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Diffstat (limited to 'doc/src/sgml/xindex.sgml')
| -rw-r--r-- | doc/src/sgml/xindex.sgml | 23 |
1 files changed, 16 insertions, 7 deletions
diff --git a/doc/src/sgml/xindex.sgml b/doc/src/sgml/xindex.sgml index 7c68bfc1009..84421de8bb7 100644 --- a/doc/src/sgml/xindex.sgml +++ b/doc/src/sgml/xindex.sgml @@ -1,4 +1,4 @@ -<!-- $PostgreSQL: pgsql/doc/src/sgml/xindex.sgml,v 1.58 2007/02/01 00:28:18 momjian Exp $ --> +<!-- $PostgreSQL: pgsql/doc/src/sgml/xindex.sgml,v 1.59 2007/02/06 04:38:31 tgl Exp $ --> <sect1 id="xindex"> <title>Interfacing Extensions To Indexes</title> @@ -139,7 +139,7 @@ </table> <para> - Hash indexes express only bitwise equality, and so they use only one + Hash indexes support only equality comparisons, and so they use only one strategy, shown in <xref linkend="xindex-hash-strat-table">. </para> @@ -162,7 +162,7 @@ </table> <para> - GiST indexes are even more flexible: they do not have a fixed set of + GiST indexes are more flexible: they do not have a fixed set of strategies at all. Instead, the <quote>consistency</> support routine of each particular GiST operator class interprets the strategy numbers however it likes. As an example, several of the built-in GiST index @@ -802,14 +802,23 @@ ALTER OPERATOR FAMILY integer_ops USING btree ADD operator in the family there must be a support function having the same two input data types as the operator. It is recommended that a family be complete, i.e., for each combination of data types, all operators are - included. An operator class should include just the non-cross-type + included. Each operator class should include just the non-cross-type operators and support function for its data type. </para> <para> - At this writing, hash indexes do not support cross-type operations, - and so there is little use for a hash operator family larger than one - operator class. This is expected to be relaxed in the future. + To build a multiple-data-type hash operator family, compatible hash + support functions must be created for each data type supported by the + family. Here compatibility means that the functions are guaranteed to + return the same hash code for any two values that are considered equal + by the family's equality operators, even when the values are of different + types. This is usually difficult to accomplish when the types have + different physical representations, but it can be done in some cases. + Notice that there is only one support function per data type, not one + per equality operator. It is recommended that a family be complete, i.e., + provide an equality operator for each combination of data types. + Each operator class should include just the non-cross-type equality + operator and the support function for its data type. </para> <para> |