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Doc: minor clarifications in xindex.sgml.

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I noticed some slightly confusing or out-of-date verbiage here
while working on the window RANGE patch.  Seems worth committing
separately.
This commit is contained in:
Tom Lane 2018-02-04 11:46:28 -05:00
parent 4ac583f36a
commit 64fb645914
1 changed files with 24 additions and 10 deletions
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34
doc/src/sgml/xindex.sgml
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@ -458,8 +458,9 @@
<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>
@ -1139,16 +1140,11 @@ ALTER OPERATOR FAMILY integer_ops USING btree ADD
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 practice it is only enough
to support array equality.
operator class only provides equality, it is only able to support grouping
not sorting.
</para>
<para>
@ -1168,7 +1164,25 @@ ALTER OPERATOR FAMILY integer_ops USING btree ADD
</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.