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

# Hyperloglog Functions

## `approx_set`

Returns the HyperLogLog sketch of the input data set of x. This data sketch underlies approx\_distinct() and can be stored and used later by calling cardinality().

*Supported Signatures*

```sql theme={null}
function approx_set(bigint) returns hyperloglog
function approx_set(double) returns hyperloglog
function approx_set(varchar) returns hyperloglog
```

[🔗 Official Documentation](https://trino.io/docs/current/functions/hyperloglog.html#approx_set)

## `empty_approx_set`

Returns an empty HyperLogLog.

*Supported Signatures*

```sql theme={null}
function empty_approx_set() returns hyperloglog
```

[🔗 Official Documentation](https://trino.io/docs/current/functions/hyperloglog.html#empty_approx_set)

## `merge`

Returns the HyperLogLog of the aggregate union of the individual hll HyperLogLog structures.

*Supported Signatures*

```sql theme={null}
function merge(hyperloglog) returns hyperloglog
function merge(qdigest) returns qdigest
function merge(tdigest) returns tdigest
```

[🔗 Official Documentation](https://trino.io/docs/current/functions/hyperloglog.html#merge)