---
title: idelta_left() | Tiger Data Docs
description: Calculate the instantaneous change at the left, or earliest, edge of a gauge aggregate
---

Early access [1.6.0](https://github.com/timescale/timescaledb-toolkit/releases/tag/1.6.0)

Calculate the instantaneous change at the left, or earliest, edge of a gauge aggregate. This is equal to the second value minus the first value.

## Samples

Get the instantaneous change at the start of each 15-minute gauge aggregate.

```
SELECT
    id,
    bucket,
    idelta_left(summary)
FROM (
    SELECT
        id,
        time_bucket('15 min'::interval, ts) AS bucket,
        gauge_agg(ts, val) AS summary
    FROM foo
    GROUP BY id, time_bucket('15 min'::interval, ts)
) t
```

## Arguments

The syntax is:

```
idelta_left(
  summary GaugeSummary
) RETURNS DOUBLE PRECISION
```

| Name      | Type           | Default | Required | Description                                                                                                             |
| --------- | -------------- | ------- | -------- | ----------------------------------------------------------------------------------------------------------------------- |
| `summary` | `GaugeSummary` | -       | ✔        | A gauge aggregate created using [`gauge_agg`](/docs/reference/toolkit/counters-and-gauges/gauge_agg/gauge_agg/index.md) |

## Returns

| Column       | Type             | Description                                                                   |
| ------------ | ---------------- | ----------------------------------------------------------------------------- |
| idelta\_left | DOUBLE PRECISION | The instantaneous delta at the left, or earliest, edge of the gauge aggregate |