versioned_docs/version-2.0/sql-manual/sql-functions/bitmap-functions/bitmap-hash.md

---
{
    "title": "BITMAP_HASH",
    "language": "en"
}
---

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## bitmap_hash

### Name

BITMAP_HASH

### Description

Calculating hash value for what your input and return a BITMAP which contain the hash value. MurMur3 is used for this function because it is high-performance with low collision rate. More important, the MurMur3 distribution is "simili-random"; the Chi-Square distribution test is used to prove it. By the way, Different hardware platforms and different SEED may change the result of MurMur3. For more information about its performance, see [Smhasher](http://rurban.github.io/smhasher/).

#### Syntax

`BITMAP BITMAP_HASH(<any_value>)`

#### Arguments

`<any_value>`
any value or expression. 

#### Return Type

BITMAP

#### Remarks

Generally, MurMurHash 32 is friendly to random, short STRING with low collision rate about one-billionth. But for longer STRING, such as your path of system, can cause more frequent collision. If you indexed your system path, you will find a lot of collisions.

The following two values are the same.

```sql
SELECT bitmap_to_string(bitmap_hash('/System/Volumes/Data/Library/Developer/CommandLineTools/SDKs/MacOSX12.3.sdk/System/Library/Frameworks/KernelManagement.framework/KernelManagement.tbd')) AS a ,
       bitmap_to_string(bitmap_hash('/System/Library/PrivateFrameworks/Install.framework/Versions/Current/Resources/es_419.lproj/Architectures.strings')) AS b;
```

Here is the result.

```text
+-----------+-----------+
| a         | b         |
+-----------+-----------+
| 282251871 | 282251871 |
+-----------+-----------+
```

### Example

If you want to calculate MurMur3 of a certain value, you can

```
select bitmap_to_array(bitmap_hash('hello'))[1];
```

Here is the result.

```text
+-------------------------------------------------------------+
| %element_extract%(bitmap_to_array(bitmap_hash('hello')), 1) |
+-------------------------------------------------------------+
|                                                  1321743225 |
+-------------------------------------------------------------+
```

If you want to `count distinct` some columns, using bitmap has higher performance in some scenes. 

```sql
select bitmap_count(bitmap_union(bitmap_hash(`word`))) from `words`;
```

Here is the result.

```text
+-------------------------------------------------+
| bitmap_count(bitmap_union(bitmap_hash(`word`))) |
+-------------------------------------------------+
|                                        33263478 |
+-------------------------------------------------+
```

### Keywords

    BITMAP_HASH,BITMAP

### Best Practice

For more information, see also:
- [BITMAP_HASH64](../bitmap-functions/bitmap-hash64)