# Stale Read

__*Stale Read*__
__*tidb_replica_read*__
__*TIDB_BOUNDED_STALENESS 函数*__

+ `tidb_replica_read`: 用于控制 TiDB 读取数据的位置
+ 函数 `TIDB_BOUNDED_STALENESS`: 在时间范围内尽可能地读取最新数据

```12
/* 10 seconds before */
SELECT *
FROM test.t_west 
  AS OF TIMESTAMP (NOW()-10);

SET SESSION tidb_replica_read = 'follower';

/* Between 20 seconds before and 5 seconds before */
SELECT *
FROM test.t_west
  AS OF TIMESTAMP TIDB_BOUNDED_STALENESS(NOW()-20, NOW()-5);
```

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tidb_replica_read: 这个变量用于控制 TiDB 读取数据的位置,有以下三个选择:

  leader:只从 leader 节点读取
  follower:只从 follower 节点读取
  leader-and-follower:从 leader 或 follower 节点读取

TIDB_BOUNDED_STALENESS 函数: 在时间范围内尽可能地读取最新数据