> ## Documentation Index
> Fetch the complete documentation index at: https://docs.risingwave.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Time travel queries

> This guide describes how to leverage the time travel feature. This feature helps access historical data at a specific time.

<Tip>
  **PREMIUM FEATURE**

  This is a premium feature. For a comprehensive overview of all premium features and their usage, please see [RisingWave premium features](/get-started/premium-features).
</Tip>

## Prerequisites

Time travel requires the meta store type to be [SQL-compatible](/deploy/risingwave-docker-compose#customize-meta-store). We recommend reserving at least 50 GB of disk space for the meta store.

The system parameter `time_travel_retention_ms` controls time travel functionality. By default, it's set to `600000`, i.e. 10 minutes. To modify retention period, you need to [alter this system parameter](/operate/view-configure-system-parameters#how-to-configure-system-parameters) to a non-zero value.

For example, you can set `time_travel_retention_ms` to `86400000` (1 day). Then historical data older than this period will be deleted and no longer accessible.

<Note>
  Increasing time travel retention period will introduce additional overhead to both the meta store and the object store.
</Note>

## Syntax

Specify `FOR SYSTEM_TIME AS OF` separately for each table accessing historical data. The following subclauses can be used:

* Unix timestamp in seconds. For example, `SELECT * FROM t_foo FOR SYSTEM_TIME AS OF 1721024455;`.
* Datetime. For example, `SELECT * FROM t_foo FOR SYSTEM_TIME AS OF '2000-02-29T12:13:14-08:30';`.
* NOW() \[ - Interval ]. For example, `SELECT * FROM t_foo FOR SYSTEM_TIME AS OF NOW() - '10' SECOND;`.

<Note>
  If you specify a point in time that is outside the time travel period, the query will return an error, like `time travel: version not found for epoch`.
</Note>

## Storage space reclamation

Stale time travel data in both the meta and object stores is automatically removed in the background, freeing up storage space.