pghashlib

The goal of pghashlib is to provide stable hashes for PostgeSQL, "stable" in the sense that their implementation does not change, they are independent from PostgeSQL version.

Some string hashes may be dependent on CPU architecture, so that they return different return on big-endian architecture from little-endian architecture. It you want to be architectures-independent, then use algorithms that don't depend on endianess.

Installation

You need PostgreSQL developent environment. Then simply:

$ make
$ make install
$ psql -d ... -c "create extension hashlib"

Functions

hash_string

hash_string(data text,  algo text [, initval int4]) returns int4
hash_string(data bytea, algo text [, initval int4]) returns int4

This hashes the data with specified algorithm, returns 32-bit result.

hash64_string

hash64_string(data text, algo text, [, iv1 int8 [, iv2 int8]]) returns int8
hash64_string(data byte, algo text, [, iv1 int8 [, iv2 int8]]) returns int8

Uses same algorithms as hash_string() but returns 64-bit result.

hash128_string

hash64_string(data text, algo text, [, initval int4]) returns int8
hash64_string(data byte, algo text, [, initval int4]) returns int8

Uses same algorithms as hash_string() but returns 128-bit result.

hash_int4

hash_int4(val int4) returns int4

Hash 32-bit integer.

hash_int8

hash_int8(val int8) returns int8

Hash 64-bit integer.

String hashing algorithms

List of currently provided algorithms.

Algorithm	CPU-indep	Bits	IV bits	Partial	Description
city64	no	64	64	no	CityHash64
city128	no	128	128	no	CityHash128
crc32	yes	32	32	yes	CRC32
lookup2	no	64	32	no	Jenkins lookup2
lookup3be	yes	64	32	no	Jenkins lookup3 big-endian
lookup3le	yes	64	32	no	Jenkins lookup3 little-endian
lookup3	no	64	32	no	Jenkins lookup3 CPU-native
murmur3	no	32	32	no	MurmurHash v3, 32-bit variant
md5	yes	128	128	no	MD5
pgsql84	no	64	0	no	Hacked lookup3 in Postgres 8.4+
siphash24	yes	64	128	no	SipHash-2-4
spooky	no	128	128	no	SpookyHash

CPU-independence

Whether hash output is independent of CPU endianess. If not, then hash result is different on little-endian machines (x86) and big-endian machines (sparc).

Bits

Maximum number of output bits that hash can output. If longer result is requested, result will be zero-padded.

IV bits

Maximum number of input bits for "initial value".

Partial hashing

Whether long string can be hashed in smaller parts, by giving last value as initval to next hash call.

Integer hashing algorithms

Algorithm	Bits	Description	Notes
wang32	32	Thomas Wang hash32shift
wang32mult	32	Thomas Wang hash32shiftmult
jenkins	32	Bob Jenkins hash with 6 shifts
wang64	64	Thomas Wang hash64shift
wang64to32	64	Thomas Wang hash6432shift	Result can be cast to int4

All algorithms here have the property that they are "reversible", that means there is 1:1 mapping between input and output.

This propery is useful for creating well-defined "random" sort order over unique integer id's. Or picking up random row from table with unique id's.

Links

• Lookup2/3 hashes by Bob Jenkins.

• MurmurHash by Austin Appleby.

• Integer hashes by Bob Jenkins.

• Integer hashes by Thomas Wang.

• Google's CityHash. 64/128/256-bit output.

• SpookyHash by Bob Jenkins. 128-bit output.

• SipHash-2-4 by Jean-Philippe Aumasson and Daniel J. Bernstein.