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* docs(hash_functions): fix xxHash32, xxHash64 * Fixes after review * Fixes after review * Update hash_functions.md * Update hash_functions.md
397 lines
12 KiB
Markdown
397 lines
12 KiB
Markdown
# Hash functions
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Hash functions can be used for the deterministic pseudo-random shuffling of elements.
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## halfMD5 {#hash_functions-halfmd5}
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[Interprets](../../query_language/functions/type_conversion_functions.md#type_conversion_functions-reinterpretAsString) all the input parameters as strings and calculates the [MD5](https://en.wikipedia.org/wiki/MD5) hash value for each of them. Then combines hashes, takes the first 8 bytes of the hash of the resulting string, and interprets them as `UInt64` in big-endian byte order.
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```sql
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halfMD5(par1, ...)
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```
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The function is relatively slow (5 million short strings per second per processor core).
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Consider using the [sipHash64](#hash_functions-siphash64) function instead.
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**Parameters**
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The function takes a variable number of input parameters. Parameters can be any of the [supported data types](../../data_types/index.md).
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**Returned Value**
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A [UInt64](../../data_types/int_uint.md) data type hash value.
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**Example**
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```sql
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SELECT halfMD5(array('e','x','a'), 'mple', 10, toDateTime('2019-06-15 23:00:00')) AS halfMD5hash, toTypeName(halfMD5hash) AS type
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```
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```text
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┌────────halfMD5hash─┬─type───┐
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│ 186182704141653334 │ UInt64 │
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└────────────────────┴────────┘
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```
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## MD5 {#hash_functions-md5}
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Calculates the MD5 from a string and returns the resulting set of bytes as FixedString(16).
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If you don't need MD5 in particular, but you need a decent cryptographic 128-bit hash, use the 'sipHash128' function instead.
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If you want to get the same result as output by the md5sum utility, use lower(hex(MD5(s))).
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## sipHash64 {#hash_functions-siphash64}
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Produces a 64-bit [SipHash](https://131002.net/siphash/) hash value.
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```sql
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sipHash64(par1,...)
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```
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This is a cryptographic hash function. It works at least three times faster than the [MD5](#hash_functions-md5) function.
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Function [interprets](../../query_language/functions/type_conversion_functions.md#type_conversion_functions-reinterpretAsString) all the input parameters as strings and calculates the hash value for each of them. Then combines hashes by the following algorithm:
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1. After hashing all the input parameters, the function gets the array of hashes.
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2. Function takes the first and the second elements and calculates a hash for the array of them.
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3. Then the function takes the hash value, calculated at the previous step, and the third element of the initial hash array, and calculates a hash for the array of them.
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4. The previous step is repeated for all the remaining elements of the initial hash array.
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**Parameters**
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The function takes a variable number of input parameters. Parameters can be any of the [supported data types](../../data_types/index.md).
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**Returned Value**
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A [UInt64](../../data_types/int_uint.md) data type hash value.
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**Example**
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```sql
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SELECT sipHash64(array('e','x','a'), 'mple', 10, toDateTime('2019-06-15 23:00:00')) AS SipHash, toTypeName(SipHash) AS type
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```
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```text
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┌──────────────SipHash─┬─type───┐
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│ 13726873534472839665 │ UInt64 │
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└──────────────────────┴────────┘
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```
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## sipHash128 {#hash_functions-siphash128}
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Calculates SipHash from a string.
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Accepts a String-type argument. Returns FixedString(16).
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Differs from sipHash64 in that the final xor-folding state is only done up to 128 bits.
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## cityHash64
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Produces a 64-bit [CityHash](https://github.com/google/cityhash) hash value.
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```sql
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cityHash64(par1,...)
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```
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This is a fast non-cryptographic hash function. It uses the CityHash algorithm for string parameters and implementation-specific fast non-cryptographic hash function for parameters with other data types. The function uses the CityHash combinator to get the final results.
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**Parameters**
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The function takes a variable number of input parameters. Parameters can be any of the [supported data types](../../data_types/index.md).
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**Returned Value**
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A [UInt64](../../data_types/int_uint.md) data type hash value.
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**Examples**
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Call example:
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```sql
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SELECT cityHash64(array('e','x','a'), 'mple', 10, toDateTime('2019-06-15 23:00:00')) AS CityHash, toTypeName(CityHash) AS type
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```
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```text
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┌─────────────CityHash─┬─type───┐
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│ 12072650598913549138 │ UInt64 │
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└──────────────────────┴────────┘
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```
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The following example shows how to compute the checksum of the entire table with accuracy up to the row order:
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```sql
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SELECT groupBitXor(cityHash64(*)) FROM table
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```
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## intHash32
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Calculates a 32-bit hash code from any type of integer.
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This is a relatively fast non-cryptographic hash function of average quality for numbers.
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## intHash64
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Calculates a 64-bit hash code from any type of integer.
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It works faster than intHash32. Average quality.
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## SHA1
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## SHA224
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## SHA256
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Calculates SHA-1, SHA-224, or SHA-256 from a string and returns the resulting set of bytes as FixedString(20), FixedString(28), or FixedString(32).
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The function works fairly slowly (SHA-1 processes about 5 million short strings per second per processor core, while SHA-224 and SHA-256 process about 2.2 million).
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We recommend using this function only in cases when you need a specific hash function and you can't select it.
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Even in these cases, we recommend applying the function offline and pre-calculating values when inserting them into the table, instead of applying it in SELECTS.
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## URLHash(url\[, N\])
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A fast, decent-quality non-cryptographic hash function for a string obtained from a URL using some type of normalization.
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`URLHash(s)` – Calculates a hash from a string without one of the trailing symbols `/`,`?` or `#` at the end, if present.
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`URLHash(s, N)` – Calculates a hash from a string up to the N level in the URL hierarchy, without one of the trailing symbols `/`,`?` or `#` at the end, if present.
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Levels are the same as in URLHierarchy. This function is specific to Yandex.Metrica.
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## farmHash64
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Produces a 64-bit [FarmHash](https://github.com/google/farmhash) hash value.
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```sql
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farmHash64(par1, ...)
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```
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The function uses the `Hash64` method from all [available methods](https://github.com/google/farmhash/blob/master/src/farmhash.h).
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**Parameters**
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The function takes a variable number of input parameters. Parameters can be any of the [supported data types](../../data_types/index.md).
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**Returned Value**
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A [UInt64](../../data_types/int_uint.md) data type hash value.
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**Example**
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```sql
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SELECT farmHash64(array('e','x','a'), 'mple', 10, toDateTime('2019-06-15 23:00:00')) AS FarmHash, toTypeName(FarmHash) AS type
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```
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```text
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┌─────────────FarmHash─┬─type───┐
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│ 17790458267262532859 │ UInt64 │
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└──────────────────────┴────────┘
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```
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## javaHash {#hash_functions-javahash}
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Calculates [JavaHash](http://hg.openjdk.java.net/jdk8u/jdk8u/jdk/file/478a4add975b/src/share/classes/java/lang/String.java#l1452) from a string. This hash function is neither fast nor having a good quality. The only reason to use it is when this algorithm is already used in another system and you have to calculate exactly the same result.
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```sql
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SELECT javaHash('');
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```
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**Returned value**
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A `Int32` data type hash value.
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Type: `javaHash`.
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**Example**
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Query:
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```sql
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SELECT javaHash('Hello, world!');
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```
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Result:
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```text
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┌─javaHash('Hello, world!')─┐
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│ -1880044555 │
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└───────────────────────────┘
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```
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## hiveHash {#hash_functions-hivehash}
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Calculates `HiveHash` from a string.
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```sql
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SELECT hiveHash('');
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```
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This is just [JavaHash](#hash_functions-javahash) with zeroed out sign bit. This function is used in [Apache Hive](https://en.wikipedia.org/wiki/Apache_Hive) for versions before 3.0. This hash function is neither fast nor having a good quality. The only reason to use it is when this algorithm is already used in another system and you have to calculate exactly the same result.
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**Returned value**
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A `Int32` data type hash value.
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Type: `hiveHash`.
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**Example**
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Query:
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```sql
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SELECT hiveHash('Hello, world!');
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```
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Result:
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```text
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┌─hiveHash('Hello, world!')─┐
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│ 267439093 │
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└───────────────────────────┘
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```
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## metroHash64
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Produces a 64-bit [MetroHash](http://www.jandrewrogers.com/2015/05/27/metrohash/) hash value.
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```sql
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metroHash64(par1, ...)
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```
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**Parameters**
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The function takes a variable number of input parameters. Parameters can be any of the [supported data types](../../data_types/index.md).
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**Returned Value**
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A [UInt64](../../data_types/int_uint.md) data type hash value.
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**Example**
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```sql
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SELECT metroHash64(array('e','x','a'), 'mple', 10, toDateTime('2019-06-15 23:00:00')) AS MetroHash, toTypeName(MetroHash) AS type
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```
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```text
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┌────────────MetroHash─┬─type───┐
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│ 14235658766382344533 │ UInt64 │
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└──────────────────────┴────────┘
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```
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## jumpConsistentHash
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Calculates JumpConsistentHash form a UInt64.
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Accepts two arguments: a UInt64-type key and the number of buckets. Returns Int32.
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For more information, see the link: [JumpConsistentHash](https://arxiv.org/pdf/1406.2294.pdf)
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## murmurHash2_32, murmurHash2_64
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Produces a [MurmurHash2](https://github.com/aappleby/smhasher) hash value.
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```sql
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murmurHash2_32(par1, ...)
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murmurHash2_64(par1, ...)
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```
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**Parameters**
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Both functions take a variable number of input parameters. Parameters can be any of the [supported data types](../../data_types/index.md).
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**Returned Value**
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- The `murmurHash2_32` function returns hash value having the [UInt32](../../data_types/int_uint.md) data type.
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- The `murmurHash2_64` function returns hash value having the [UInt64](../../data_types/int_uint.md) data type.
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**Example**
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```sql
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SELECT murmurHash2_64(array('e','x','a'), 'mple', 10, toDateTime('2019-06-15 23:00:00')) AS MurmurHash2, toTypeName(MurmurHash2) AS type
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```
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```text
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┌──────────MurmurHash2─┬─type───┐
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│ 11832096901709403633 │ UInt64 │
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└──────────────────────┴────────┘
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```
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## murmurHash3_32, murmurHash3_64
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Produces a [MurmurHash3](https://github.com/aappleby/smhasher) hash value.
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```sql
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murmurHash3_32(par1, ...)
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murmurHash3_64(par1, ...)
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```
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**Parameters**
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Both functions take a variable number of input parameters. Parameters can be any of the [supported data types](../../data_types/index.md).
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**Returned Value**
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- The `murmurHash3_32` function returns a [UInt32](../../data_types/int_uint.md) data type hash value.
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- The `murmurHash3_64` function returns a [UInt64](../../data_types/int_uint.md) data type hash value.
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**Example**
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```sql
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SELECT murmurHash3_32(array('e','x','a'), 'mple', 10, toDateTime('2019-06-15 23:00:00')) AS MurmurHash3, toTypeName(MurmurHash3) AS type
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```
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```text
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┌─MurmurHash3─┬─type───┐
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│ 2152717 │ UInt32 │
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└─────────────┴────────┘
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```
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## murmurHash3_128
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Produces a 128-bit [MurmurHash3](https://github.com/aappleby/smhasher) hash value.
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```sql
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murmurHash3_128( expr )
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```
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**Parameters**
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- `expr` — [Expressions](../syntax.md#syntax-expressions) returning a [String](../../data_types/string.md)-type value.
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**Returned Value**
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A [FixedString(16)](../../data_types/fixedstring.md) data type hash value.
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**Example**
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```sql
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SELECT murmurHash3_128('example_string') AS MurmurHash3, toTypeName(MurmurHash3) AS type
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```
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```text
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┌─MurmurHash3──────┬─type────────────┐
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│ 6<>1 <1C>4"S5KT<4B>~~q │ FixedString(16) │
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└──────────────────┴─────────────────┘
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```
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## xxHash32, xxHash64 {#hash_functions-xxhash32}
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Calculates `xxHash` from a string. It is proposed in two flavors, 32 and 64 bits.
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```sql
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SELECT xxHash32('');
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OR
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SELECT xxHash64('');
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```
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**Returned value**
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A `Uint32` or `Uint64` data type hash value.
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Type: `xxHash`.
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**Example**
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Query:
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```sql
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SELECT xxHash32('Hello, world!');
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```
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Result:
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```text
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┌─xxHash32('Hello, world!')─┐
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│ 834093149 │
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└───────────────────────────┘
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```
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**See Also**
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- [xxHash](http://cyan4973.github.io/xxHash/).
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[Original article](https://clickhouse.yandex/docs/en/query_language/functions/hash_functions/) <!--hide-->
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