264 lines
8.0 KiB
Plaintext
264 lines
8.0 KiB
Plaintext
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$PostgreSQL: pgsql/doc/src/sgml/gist.sgml,v 1.14 2003/11/29 19:51:37 pgsql Exp $
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<chapter Id="GiST">
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<title>GiST Indexes</title>
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<sect1 id="intro">
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<title>Introduction</title>
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<para>
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<acronym>GiST</acronym> stands for Generalized Search Tree. It is a
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balanced, tree-structured access method, that acts as a base template in
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which to implement arbitrary indexing schemes. B+-trees, R-trees and many
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other indexing schemes can be implemented in <acronym>GiST</acronym>.
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</para>
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<para>
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One advantage of <acronym>GiST</acronym> is that it allows the development
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of custom data types with the appropriate access methods, by
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an expert in the domain of the data type, rather than a database expert.
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</para>
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<para>
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Some of the information here is derived from <ulink
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url="http://gist.cs.berkeley.edu/">the University of California at
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Berkeley's GiST Indexing Project web site</ulink> and Marcel Kornacker's
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thesis,
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<ulink url="http://citeseer.nj.nec.com/448594.html">Access Methods for
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Next-Generation Database Systems</ulink>. The <acronym>GiST</acronym>
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implementation in <productname>PostgreSQL</productname> is primarily
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maintained by Teodor Sigaev and Oleg Bartunov, and there is more
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information on their website: <ulink
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url="http://www.sai.msu.su/~megera/postgres/gist/"></>.
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</para>
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</sect1>
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<sect1 id="extensibility">
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<title>Extensibility</title>
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<para>
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Traditionally, implementing a new index access method meant a lot of
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difficult work. It was necessary to understand the inner workings of the
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database, such as the lock manager and Write-Ahead Log. The
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<acronym>GiST</acronym> interface has a high level of abstraction,
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requiring the access method implementor to only implement the semantics of
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the data type being accessed. The <acronym>GiST</acronym> layer itself
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takes care of concurrency, logging and searching the tree structure.
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</para>
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<para>
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This extensibility should not be confused with the extensibility of the
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other standard search trees in terms of the data they can handle. For
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example, <productname>PostgreSQL</productname> supports extensible B+-trees
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and R-trees. That means that you can use
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<productname>PostgreSQL</productname> to build a B+-tree or R-tree over any
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data type you want. But B+-trees only support range predicates
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(<literal><</literal>, <literal>=</literal>, <literal>></literal>),
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and R-trees only support n-D range queries (contains, contained, equals).
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</para>
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<para>
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So if you index, say, an image collection with a
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<productname>PostgreSQL</productname> B+-tree, you can only issue queries
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such as <quote>is imagex equal to imagey</quote>, <quote>is imagex less
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than imagey</quote> and <quote>is imagex greater than imagey</quote>?
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Depending on how you define <quote>equals</quote>, <quote>less than</quote>
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and <quote>greater than</quote> in this context, this could be useful.
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However, by using a <acronym>GiST</acronym> based index, you could create
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ways to ask domain-specific questions, perhaps <quote>find all images of
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horses</quote> or <quote>find all over-exposed images</quote>.
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</para>
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<para>
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All it takes to get a <acronym>GiST</acronym> access method up and running
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is to implement seven user-defined methods, which define the behavior of
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keys in the tree. Of course these methods have to be pretty fancy to
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support fancy queries, but for all the standard queries (B+-trees,
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R-trees, etc.) they're relatively straightforward. In short,
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<acronym>GiST</acronym> combines extensibility along with generality, code
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reuse, and a clean interface.
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</para>
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</sect1>
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<sect1 id="implementation">
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<title>Implementation</title>
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<para>
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There are seven methods that an index operator class for
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<acronym>GiST</acronym> must provide:
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</para>
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<variablelist>
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<varlistentry>
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<term>consistent</term>
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<listitem>
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<para>
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Given a predicate <literal>p</literal> on a tree page, and a user
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query, <literal>q</literal>, this method will return false if it is
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certain that both <literal>p</literal> and <literal>q</literal> cannot
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be true for a given data item.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>union</term>
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<listitem>
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<para>
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This method consolidates information in the tree. Given a set of
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entries, this function generates a new predicate that is true for all
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the entries.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>compress</term>
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<listitem>
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<para>
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Converts the data item into a format suitable for physical storage in
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an index page.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>decompress</term>
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<listitem>
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<para>
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The reverse of the <function>compress</function> method. Converts the
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index representation of the data item into a format that can be
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manipulated by the database.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>penalty</term>
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<listitem>
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<para>
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Returns a value indicating the <quote>cost</quote> of inserting the new
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entry into a particular branch of the tree. items will be inserted
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down the path of least <function>penalty</function> in the tree.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>picksplit</term>
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<listitem>
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<para>
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When a page split is necessary, this function decides which entries on
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the page are to stay on the old page, and which are to move to the new
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page.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>same</term>
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<listitem>
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<para>
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Returns true if two entries are identical, false otherwise.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</sect1>
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<sect1 id="limitations">
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<title>Limitations</title>
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<para>
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The current implementation of <acronym>GiST</acronym> within
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<productname>PostgreSQL</productname> has some major limitations:
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<acronym>GiST</acronym> access is not concurrent; the
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<acronym>GiST</acronym> interface doesn't allow the development of certain
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data types, such as digital trees (see papers by Aoki et al); and there
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is not yet any support for write-ahead logging of updates in
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<acronym>GiST</acronym> indexes.
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</para>
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<para>
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Solutions to the concurrency problems appear in Marcel Kornacker's
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thesis; however these ideas have not yet been put into practice in the
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<productname>PostgreSQL</productname> implementation.
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</para>
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<para>
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The lack of write-ahead logging is just a small matter of programming,
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but since it isn't done yet, a crash could render a <acronym>GiST</acronym>
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index inconsistent, forcing a REINDEX.
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</para>
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</sect1>
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<sect1 id="examples">
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<title>Examples</title>
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<para>
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To see example implementations of index methods implemented using
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<acronym>GiST</acronym>, examine the following contrib modules:
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</para>
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<variablelist>
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<varlistentry>
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<term>btree_gist</term>
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<listitem>
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<para>B-Tree</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>cube</term>
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<listitem>
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<para>Indexing for multi-dimensional cubes</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>intarray</term>
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<listitem>
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<para>RD-Tree for one-dimensional array of int4 values</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>ltree</term>
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<listitem>
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<para>Indexing for tree-like stuctures</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>rtree_gist</term>
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<listitem>
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<para>R-Tree</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>seg</term>
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<listitem>
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<para>Storage and indexed access for <quote>float ranges</quote></para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>tsearch and tsearch2</term>
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<listitem>
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<para>Full text indexing</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</sect1>
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</chapter>
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