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Introduce 64-bit hash functions with a 64-bit seed.

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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
This commit is contained in:
Robert Haas
2017-08-31 22:21:21 -04:00
parent 2d44c58c79
commit 81c5e46c49
33 changed files with 1555 additions and 42 deletions
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13
doc/src/sgml/xindex.sgml
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@ -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>