diff options
| -rw-r--r-- | doc/src/sgml/xindex.sgml | 34 |
1 files changed, 24 insertions, 10 deletions
diff --git a/doc/src/sgml/xindex.sgml b/doc/src/sgml/xindex.sgml index 2b4298065cd..81c0cdc4f8d 100644 --- a/doc/src/sgml/xindex.sgml +++ b/doc/src/sgml/xindex.sgml @@ -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> @@ -1140,15 +1141,10 @@ ALTER OPERATOR FAMILY integer_ops USING btree ADD </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 ~<~; +</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. |