# [Known Issues] Dumpling 备份历史快照时报错 Unknown databases

## Key Words

Dumpling, snapshot, unknown databases

## Description

Drop database 后，使用 dumpling 指定 --snapshot 为 drop 之前的历史时间点备份库时报错 Unknown databases，但通过 session 设置 tidb_snapshot 可以看到已删除的 database

## Root Cause

TiDB bug 导致，设置任何非 snapshot/stale 的 session 变量，都会使已经设置的 tidb_snapshot 失效

## Diagnostic Steps

```sql
mysql> select now();
+---------------------+
| now()               |
+---------------------+
| 2021-12-15 15:12:25 |
+---------------------+
1 row in set (0.00 sec)

mysql> drop database test;
Query OK, 0 rows affected (0.21 sec)

mysql> set tidb_snapshot='2021-12-15 15:12:25';
Query OK, 0 rows affected (0.01 sec)

mysql> show databases;
+--------------------+
| Database           |
+--------------------+
| INFORMATION_SCHEMA |
| METRICS_SCHEMA     |
| PERFORMANCE_SCHEMA |
| mysql              |
| test               |
+--------------------+
5 rows in set (0.00 sec)

mysql> set session autocommit=0;
Query OK, 0 rows affected (0.00 sec)

mysql> show databases;
+--------------------+
| Database           |
+--------------------+
| INFORMATION_SCHEMA |
| METRICS_SCHEMA     |
| PERFORMANCE_SCHEMA |
| mysql              |
+--------------------+
4 rows in set (0.00 sec)
```

## Resolution

PR [##28677](https://github.com/pingcap/tidb/pull/28677) 已修复，将 TiDB 升级至 v5.2.2 及以上版本解决问题

## Workaround

dumpling PR [##357](https://github.com/pingcap/dumpling/pull/357) 已修复适配这个报错，可以使用 v5.2.2 及以上版本的 dumpling 进行备份

## Affected Versions

v5.1.0, v5.1.1, v5.1.2,v5.1.3

v5.2.0, v5.2.1