<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

the salt is 0, the low 32 bits of the result must match the value

that would have been computed by function 1

(optional)

</entry>

<entry>2</entry>

</row>

ordering.

</para>

<para>

Comparison of arrays of user-defined types also relies on the semantics

defined by the default B-tree operator class.

</para>

<para>

If there is no default B-tree operator class for a data type, the system

will look for a default hash operator class. But since that kind of

operator class only provides equality,in practiceit is onlyenough

to support array equality.

operator class only provides equality, it is onlyable to support grouping

not sorting.

</para>

<para>

</note>

<para>

Another important point is that an operator that

Sorting by a non-default B-tree operator class is possible by specifying

the class's less-than operator in a <literal>USING</literal> option,

for example

<programlisting>

SELECT * FROM mytable ORDER BY somecol USING ~&lt;~;

</programlisting>

Alternatively, specifying the class's greater-than operator

in <literal>USING</literal> selects a descending-order sort.

</para>

<para>

Comparison of arrays of a user-defined type also relies on the semantics

defined by the type's default B-tree operator class. If there is no

default B-tree operator class, but there is a default hash operator class,

then array equality is supported, but not ordering comparisons.

</para>

<para>

Another important point is that an equality operator that

appears in a hash operator family is a candidate for hash joins,

hash aggregation, and related optimizations. The hash operator family

is essential here since it identifies the hash function(s) to use.