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67 lines
3.2 KiB
Markdown
67 lines
3.2 KiB
Markdown
# Hash functions
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Hash functions can be used for deterministic pseudo-random shuffling of elements.
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## halfMD5
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Calculates the MD5 from a string. Then it takes the first 8 bytes of the hash and interprets them as UInt64 in big endian.
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Accepts a String-type argument. Returns UInt64.
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This function works fairly slowly (5 million short strings per second per processor core).
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If you don't need MD5 in particular, use the 'sipHash64' function instead.
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## 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
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Calculates SipHash from a string.
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Accepts a String-type argument. Returns UInt64.
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SipHash is a cryptographic hash function. It works at least three times faster than MD5.
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For more information, see the link: <https://131002.net/siphash/>
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## 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 bytes.
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## cityHash64
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Calculates CityHash64 from a string or a similar hash function for any number of any type of arguments.
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For String-type arguments, CityHash is used. This is a fast non-cryptographic hash function for strings with decent quality.
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For other types of arguments, a decent implementation-specific fast non-cryptographic hash function is used.
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If multiple arguments are passed, the function is calculated using the same rules and chain combinations using the CityHash combinator.
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For example, you can compute the checksum of an entire table with accuracy up to the row order: `SELECT sum(cityHash64(*)) FROM table`.
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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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