mirror of
https://github.com/ClickHouse/ClickHouse.git
synced 2024-12-15 02:41:59 +00:00
67 lines
3.2 KiB
Markdown
67 lines
3.2 KiB
Markdown
# Hash functions
|
||
|
||
Hash functions can be used for deterministic pseudo-random shuffling of elements.
|
||
|
||
## halfMD5
|
||
|
||
Calculates the MD5 from a string. Then it takes the first 8 bytes of the hash and interprets them as UInt64 in big endian.
|
||
Accepts a String-type argument. Returns UInt64.
|
||
This function works fairly slowly (5 million short strings per second per processor core).
|
||
If you don't need MD5 in particular, use the 'sipHash64' function instead.
|
||
|
||
## MD5
|
||
|
||
Calculates the MD5 from a string and returns the resulting set of bytes as FixedString(16).
|
||
If you don't need MD5 in particular, but you need a decent cryptographic 128-bit hash, use the 'sipHash128' function instead.
|
||
If you want to get the same result as output by the md5sum utility, use lower(hex(MD5(s))).
|
||
|
||
## sipHash64
|
||
|
||
Calculates SipHash from a string.
|
||
Accepts a String-type argument. Returns UInt64.
|
||
SipHash is a cryptographic hash function. It works at least three times faster than MD5.
|
||
For more information, see the link: <https://131002.net/siphash/>
|
||
|
||
## sipHash128
|
||
|
||
Calculates SipHash from a string.
|
||
Accepts a String-type argument. Returns FixedString(16).
|
||
Differs from sipHash64 in that the final xor-folding state is only done up to 128 bytes.
|
||
|
||
## cityHash64
|
||
|
||
Calculates CityHash64 from a string or a similar hash function for any number of any type of arguments.
|
||
For String-type arguments, CityHash is used. This is a fast non-cryptographic hash function for strings with decent quality.
|
||
For other types of arguments, a decent implementation-specific fast non-cryptographic hash function is used.
|
||
If multiple arguments are passed, the function is calculated using the same rules and chain combinations using the CityHash combinator.
|
||
For example, you can compute the checksum of an entire table with accuracy up to the row order: `SELECT sum(cityHash64(*)) FROM table`.
|
||
|
||
## intHash32
|
||
|
||
Calculates a 32-bit hash code from any type of integer.
|
||
This is a relatively fast non-cryptographic hash function of average quality for numbers.
|
||
|
||
## intHash64
|
||
|
||
Calculates a 64-bit hash code from any type of integer.
|
||
It works faster than intHash32. Average quality.
|
||
|
||
## SHA1
|
||
|
||
## SHA224
|
||
|
||
## SHA256
|
||
|
||
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).
|
||
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).
|
||
We recommend using this function only in cases when you need a specific hash function and you can't select it.
|
||
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.
|
||
|
||
## URLHash(url\[, N\])
|
||
|
||
A fast, decent-quality non-cryptographic hash function for a string obtained from a URL using some type of normalization.
|
||
`URLHash(s)` – Calculates a hash from a string without one of the trailing symbols `/`,`?` or `#` at the end, if present.
|
||
`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.
|
||
Levels are the same as in URLHierarchy. This function is specific to Yandex.Metrica.
|
||
|