# HLL\$1SKETCH\$1ESTIMATE function
<a name="HLL_SKETCH_ESTIMATE"></a>

The HLL\$1SKETCH\$1ESTIMATE function takes an HLL sketch and estimates the number of unique elements represented by the sketch. It uses the HyperLogLog (HLL) algorithm to count a probabilistic approximation of the number of unique values in a given column, consuming a binary representation known as a sketch buffer previously generated by the HLL\$1SKETCH\$1AGG function and returning the result as a big integer. 

The HLL sketching algorithm provides an efficient way to estimate the number of unique elements, even for large datasets, without having to store the full set of unique values.

The `hll_union` and `hll_union_agg` functions can also combine sketches together by consuming and merging these buffers as inputs.

## Syntax
<a name="HLL_SKETCH_ESTIMATE-synopsis"></a>

```
HLL_SKETCH_ESTIMATE (hllsketch_expression)
```

## Argument
<a name="HLL_SKETCH_ESTIMATE-argument"></a>

 *hllsketch\$1expression*   
A `BINARY` expression holding a sketch generated by HLL\$1SKETCH\$1AGG

## Return type
<a name="HLL_SKETCH_ESTIMATE-return-type"></a>

The HLL\$1SKETCH\$1ESTIMATE function returns a BIGINT value that is the approximate distinct count represented by the input sketch.

## Examples
<a name="HLL_SKETCH_ESTIMATE-examples"></a>

The following examples use the HyperLogLog (HLL) sketching algorithm to estimate the cardinality (unique count) of values in the `col` column. The `hll_sketch_agg(col, 12)` function takes the `col` column and creates an HLL sketch using a precision of 12 bits. The HLL sketch is an approximate data structure that can efficiently estimate the number of unique elements in a set. The `hll_sketch_estimate()` function takes the HLL sketch created by `hll_sketch_agg` and estimates the cardinality (unique count) of the values represented by the sketch. The `FROM VALUES (1), (1), (2), (2), (3) tab(col);` generates a test dataset with 5 rows, where the `col` column contains the values 1, 1, 2, 2, and 3. The result of this query is the estimated unique count of the values in the `col` column, which is 3.

```
SELECT hll_sketch_estimate(hll_sketch_agg(col, 12))
    FROM VALUES (1), (1), (2), (2), (3) tab(col);
  3
```

The difference between the following example and the previous one is that the precision parameter (12 bits) isn't specified in the `hll_sketch_agg` function call. In this case, the default precision of 14 bits is used, which may provide a more accurate estimate for the unique count compared to the previous example that used 12 bits of precision.

```
SELECT hll_sketch_estimate(hll_sketch_agg(col))
FROM VALUES (1), (1), (2), (2), (3) tab(col);
3
```