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

Since [1.3.0](https://github.com/timescale/timescaledb-toolkit/releases/tag/1.3.0)

Calculate the instantaneous change at the left, or earliest, edge of a counter aggregate. This is equal to the second value minus the first value, after accounting for resets.

## Samples

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

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

## Arguments

The syntax is:

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

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

## Returns

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