diff options
| author | Robert Haas | 2017-09-01 02:21:21 +0000 |
|---|---|---|
| committer | Robert Haas | 2017-09-01 02:21:21 +0000 |
| commit | 81c5e46c490e2426db243eada186995da5bb0ba7 (patch) | |
| tree | a6cb745131c45a06fa43746a17a69e1dc9daa44a /doc/src | |
| parent | 2d44c58c79aeef2d376be0141057afbb9ec6b5bc (diff) | |
Introduce 64-bit hash functions with a 64-bit seed.
This will be useful for hash partitioning, which needs a way to seed
the hash functions to avoid problems such as a hash index on a hash
partitioned table clumping all values into a small portion of the
bucket space; it's also useful for anything that wants a 64-bit hash
value rather than a 32-bit hash value.
Just in case somebody wants a 64-bit hash value that is compatible
with the existing 32-bit hash values, make the low 32-bits of the
64-bit hash value match the 32-bit hash value when the seed is 0.
Robert Haas and Amul Sul
Discussion: http://postgr.es/m/CA+Tgmoafx2yoJuhCQQOL5CocEi-w_uG4S2xT0EtgiJnPGcHW3g@mail.gmail.com
Diffstat (limited to 'doc/src')
| -rw-r--r-- | doc/src/sgml/xindex.sgml | 13 |
1 files changed, 11 insertions, 2 deletions
diff --git a/doc/src/sgml/xindex.sgml b/doc/src/sgml/xindex.sgml index 333a36c4562..745b4d5619a 100644 --- a/doc/src/sgml/xindex.sgml +++ b/doc/src/sgml/xindex.sgml @@ -436,7 +436,8 @@ </table> <para> - Hash indexes require one support function, shown in <xref + Hash indexes require one support function, and allow a second one to be + supplied at the operator class author's option, as shown in <xref linkend="xindex-hash-support-table">. </para> @@ -451,9 +452,17 @@ </thead> <tbody> <row> - <entry>Compute the hash value for a key</entry> + <entry>Compute the 32-bit hash value for a key</entry> <entry>1</entry> </row> + <row> + <entry> + Compute the 64-bit hash value for a key given a 64-bit salt; if + the salt is 0, the low 32 bits will match the value that would + have been computed by function 1 + </entry> + <entry>2</entry> + </row> </tbody> </tgroup> </table> |