gin.sgml

<!-- $PostgreSQL: pgsql/doc/src/sgml/gin.sgml,v 2.12 2007/11/13 23:36:26 tgl Exp $ -->

<chapter id="GIN">
<title>GIN Indexes</title>

   <indexterm>
    <primary>index</primary>
    <secondary>GIN</secondary>
   </indexterm>

<sect1 id="gin-intro">
 <title>Introduction</title>

 <para>
   <acronym>GIN</acronym> stands for Generalized Inverted Index.  It is
   an index structure storing a set of (key, posting list) pairs, where
   a <quote>posting list</> is a set of rows in which the key occurs. Each
   indexed value can contain many keys, so the same row ID can appear in
   multiple posting lists.
 </para>

 <para>
   It is generalized in the sense that a <acronym>GIN</acronym> index
   does not need to be aware of the operation that it accelerates.
   Instead, it uses custom strategies defined for particular data types.
 </para>

 <para>
  One advantage of <acronym>GIN</acronym> is that it allows the development
  of custom data types with the appropriate access methods, by
  an expert in the domain of the data type, rather than a database expert.
  This is much the same advantage as using <acronym>GiST</acronym>.
 </para>

 <para>
  The <acronym>GIN</acronym>
  implementation in <productname>PostgreSQL</productname> is primarily
  maintained by Teodor Sigaev and Oleg Bartunov. There is more
  information about <acronym>GIN</acronym> on their
  <ulink url="http://www.sai.msu.su/~megera/oddmuse/index.cgi/Gin">website</ulink>.
 </para>
</sect1>

<sect1 id="gin-extensibility">
 <title>Extensibility</title>

 <para>
   The <acronym>GIN</acronym> interface has a high level of abstraction,
   requiring the access method implementer only to implement the semantics of
   the data type being accessed.  The <acronym>GIN</acronym> layer itself
   takes care of concurrency, logging and searching the tree structure.
 </para>

 <para>
   All it takes to get a <acronym>GIN</acronym> access method working
   is to implement four user-defined methods, which define the behavior of
   keys in the tree and the relationships between keys, indexed values,
   and indexable queries. In short, <acronym>GIN</acronym> combines
   extensibility with generality, code reuse, and a clean interface.
 </para>

 <para>
   The four methods that an index operator class for
   <acronym>GIN</acronym> must provide are:
 </para>

 <variablelist>
    <varlistentry>
     <term>int compare(Datum a, Datum b)</term>
     <listitem>
      <para>
       Compares keys (not indexed values!) and returns an integer less than
       zero, zero, or greater than zero, indicating whether the first key is
       less than, equal to, or greater than the second.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>Datum* extractValue(Datum inputValue, int32 *nkeys)</term>
     <listitem>
      <para>
       Returns an array of keys given a value to be indexed.  The
       number of returned keys must be stored into <literal>*nkeys</>.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>Datum* extractQuery(Datum query, int32 *nkeys,
        StrategyNumber n)</term>
     <listitem>
      <para>
       Returns an array of keys given a value to be queried; that is,
       <literal>query</> is the value on the right-hand side of an
       indexable operator whose left-hand side is the indexed column.
       <literal>n</> is the strategy number of the operator within the
       operator class (see <xref linkend="xindex-strategies">).
       Often, <function>extractQuery</> will need
       to consult <literal>n</> to determine the data type of
       <literal>query</> and the key values that need to be extracted.
       The number of returned keys must be stored into <literal>*nkeys</>.
       If number of keys is equal to zero then <function>extractQuery</> 
       should store 0 or -1 into <literal>*nkeys</>. 0 means that any 
       row matches the <literal>query</> and sequence scan should be 
       produced. -1 means nothing can satisfy <literal>query</>. 
       Choice of value should be based on semantics meaning of operation with 
       given strategy number.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>bool consistent(bool check[], StrategyNumber n, Datum query)</term>
     <listitem>
      <para>
       Returns TRUE if the indexed value satisfies the query operator with
       strategy number <literal>n</> (or would satisfy, if the operator is
       marked RECHECK in the operator class).  The <literal>check</> array has
       the same length as the number of keys previously returned by
       <function>extractQuery</> for this query.  Each element of the
       <literal>check</> array is TRUE if the indexed value contains the
       corresponding query key, ie, if (check[i] == TRUE) the i-th key of the
       <function>extractQuery</> result array is present in the indexed value.
       The original <literal>query</> datum (not the extracted key array!) is
       passed in case the <function>consistent</> method needs to consult it.
      </para>
     </listitem>
    </varlistentry>

  </variablelist>

</sect1>

<sect1 id="gin-implementation">
 <title>Implementation</title>

 <para>
  Internally, a <acronym>GIN</acronym> index contains a B-tree index
  constructed over keys, where each key is an element of the indexed value
  (a member of an array, for example) and where each tuple in a leaf page is
  either a pointer to a B-tree over heap pointers (PT, posting tree), or a
  list of heap pointers (PL, posting list) if the list is small enough.
 </para>

</sect1>

<sect1 id="gin-tips">
<title>GIN tips and tricks</title>

 <variablelist>
  <varlistentry>
   <term>Create vs insert</term>
   <listitem>
    <para>
     In most cases, insertion into a <acronym>GIN</acronym> index is slow
     due to the likelihood of many keys being inserted for each value.
     So, for bulk insertions into a table it is advisable to drop the GIN
     index and recreate it after finishing bulk insertion.
    </para>
   </listitem>
  </varlistentry>

  <varlistentry>
   <term><xref linkend="guc-gin-fuzzy-search-limit"></term>
   <listitem>
    <para>
     The primary goal of developing <acronym>GIN</acronym> indexes was
     to create support for highly scalable, full-text search in
     <productname>PostgreSQL</productname>, and there are often situations when
     a full-text search returns a very large set of results.  Moreover, this
     often happens when the query contains very frequent words, so that the
     large result set is not even useful.  Since reading many
     tuples from the disk and sorting them could take a lot of time, this is
     unacceptable for production.  (Note that the index search itself is very
     fast.)
    </para>
    <para>
     To facilitate controlled execution of such queries
     <acronym>GIN</acronym> has a configurable soft upper limit on the size
     of the returned set, the
     <varname>gin_fuzzy_search_limit</varname> configuration parameter.
     It is set to 0 (meaning no limit) by default.
     If a non-zero limit is set, then the returned set is a subset of
     the whole result set, chosen at random.
    </para>
    <para>
     <quote>Soft</quote> means that the actual number of returned results
     could differ slightly from the specified limit, depending on the query
     and the quality of the system's random number generator.
    </para>
   </listitem>
  </varlistentry>
 </variablelist>

</sect1>

<sect1 id="gin-limit">
 <title>Limitations</title>

 <para>
  <acronym>GIN</acronym> doesn't support full index scans: because there are
  often many keys per value, each heap pointer would be returned many times,
  and there is no easy way to prevent this.
 </para>

 <para>
  When <function>extractQuery</function> returns zero keys,
  <acronym>GIN</acronym> will emit an error.  Depending on the operator,
  a void query might match all, some, or none of the indexed values (for
  example, every array contains the empty array, but does not overlap the
  empty array), and <acronym>GIN</acronym> cannot determine the correct
  answer, nor produce a full-index-scan result if it could determine that
  that was correct.
 </para>

 <para>
  It is not an error for <function>extractValue</> to return zero keys,
  but in this case the indexed value will be unrepresented in the index.
  This is another reason why full index scan is not useful &mdash; it would
  miss such rows.
 </para>

 <para>
  <acronym>GIN</acronym> searches keys only by equality matching.  This might
  be improved in future.
 </para>
</sect1>

<sect1 id="gin-examples">
 <title>Examples</title>

 <para>
  The <productname>PostgreSQL</productname> source distribution includes
  <acronym>GIN</acronym> operator classes for <type>tsvector</> and
  for one-dimensional arrays of all internal types.  The following
  <filename>contrib</> modules also contain <acronym>GIN</acronym>
  operator classes:
 </para>

 <variablelist>
  <varlistentry>
   <term>hstore</term>
   <listitem>
    <para>Module for storing (key, value) pairs</para>
   </listitem>
  </varlistentry>

  <varlistentry>
   <term>intarray</term>
   <listitem>
    <para>Enhanced support for int4[]</para>
   </listitem>
  </varlistentry>

  <varlistentry>
   <term>pg_trgm</term>
   <listitem>
    <para>Text similarity using trigram matching</para>
   </listitem>
  </varlistentry>
 </variablelist>
</sect1>

</chapter>