {
      "using_temporary_table": true,
      "using_filesort": true,
      "cost_info": {
        "sort_cost": "1.78"
      },
      "nested_loop": [
        {
          "table": {
            "table_name": "cscart_categories",
            "access_type": "ALL",
            "possible_keys": [
              "PRIMARY",
              "c_status",
              "p_category_id"
            ],
            "rows_examined_per_scan": 46,
            "rows_produced_per_join": 1,
            "filtered": "4.00",
            "cost_info": {
              "read_cost": "5.15",
              "eval_cost": "0.18",
              "prefix_cost": "5.33",
              "data_read_per_join": "7K"
            },
            "used_columns": [
              "category_id",
              "storefront_id",
              "usergroup_ids",
              "status"
            ],
            "attached_condition": "((`test2_gaseus`.`cscart_categories`.`storefront_id` in (0,1)) and ((`test2_gaseus`.`cscart_categories`.`usergroup_ids` = '') or (0 <> find_in_set(0,`test2_gaseus`.`cscart_categories`.`usergroup_ids`)) or (0 <> find_in_set(1,`test2_gaseus`.`cscart_categories`.`usergroup_ids`))) and (`test2_gaseus`.`cscart_categories`.`status` in ('A','H')))"
          }
        },
        {
          "table": {
            "table_name": "cscart_products_categories",
            "access_type": "ref",
            "possible_keys": [
              "PRIMARY",
              "pt"
            ],
            "key": "PRIMARY",
            "used_key_parts": [
              "category_id"
            ],
            "key_length": "3",
            "ref": [
              "test2_gaseus.cscart_categories.category_id"
            ],
            "rows_examined_per_scan": 83,
            "rows_produced_per_join": 1,
            "filtered": "1.16",
            "index_condition": "(`test2_gaseus`.`cscart_products_categories`.`product_id` in (3820,3821,3822,3823,3824,3825,3826,3827,3828,3829,3830,3831,3832,3778,2103,2104,2105,2106,2107,2108,2109,2110,2111,2112,2113,2114,2115,2116,2117,2118,2119,2120,2121,2122,2123,2124,2125,2126,2127,2128,2129,3752,3753,3754,3755,3756,3757,3758,3759,3760,3761,3762,3763,3764,3765,3766,3767,3768,3769,3770,3771,3772,3773,3774,3775,3776,3777,1143,1139,1140,1141,1142,3681,3682,3683,3684,3685,2207,2208,2209,2210,2211,2212,2213,2214,2215,2216,5808,5809,5810,5811,5812,5813,5814,5815,5816))",
            "cost_info": {
              "read_cost": "38.18",
              "eval_cost": "0.18",
              "prefix_cost": "58.78",
              "data_read_per_join": "28"
            },
            "used_columns": [
              "product_id",
              "category_id",
              "link_type"
            ]
          }
        },
        {
          "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",
              "test2_gaseus.cscart_products_categories.product_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 1,
            "filtered": "100.00",
            "cost_info": {
              "read_cost": "0.44",
              "eval_cost": "0.18",
              "prefix_cost": "59.40",
              "data_read_per_join": "28"
            },
            "used_columns": [
              "product_id",
              "category_id",
              "position"
            ]
          }
        }
      ]
    }
  }
}