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Update TiDB specific function docs (#17980) #18204

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Jul 30, 2024
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Update TiDB specific function docs (#17980) #18204

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Expand Up @@ -23,7 +23,7 @@ TiDB 支持通过标准 SQL 接口，即通过 `AS OF TIMESTAMP` SQL 语法的

如果你想要指定一个精确的时间点，可在 `AS OF TIMESTAMP` 中使用日期时间和时间函数，日期时间的格式为："2016-10-08 16:45:26.999"，最小时间精度范围为毫秒，通常可只写到秒，例如 "2016-10-08 16:45:26"。你也可以通过 `NOW(3)` 函数获得精确到毫秒的当前时间。如果想读取几秒前的数据，推荐使用例如 `NOW() - INTERVAL 10 SECOND` 的表达式。（推荐）

如果你想要指定一个时间范围，需要使用 `TIDB_BOUNDED_STALENESS()` 函数。使用该函数，TiDB 会在指定的时间范围内选择一个合适的时间戳，该时间戳能保证所访问的副本上不存在开始于这个时间戳之前且还没有提交的相关事务，即能保证所访问的可用副本上执行读取操作而且不会被阻塞。用法为 `TIDB_BOUNDED_STALENESS(t1, t2)`，其中 `t1` 和 `t2` 为时间范围的两端，支持使用日期时间和时间函数。
如果你想要指定一个时间范围，需要使用 [`TIDB_BOUNDED_STALENESS()`](/functions-and-operators/tidb-functions.md#tidb_bounded_staleness) 函数。使用该函数，TiDB 会在指定的时间范围内选择一个合适的时间戳，该时间戳能保证所访问的副本上不存在开始于这个时间戳之前且还没有提交的相关事务，即能保证所访问的可用副本上执行读取操作而且不会被阻塞。用法为 `TIDB_BOUNDED_STALENESS(t1, t2)`，其中 `t1` 和 `t2` 为时间范围的两端，支持使用日期时间和时间函数。

示例如下：

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