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# interpolated_downtime()

> Get the total time dead from a heartbeat aggregate and predecessor

<Icon icon="tag" iconType="duotone" /> Since [1.15.0][toolkit-1.15.0]

Calculate downtime from a heartbeat aggregate, taking the heartbeat aggregate from the preceding interval to interpolate
values at the boundary. This checks when the last heartbeat in the predecessor was received and makes sure not to
consider the heartbeat interval after that time as unhealthy, even if it extends into the current aggregate prior to the
first heartbeat.

## Samples

Given a table called `liveness` containing weekly heartbeat aggregates in column `health` with timestamp column `date`,
use this command to get the total interpolated downtime of the system during the week of Jan 9, 2022.

```sql theme={"dark"}
SELECT interpolated_downtime(
  health,
  LAG(health) OVER (ORDER BY date)
)
FROM liveness
WHERE date = '01-9-2022 UTC'
```

Returns:

```
interpolated_downtime
---------------------
       00:03:55
```

## Arguments

The syntax is:

```sql theme={"dark"}
interpolated_downtime(
    agg HEARTBEATAGG,
    pred HEARTBEATAGG
) RETURNS INTERVAL
```

| Name | Type         | Default | Required | Description                                                                           |
| ---- | ------------ | ------- | -------- | ------------------------------------------------------------------------------------- |
| agg  | HeartbeatAgg | -       | ✔        | A heartbeat aggregate to get the liveness data from                                   |
| pred | HeartbeatAgg | -       |          | The heartbeat aggregate for the interval before the one being measured, if one exists |

## Returns

| Column                 | Type     | Description                                                                                                                   |
| ---------------------- | -------- | ----------------------------------------------------------------------------------------------------------------------------- |
| interpolated\_downtime | INTERVAL | The sum of all the unhealthy ranges in the aggregate, excluding those covered by the last heartbeat of the previous interval. |

[toolkit-1.15.0]: https://github.com/timescale/timescaledb-toolkit/releases/tag/1.15.0