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R-tree is dead ... long live GiST.

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Tom Lane
2005-11-07 17:36:47 +00:00
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doc/src/sgml/gist.sgml
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<!--
$PostgreSQL: pgsql/doc/src/sgml/gist.sgml,v 1.24 2005/11/04 23:14:00 petere Exp $
$PostgreSQL: pgsql/doc/src/sgml/gist.sgml,v 1.25 2005/11/07 17:36:44 tgl Exp $
-->
<chapter id="GiST">
<title>GiST Indexes</title>
<sect1 id="gist-intro">
<title>Introduction</title>
<para>
<indexterm>
<primary>index</primary>
<secondary>GiST</secondary>
</indexterm>
<indexterm>
<primary>GiST</primary>
<see>index</see>
</indexterm>
<sect1 id="gist-intro">
<title>Introduction</title>
<para>
<acronym>GiST</acronym> stands for Generalized Search Tree. It is a
balanced, tree-structured access method, that acts as a base template in
which to implement arbitrary indexing schemes. B+-trees, R-trees and many
which to implement arbitrary indexing schemes. B-trees, R-trees and many
other indexing schemes can be implemented in <acronym>GiST</acronym>.
</para>
@ -60,17 +57,17 @@ $PostgreSQL: pgsql/doc/src/sgml/gist.sgml,v 1.24 2005/11/04 23:14:00 petere Exp
<para>
This extensibility should not be confused with the extensibility of the
other standard search trees in terms of the data they can handle. For
example, <productname>PostgreSQL</productname> supports extensible B+-trees
and R-trees. That means that you can use
<productname>PostgreSQL</productname> to build a B+-tree or R-tree over any
data type you want. But B+-trees only support range predicates
example, <productname>PostgreSQL</productname> supports extensible B-trees
and hash indexes. That means that you can use
<productname>PostgreSQL</productname> to build a B-tree or hash over any
data type you want. But B-trees only support range predicates
(<literal>&lt;</literal>, <literal>=</literal>, <literal>&gt;</literal>),
and R-trees only support n-D range queries (contains, contained, equals).
and hash indexes only support equality queries.
</para>
<para>
So if you index, say, an image collection with a
<productname>PostgreSQL</productname> B+-tree, you can only issue queries
<productname>PostgreSQL</productname> B-tree, you can only issue queries
such as <quote>is imagex equal to imagey</quote>, <quote>is imagex less
than imagey</quote> and <quote>is imagex greater than imagey</quote>?
Depending on how you define <quote>equals</quote>, <quote>less than</quote>
@ -84,7 +81,7 @@ $PostgreSQL: pgsql/doc/src/sgml/gist.sgml,v 1.24 2005/11/04 23:14:00 petere Exp
All it takes to get a <acronym>GiST</acronym> access method up and running
is to implement seven user-defined methods, which define the behavior of
keys in the tree. Of course these methods have to be pretty fancy to
support fancy queries, but for all the standard queries (B+-trees,
support fancy queries, but for all the standard queries (B-trees,
R-trees, etc.) they're relatively straightforward. In short,
<acronym>GiST</acronym> combines extensibility along with generality, code
reuse, and a clean interface.