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 = 302 
WHERE 
  cscart_products_categories.product_id IN (
    2321, 2859, 1708, 631, 626, 632, 627, 
    633, 2442, 766, 1891, 1932, 2426, 2427, 
    1933, 1883, 1882, 2428, 726, 727
  ) 
GROUP BY 
  cscart_products_categories.product_id

Query time 0.00125

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "29.16"
    },
    "grouping_operation": {
      "using_filesort": false,
      "nested_loop": [
        {
          "table": {
            "table_name": "cscart_products_categories",
            "access_type": "range",
            "possible_keys": [
              "PRIMARY",
              "pt"
            ],
            "key": "pt",
            "used_key_parts": [
              "product_id"
            ],
            "key_length": "3",
            "rows_examined_per_scan": 21,
            "rows_produced_per_join": 21,
            "filtered": "100.00",
            "index_condition": "(`zdrowy_db`.`cscart_products_categories`.`product_id` in (2321,2859,1708,631,626,632,627,633,2442,766,1891,1932,2426,2427,1933,1883,1882,2428,726,727))",
            "cost_info": {
              "read_cost": "12.36",
              "eval_cost": "2.10",
              "prefix_cost": "14.46",
              "data_read_per_join": "336"
            },
            "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",
              "zdrowy_db.cscart_products_categories.product_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 21,
            "filtered": "100.00",
            "cost_info": {
              "read_cost": "5.25",
              "eval_cost": "2.10",
              "prefix_cost": "21.81",
              "data_read_per_join": "336"
            },
            "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": [
              "zdrowy_db.cscart_products_categories.category_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 1,
            "filtered": "5.00",
            "cost_info": {
              "read_cost": "5.25",
              "eval_cost": "0.11",
              "prefix_cost": "29.16",
              "data_read_per_join": "6K"
            },
            "used_columns": [
              "category_id",
              "usergroup_ids",
              "status",
              "storefront_id"
            ],
            "attached_condition": "((`zdrowy_db`.`cscart_categories`.`storefront_id` in (0,1)) and ((`zdrowy_db`.`cscart_categories`.`usergroup_ids` = '') or (0 <> find_in_set(0,`zdrowy_db`.`cscart_categories`.`usergroup_ids`)) or (0 <> find_in_set(1,`zdrowy_db`.`cscart_categories`.`usergroup_ids`))) and (`zdrowy_db`.`cscart_categories`.`status` in ('A','H')))"
          }
        }
      ]
    }
  }
}