mirror of https://github.com/postgres/postgres
Bruce Momjian
19 years ago
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<!-- $PostgreSQL: pgsql/doc/src/sgml/gin.sgml,v 1.1 2006/09/04 20:10:53 momjian Exp $ --> |
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<chapter id="GIN"> |
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<title>GIN Indexes</title> |
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<indexterm> |
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<primary>index</primary> |
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<secondary>GIN</secondary> |
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</indexterm> |
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<sect1 id="gin-intro"> |
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<title>Introduction</title> |
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<para> |
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<acronym>GIN</acronym> stands for Generalized Inverted Index. It is |
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an index structure storing a set of (key, posting list) pairs, where |
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'posting list' is a set of documents in which the key occurs. |
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</para> |
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<para> |
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It is generalized in the sense that a <acronym>GIN</acronym> index |
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does not need to be aware of the operation that it accelerates. |
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Instead, it uses custom strategies defined for particular data types. |
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</para> |
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<para> |
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One advantage of <acronym>GIN</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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This is much the same advantage as using <acronym>GiST</acronym>. |
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</para> |
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<para> |
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The <acronym>GIN</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 |
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<ulink url="http://www.sai.msu.su/~megera/oddmuse/index.cgi/Gin">website</ulink>. |
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</para> |
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</sect1> |
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<sect1 id="gin-extensibility"> |
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<title>Extensibility</title> |
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<para> |
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The <acronym>GIN</acronym> interface has a high level of abstraction, |
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requiring the access method implementer to only implement the semantics of |
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the data type being accessed. The <acronym>GIN</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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All it takes to get a <acronym>GIN</acronym> access method working |
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is to implement four user-defined methods, which define the behavior of |
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keys in the tree. In short, <acronym>GIN</acronym> combines extensibility |
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along with generality, code reuse, and a clean interface. |
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</para> |
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</sect1> |
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<sect1 id="gin-implementation"> |
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<title>Implementation</title> |
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<para> |
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There are four methods that an index operator class for |
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<acronym>GIN</acronym> must provide: |
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</para> |
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<variablelist> |
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<varlistentry> |
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<term>compare</term> |
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<listitem> |
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<para> |
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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>extract value</term> |
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<listitem> |
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<para> |
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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>extract query</term> |
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<listitem> |
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<para> |
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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>consistent</term> |
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<listitem> |
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<para> |
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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="gin-examples"> |
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<title>Examples</title> |
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<para> |
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The <productname>PostgreSQL</productname> source distribution includes |
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<acronym>GIN</acronym> classes for one-dimensional arrays of all internal |
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types. The following |
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<filename>contrib</> modules also contain <acronym>GIN</acronym> |
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operator classes: |
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</para> |
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<variablelist> |
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<varlistentry> |
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<term>intarray</term> |
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<listitem> |
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<para>Enhanced support for int4[]</para> |
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</listitem> |
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</varlistentry> |
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<varlistentry> |
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<term>tsearch2</term> |
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<listitem> |
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<para>Support for inverted text indexing. This is much faster for very |
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large, mostly-static sets of documents. |
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</para> |
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</listitem> |
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</varlistentry> |
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</chapter> |
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