mirror of
https://github.com/postgres/postgres.git
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36d1a848a8
gist.sgml and xindex.sgml hadn't been fully updated for the addition of a sortsupport support function (commit 16fa9b2b3). xindex.sgml also missed that the compress and decompress support functions are optional, an apparently far older oversight. In passing, fix gratuitous inconsistencies in wording and capitalization. Noted by E. Rogov. Back-patch to v14; the residual issues before that aren't significant enough to bother with. Discussion: https://postgr.es/m/163335322905.12519.5711557029494638051@wrigleys.postgresql.org
1313 lines
50 KiB
Plaintext
1313 lines
50 KiB
Plaintext
<!-- doc/src/sgml/gist.sgml -->
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<chapter id="gist">
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<title>GiST Indexes</title>
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<indexterm>
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<primary>index</primary>
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<secondary>GiST</secondary>
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</indexterm>
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<sect1 id="gist-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 the University of California
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at Berkeley's GiST Indexing Project
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<ulink url="http://gist.cs.berkeley.edu/">web site</ulink> and
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Marcel Kornacker's thesis,
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<ulink url="http://www.sai.msu.su/~megera/postgres/gist/papers/concurrency/access-methods-for-next-generation.pdf.gz">
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Access Methods for Next-Generation Database Systems</ulink>.
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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
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<ulink url="http://www.sai.msu.su/~megera/postgres/gist/">web site</ulink>.
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</para>
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</sect1>
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<sect1 id="gist-builtin-opclasses">
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<title>Built-in Operator Classes</title>
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<para>
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The core <productname>PostgreSQL</productname> distribution
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includes the <acronym>GiST</acronym> operator classes shown in
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<xref linkend="gist-builtin-opclasses-table"/>.
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(Some of the optional modules described in <xref linkend="contrib"/>
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provide additional <acronym>GiST</acronym> operator classes.)
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</para>
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<table id="gist-builtin-opclasses-table">
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<title>Built-in <acronym>GiST</acronym> Operator Classes</title>
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<tgroup cols="3">
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<colspec colname="col1" colwidth="2*"/>
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<colspec colname="col2" colwidth="3*"/>
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<colspec colname="col3" colwidth="2*"/>
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<thead>
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<row>
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<entry>Name</entry>
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<entry>Indexable Operators</entry>
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<entry>Ordering Operators</entry>
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</row>
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</thead>
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<tbody>
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<row>
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<entry valign="middle" morerows="13"><literal>box_ops</literal></entry>
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<entry><literal><< (box, box)</literal></entry>
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<entry valign="middle" morerows="13"><literal><-> (box, point)</literal></entry>
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</row>
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<row><entry><literal>&< (box, box)</literal></entry></row>
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<row><entry><literal>&& (box, box)</literal></entry></row>
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<row><entry><literal>&> (box, box)</literal></entry></row>
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<row><entry><literal>>> (box, box)</literal></entry></row>
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<row><entry><literal>~= (box, box)</literal></entry></row>
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<row><entry><literal>@> (box, box)</literal></entry></row>
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<row><entry><literal><@ (box, box)</literal></entry></row>
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<row><entry><literal>&<| (box, box)</literal></entry></row>
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<row><entry><literal><<| (box, box)</literal></entry></row>
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<row><entry><literal>|>> (box, box)</literal></entry></row>
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<row><entry><literal>|&> (box, box)</literal></entry></row>
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<row><entry><literal>~ (box, box)</literal></entry></row>
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<row><entry><literal>@ (box, box)</literal></entry></row>
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<row>
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<entry valign="middle" morerows="13"><literal>circle_ops</literal></entry>
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<entry><literal><< (circle, circle)</literal></entry>
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<entry valign="middle" morerows="13"><literal><-> (circle, point)</literal></entry>
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</row>
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<row><entry><literal>&< (circle, circle)</literal></entry></row>
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<row><entry><literal>&> (circle, circle)</literal></entry></row>
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<row><entry><literal>>> (circle, circle)</literal></entry></row>
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<row><entry><literal><@ (circle, circle)</literal></entry></row>
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<row><entry><literal>@> (circle, circle)</literal></entry></row>
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<row><entry><literal>~= (circle, circle)</literal></entry></row>
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<row><entry><literal>&& (circle, circle)</literal></entry></row>
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<row><entry><literal>|>> (circle, circle)</literal></entry></row>
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<row><entry><literal><<| (circle, circle)</literal></entry></row>
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<row><entry><literal>&<| (circle, circle)</literal></entry></row>
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<row><entry><literal>|&> (circle, circle)</literal></entry></row>
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<row><entry><literal>@ (circle, circle)</literal></entry></row>
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<row><entry><literal>~ (circle, circle)</literal></entry></row>
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<row>
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<entry valign="middle" morerows="10"><literal>inet_ops</literal></entry>
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<entry><literal><< (inet, inet)</literal></entry>
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<entry valign="middle" morerows="10"></entry>
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</row>
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<row><entry><literal><<= (inet, inet)</literal></entry></row>
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<row><entry><literal>>> (inet, inet)</literal></entry></row>
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<row><entry><literal>>>= (inet, inet)</literal></entry></row>
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<row><entry><literal>= (inet, inet)</literal></entry></row>
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<row><entry><literal><> (inet, inet)</literal></entry></row>
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<row><entry><literal>< (inet, inet)</literal></entry></row>
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<row><entry><literal><= (inet, inet)</literal></entry></row>
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<row><entry><literal>> (inet, inet)</literal></entry></row>
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<row><entry><literal>>= (inet, inet)</literal></entry></row>
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<row><entry><literal>&& (inet, inet)</literal></entry></row>
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<row>
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<entry valign="middle" morerows="17"><literal>multirange_ops</literal></entry>
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<entry><literal>= (anymultirange, anymultirange)</literal></entry>
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<entry valign="middle" morerows="17"></entry>
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</row>
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<row><entry><literal>&& (anymultirange, anymultirange)</literal></entry></row>
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<row><entry><literal>&& (anymultirange, anyrange)</literal></entry></row>
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<row><entry><literal>@> (anymultirange, anyelement)</literal></entry></row>
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<row><entry><literal>@> (anymultirange, anymultirange)</literal></entry></row>
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<row><entry><literal>@> (anymultirange, anyrange)</literal></entry></row>
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<row><entry><literal><@ (anymultirange, anymultirange)</literal></entry></row>
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<row><entry><literal><@ (anymultirange, anyrange)</literal></entry></row>
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<row><entry><literal><< (anymultirange, anymultirange)</literal></entry></row>
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<row><entry><literal><< (anymultirange, anyrange)</literal></entry></row>
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<row><entry><literal>>> (anymultirange, anymultirange)</literal></entry></row>
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<row><entry><literal>>> (anymultirange, anyrange)</literal></entry></row>
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<row><entry><literal>&< (anymultirange, anymultirange)</literal></entry></row>
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<row><entry><literal>&< (anymultirange, anyrange)</literal></entry></row>
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<row><entry><literal>&> (anymultirange, anymultirange)</literal></entry></row>
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<row><entry><literal>&> (anymultirange, anyrange)</literal></entry></row>
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<row><entry><literal>-|- (anymultirange, anymultirange)</literal></entry></row>
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<row><entry><literal>-|- (anymultirange, anyrange)</literal></entry></row>
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<row>
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<entry valign="middle" morerows="7"><literal>point_ops</literal></entry>
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<entry><literal>|>> (point, point)</literal></entry>
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<entry valign="middle" morerows="7"><literal><-> (point, point)</literal></entry>
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</row>
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<row><entry><literal><< (point, point)</literal></entry></row>
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<row><entry><literal>>> (point, point)</literal></entry></row>
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<row><entry><literal><<| (point, point)</literal></entry></row>
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<row><entry><literal>~= (point, point)</literal></entry></row>
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<row><entry><literal><@ (point, box)</literal></entry></row>
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<row><entry><literal><@ (point, polygon)</literal></entry></row>
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<row><entry><literal><@ (point, circle)</literal></entry></row>
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<row>
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<entry valign="middle" morerows="13"><literal>poly_ops</literal></entry>
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<entry><literal><< (polygon, polygon)</literal></entry>
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<entry valign="middle" morerows="13"><literal><-> (polygon, point)</literal></entry>
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</row>
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<row><entry><literal>&< (polygon, polygon)</literal></entry></row>
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<row><entry><literal>&> (polygon, polygon)</literal></entry></row>
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<row><entry><literal>>> (polygon, polygon)</literal></entry></row>
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<row><entry><literal><@ (polygon, polygon)</literal></entry></row>
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<row><entry><literal>@> (polygon, polygon)</literal></entry></row>
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<row><entry><literal>~= (polygon, polygon)</literal></entry></row>
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<row><entry><literal>&& (polygon, polygon)</literal></entry></row>
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<row><entry><literal><<| (polygon, polygon)</literal></entry></row>
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<row><entry><literal>&<| (polygon, polygon)</literal></entry></row>
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<row><entry><literal>|&> (polygon, polygon)</literal></entry></row>
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<row><entry><literal>|>> (polygon, polygon)</literal></entry></row>
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<row><entry><literal>@ (polygon, polygon)</literal></entry></row>
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<row><entry><literal>~ (polygon, polygon)</literal></entry></row>
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<row>
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<entry valign="middle" morerows="17"><literal>range_ops</literal></entry>
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<entry><literal>= (anyrange, anyrange)</literal></entry>
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<entry valign="middle" morerows="17"></entry>
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</row>
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<row><entry><literal>&& (anyrange, anyrange)</literal></entry></row>
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<row><entry><literal>&& (anyrange, anymultirange)</literal></entry></row>
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<row><entry><literal>@> (anyrange, anyelement)</literal></entry></row>
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<row><entry><literal>@> (anyrange, anyrange)</literal></entry></row>
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<row><entry><literal>@> (anyrange, anymultirange)</literal></entry></row>
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<row><entry><literal><@ (anyrange, anyrange)</literal></entry></row>
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<row><entry><literal><@ (anyrange, anymultirange)</literal></entry></row>
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<row><entry><literal><< (anyrange, anyrange)</literal></entry></row>
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<row><entry><literal><< (anyrange, anymultirange)</literal></entry></row>
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<row><entry><literal>>> (anyrange, anyrange)</literal></entry></row>
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<row><entry><literal>>> (anyrange, anymultirange)</literal></entry></row>
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<row><entry><literal>&< (anyrange, anyrange)</literal></entry></row>
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<row><entry><literal>&< (anyrange, anymultirange)</literal></entry></row>
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<row><entry><literal>&> (anyrange, anyrange)</literal></entry></row>
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<row><entry><literal>&> (anyrange, anymultirange)</literal></entry></row>
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<row><entry><literal>-|- (anyrange, anyrange)</literal></entry></row>
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<row><entry><literal>-|- (anyrange, anymultirange)</literal></entry></row>
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<row>
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<entry valign="middle" morerows="1"><literal>tsquery_ops</literal></entry>
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<entry><literal><@ (tsquery, tsquery)</literal></entry>
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<entry valign="middle" morerows="1"></entry>
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</row>
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<row><entry><literal>@> (tsquery, tsquery)</literal></entry></row>
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<row>
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<entry valign="middle"><literal>tsvector_ops</literal></entry>
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<entry><literal>@@ (tsvector, tsquery)</literal></entry>
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<entry></entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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<para>
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For historical reasons, the <literal>inet_ops</literal> operator class is
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not the default class for types <type>inet</type> and <type>cidr</type>.
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To use it, mention the class name in <command>CREATE INDEX</command>,
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for example
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<programlisting>
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CREATE INDEX ON my_table USING GIST (my_inet_column inet_ops);
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</programlisting>
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</para>
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</sect1>
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<sect1 id="gist-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 implementer only to 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 hash indexes. That means that you can use
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<productname>PostgreSQL</productname> to build a B-tree or hash 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 hash indexes only support equality queries.
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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 several 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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<para>
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There are five methods that an index operator class for
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<acronym>GiST</acronym> must provide, and six that are optional.
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Correctness of the index is ensured
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by proper implementation of the <function>same</function>, <function>consistent</function>
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and <function>union</function> methods, while efficiency (size and speed) of the
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index will depend on the <function>penalty</function> and <function>picksplit</function>
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methods.
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Two optional methods are <function>compress</function> and
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<function>decompress</function>, which allow an index to have internal tree data of
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a different type than the data it indexes. The leaves are to be of the
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indexed data type, while the other tree nodes can be of any C struct (but
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you still have to follow <productname>PostgreSQL</productname> data type rules here,
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see about <literal>varlena</literal> for variable sized data). If the tree's
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internal data type exists at the SQL level, the <literal>STORAGE</literal> option
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of the <command>CREATE OPERATOR CLASS</command> command can be used.
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The optional eighth method is <function>distance</function>, which is needed
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if the operator class wishes to support ordered scans (nearest-neighbor
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searches). The optional ninth method <function>fetch</function> is needed if the
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operator class wishes to support index-only scans, except when the
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<function>compress</function> method is omitted. The optional tenth method
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<function>options</function> is needed if the operator class has
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user-specified parameters.
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The optional eleventh method <function>sortsupport</function> is used to
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speed up building a <acronym>GiST</acronym> index.
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</para>
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<variablelist>
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<varlistentry>
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<term><function>consistent</function></term>
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<listitem>
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<para>
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Given an index entry <literal>p</literal> and a query value <literal>q</literal>,
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this function determines whether the index entry is
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<quote>consistent</quote> with the query; that is, could the predicate
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<quote><replaceable>indexed_column</replaceable>
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<replaceable>indexable_operator</replaceable> <literal>q</literal></quote> be true for
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any row represented by the index entry? For a leaf index entry this is
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equivalent to testing the indexable condition, while for an internal
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tree node this determines whether it is necessary to scan the subtree
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of the index represented by the tree node. When the result is
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<literal>true</literal>, a <literal>recheck</literal> flag must also be returned.
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This indicates whether the predicate is certainly true or only possibly
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true. If <literal>recheck</literal> = <literal>false</literal> then the index has
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tested the predicate condition exactly, whereas if <literal>recheck</literal>
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= <literal>true</literal> the row is only a candidate match. In that case the
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system will automatically evaluate the
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<replaceable>indexable_operator</replaceable> against the actual row value to see
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if it is really a match. This convention allows
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<acronym>GiST</acronym> to support both lossless and lossy index
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structures.
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</para>
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<para>
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The <acronym>SQL</acronym> declaration of the function must look like this:
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<programlisting>
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CREATE OR REPLACE FUNCTION my_consistent(internal, data_type, smallint, oid, internal)
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RETURNS bool
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AS 'MODULE_PATHNAME'
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LANGUAGE C STRICT;
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</programlisting>
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And the matching code in the C module could then follow this skeleton:
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<programlisting>
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PG_FUNCTION_INFO_V1(my_consistent);
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Datum
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my_consistent(PG_FUNCTION_ARGS)
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{
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GISTENTRY *entry = (GISTENTRY *) PG_GETARG_POINTER(0);
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data_type *query = PG_GETARG_DATA_TYPE_P(1);
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StrategyNumber strategy = (StrategyNumber) PG_GETARG_UINT16(2);
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/* Oid subtype = PG_GETARG_OID(3); */
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bool *recheck = (bool *) PG_GETARG_POINTER(4);
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data_type *key = DatumGetDataType(entry->key);
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bool retval;
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/*
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* determine return value as a function of strategy, key and query.
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*
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* Use GIST_LEAF(entry) to know where you're called in the index tree,
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|
* which comes handy when supporting the = operator for example (you could
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|
* check for non empty union() in non-leaf nodes and equality in leaf
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* nodes).
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*/
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*recheck = true; /* or false if check is exact */
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PG_RETURN_BOOL(retval);
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}
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</programlisting>
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Here, <varname>key</varname> is an element in the index and <varname>query</varname>
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the value being looked up in the index. The <literal>StrategyNumber</literal>
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parameter indicates which operator of your operator class is being
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|
applied — it matches one of the operator numbers in the
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<command>CREATE OPERATOR CLASS</command> command.
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|
</para>
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<para>
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Depending on which operators you have included in the class, the data
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type of <varname>query</varname> could vary with the operator, since it will
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be whatever type is on the right-hand side of the operator, which might
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be different from the indexed data type appearing on the left-hand side.
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(The above code skeleton assumes that only one type is possible; if
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not, fetching the <varname>query</varname> argument value would have to depend
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|
on the operator.) It is recommended that the SQL declaration of
|
|
the <function>consistent</function> function use the opclass's indexed data
|
|
type for the <varname>query</varname> argument, even though the actual type
|
|
might be something else depending on the operator.
|
|
</para>
|
|
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><function>union</function></term>
|
|
<listitem>
|
|
<para>
|
|
This method consolidates information in the tree. Given a set of
|
|
entries, this function generates a new index entry that represents
|
|
all the given entries.
|
|
</para>
|
|
|
|
<para>
|
|
The <acronym>SQL</acronym> declaration of the function must look like this:
|
|
|
|
<programlisting>
|
|
CREATE OR REPLACE FUNCTION my_union(internal, internal)
|
|
RETURNS storage_type
|
|
AS 'MODULE_PATHNAME'
|
|
LANGUAGE C STRICT;
|
|
</programlisting>
|
|
|
|
And the matching code in the C module could then follow this skeleton:
|
|
|
|
<programlisting>
|
|
PG_FUNCTION_INFO_V1(my_union);
|
|
|
|
Datum
|
|
my_union(PG_FUNCTION_ARGS)
|
|
{
|
|
GistEntryVector *entryvec = (GistEntryVector *) PG_GETARG_POINTER(0);
|
|
GISTENTRY *ent = entryvec->vector;
|
|
data_type *out,
|
|
*tmp,
|
|
*old;
|
|
int numranges,
|
|
i = 0;
|
|
|
|
numranges = entryvec->n;
|
|
tmp = DatumGetDataType(ent[0].key);
|
|
out = tmp;
|
|
|
|
if (numranges == 1)
|
|
{
|
|
out = data_type_deep_copy(tmp);
|
|
|
|
PG_RETURN_DATA_TYPE_P(out);
|
|
}
|
|
|
|
for (i = 1; i < numranges; i++)
|
|
{
|
|
old = out;
|
|
tmp = DatumGetDataType(ent[i].key);
|
|
out = my_union_implementation(out, tmp);
|
|
}
|
|
|
|
PG_RETURN_DATA_TYPE_P(out);
|
|
}
|
|
</programlisting>
|
|
</para>
|
|
|
|
<para>
|
|
As you can see, in this skeleton we're dealing with a data type
|
|
where <literal>union(X, Y, Z) = union(union(X, Y), Z)</literal>. It's easy
|
|
enough to support data types where this is not the case, by
|
|
implementing the proper union algorithm in this
|
|
<acronym>GiST</acronym> support method.
|
|
</para>
|
|
|
|
<para>
|
|
The result of the <function>union</function> function must be a value of the
|
|
index's storage type, whatever that is (it might or might not be
|
|
different from the indexed column's type). The <function>union</function>
|
|
function should return a pointer to newly <function>palloc()</function>ed
|
|
memory. You can't just return the input value as-is, even if there is
|
|
no type change.
|
|
</para>
|
|
|
|
<para>
|
|
As shown above, the <function>union</function> function's
|
|
first <type>internal</type> argument is actually
|
|
a <structname>GistEntryVector</structname> pointer. The second argument is a
|
|
pointer to an integer variable, which can be ignored. (It used to be
|
|
required that the <function>union</function> function store the size of its
|
|
result value into that variable, but this is no longer necessary.)
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><function>compress</function></term>
|
|
<listitem>
|
|
<para>
|
|
Converts a data item into a format suitable for physical storage in
|
|
an index page.
|
|
If the <function>compress</function> method is omitted, data items are stored
|
|
in the index without modification.
|
|
</para>
|
|
|
|
<para>
|
|
The <acronym>SQL</acronym> declaration of the function must look like this:
|
|
|
|
<programlisting>
|
|
CREATE OR REPLACE FUNCTION my_compress(internal)
|
|
RETURNS internal
|
|
AS 'MODULE_PATHNAME'
|
|
LANGUAGE C STRICT;
|
|
</programlisting>
|
|
|
|
And the matching code in the C module could then follow this skeleton:
|
|
|
|
<programlisting>
|
|
PG_FUNCTION_INFO_V1(my_compress);
|
|
|
|
Datum
|
|
my_compress(PG_FUNCTION_ARGS)
|
|
{
|
|
GISTENTRY *entry = (GISTENTRY *) PG_GETARG_POINTER(0);
|
|
GISTENTRY *retval;
|
|
|
|
if (entry->leafkey)
|
|
{
|
|
/* replace entry->key with a compressed version */
|
|
compressed_data_type *compressed_data = palloc(sizeof(compressed_data_type));
|
|
|
|
/* fill *compressed_data from entry->key ... */
|
|
|
|
retval = palloc(sizeof(GISTENTRY));
|
|
gistentryinit(*retval, PointerGetDatum(compressed_data),
|
|
entry->rel, entry->page, entry->offset, FALSE);
|
|
}
|
|
else
|
|
{
|
|
/* typically we needn't do anything with non-leaf entries */
|
|
retval = entry;
|
|
}
|
|
|
|
PG_RETURN_POINTER(retval);
|
|
}
|
|
</programlisting>
|
|
</para>
|
|
|
|
<para>
|
|
You have to adapt <replaceable>compressed_data_type</replaceable> to the specific
|
|
type you're converting to in order to compress your leaf nodes, of
|
|
course.
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><function>decompress</function></term>
|
|
<listitem>
|
|
<para>
|
|
Converts the stored representation of a data item into a format that
|
|
can be manipulated by the other GiST methods in the operator class.
|
|
If the <function>decompress</function> method is omitted, it is assumed that
|
|
the other GiST methods can work directly on the stored data format.
|
|
(<function>decompress</function> is not necessarily the reverse of
|
|
the <function>compress</function> method; in particular,
|
|
if <function>compress</function> is lossy then it's impossible
|
|
for <function>decompress</function> to exactly reconstruct the original
|
|
data. <function>decompress</function> is not necessarily equivalent
|
|
to <function>fetch</function>, either, since the other GiST methods might not
|
|
require full reconstruction of the data.)
|
|
</para>
|
|
|
|
<para>
|
|
The <acronym>SQL</acronym> declaration of the function must look like this:
|
|
|
|
<programlisting>
|
|
CREATE OR REPLACE FUNCTION my_decompress(internal)
|
|
RETURNS internal
|
|
AS 'MODULE_PATHNAME'
|
|
LANGUAGE C STRICT;
|
|
</programlisting>
|
|
|
|
And the matching code in the C module could then follow this skeleton:
|
|
|
|
<programlisting>
|
|
PG_FUNCTION_INFO_V1(my_decompress);
|
|
|
|
Datum
|
|
my_decompress(PG_FUNCTION_ARGS)
|
|
{
|
|
PG_RETURN_POINTER(PG_GETARG_POINTER(0));
|
|
}
|
|
</programlisting>
|
|
|
|
The above skeleton is suitable for the case where no decompression
|
|
is needed. (But, of course, omitting the method altogether is even
|
|
easier, and is recommended in such cases.)
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><function>penalty</function></term>
|
|
<listitem>
|
|
<para>
|
|
Returns a value indicating the <quote>cost</quote> of inserting the new
|
|
entry into a particular branch of the tree. Items will be inserted
|
|
down the path of least <function>penalty</function> in the tree.
|
|
Values returned by <function>penalty</function> should be non-negative.
|
|
If a negative value is returned, it will be treated as zero.
|
|
</para>
|
|
|
|
<para>
|
|
The <acronym>SQL</acronym> declaration of the function must look like this:
|
|
|
|
<programlisting>
|
|
CREATE OR REPLACE FUNCTION my_penalty(internal, internal, internal)
|
|
RETURNS internal
|
|
AS 'MODULE_PATHNAME'
|
|
LANGUAGE C STRICT; -- in some cases penalty functions need not be strict
|
|
</programlisting>
|
|
|
|
And the matching code in the C module could then follow this skeleton:
|
|
|
|
<programlisting>
|
|
PG_FUNCTION_INFO_V1(my_penalty);
|
|
|
|
Datum
|
|
my_penalty(PG_FUNCTION_ARGS)
|
|
{
|
|
GISTENTRY *origentry = (GISTENTRY *) PG_GETARG_POINTER(0);
|
|
GISTENTRY *newentry = (GISTENTRY *) PG_GETARG_POINTER(1);
|
|
float *penalty = (float *) PG_GETARG_POINTER(2);
|
|
data_type *orig = DatumGetDataType(origentry->key);
|
|
data_type *new = DatumGetDataType(newentry->key);
|
|
|
|
*penalty = my_penalty_implementation(orig, new);
|
|
PG_RETURN_POINTER(penalty);
|
|
}
|
|
</programlisting>
|
|
|
|
For historical reasons, the <function>penalty</function> function doesn't
|
|
just return a <type>float</type> result; instead it has to store the value
|
|
at the location indicated by the third argument. The return
|
|
value per se is ignored, though it's conventional to pass back the
|
|
address of that argument.
|
|
</para>
|
|
|
|
<para>
|
|
The <function>penalty</function> function is crucial to good performance of
|
|
the index. It'll get used at insertion time to determine which branch
|
|
to follow when choosing where to add the new entry in the tree. At
|
|
query time, the more balanced the index, the quicker the lookup.
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><function>picksplit</function></term>
|
|
<listitem>
|
|
<para>
|
|
When an index page split is necessary, this function decides which
|
|
entries on the page are to stay on the old page, and which are to move
|
|
to the new page.
|
|
</para>
|
|
|
|
<para>
|
|
The <acronym>SQL</acronym> declaration of the function must look like this:
|
|
|
|
<programlisting>
|
|
CREATE OR REPLACE FUNCTION my_picksplit(internal, internal)
|
|
RETURNS internal
|
|
AS 'MODULE_PATHNAME'
|
|
LANGUAGE C STRICT;
|
|
</programlisting>
|
|
|
|
And the matching code in the C module could then follow this skeleton:
|
|
|
|
<programlisting>
|
|
PG_FUNCTION_INFO_V1(my_picksplit);
|
|
|
|
Datum
|
|
my_picksplit(PG_FUNCTION_ARGS)
|
|
{
|
|
GistEntryVector *entryvec = (GistEntryVector *) PG_GETARG_POINTER(0);
|
|
GIST_SPLITVEC *v = (GIST_SPLITVEC *) PG_GETARG_POINTER(1);
|
|
OffsetNumber maxoff = entryvec->n - 1;
|
|
GISTENTRY *ent = entryvec->vector;
|
|
int i,
|
|
nbytes;
|
|
OffsetNumber *left,
|
|
*right;
|
|
data_type *tmp_union;
|
|
data_type *unionL;
|
|
data_type *unionR;
|
|
GISTENTRY **raw_entryvec;
|
|
|
|
maxoff = entryvec->n - 1;
|
|
nbytes = (maxoff + 1) * sizeof(OffsetNumber);
|
|
|
|
v->spl_left = (OffsetNumber *) palloc(nbytes);
|
|
left = v->spl_left;
|
|
v->spl_nleft = 0;
|
|
|
|
v->spl_right = (OffsetNumber *) palloc(nbytes);
|
|
right = v->spl_right;
|
|
v->spl_nright = 0;
|
|
|
|
unionL = NULL;
|
|
unionR = NULL;
|
|
|
|
/* Initialize the raw entry vector. */
|
|
raw_entryvec = (GISTENTRY **) malloc(entryvec->n * sizeof(void *));
|
|
for (i = FirstOffsetNumber; i <= maxoff; i = OffsetNumberNext(i))
|
|
raw_entryvec[i] = &(entryvec->vector[i]);
|
|
|
|
for (i = FirstOffsetNumber; i <= maxoff; i = OffsetNumberNext(i))
|
|
{
|
|
int real_index = raw_entryvec[i] - entryvec->vector;
|
|
|
|
tmp_union = DatumGetDataType(entryvec->vector[real_index].key);
|
|
Assert(tmp_union != NULL);
|
|
|
|
/*
|
|
* Choose where to put the index entries and update unionL and unionR
|
|
* accordingly. Append the entries to either v->spl_left or
|
|
* v->spl_right, and care about the counters.
|
|
*/
|
|
|
|
if (my_choice_is_left(unionL, curl, unionR, curr))
|
|
{
|
|
if (unionL == NULL)
|
|
unionL = tmp_union;
|
|
else
|
|
unionL = my_union_implementation(unionL, tmp_union);
|
|
|
|
*left = real_index;
|
|
++left;
|
|
++(v->spl_nleft);
|
|
}
|
|
else
|
|
{
|
|
/*
|
|
* Same on the right
|
|
*/
|
|
}
|
|
}
|
|
|
|
v->spl_ldatum = DataTypeGetDatum(unionL);
|
|
v->spl_rdatum = DataTypeGetDatum(unionR);
|
|
PG_RETURN_POINTER(v);
|
|
}
|
|
</programlisting>
|
|
|
|
Notice that the <function>picksplit</function> function's result is delivered
|
|
by modifying the passed-in <structname>v</structname> structure. The return
|
|
value per se is ignored, though it's conventional to pass back the
|
|
address of <structname>v</structname>.
|
|
</para>
|
|
|
|
<para>
|
|
Like <function>penalty</function>, the <function>picksplit</function> function
|
|
is crucial to good performance of the index. Designing suitable
|
|
<function>penalty</function> and <function>picksplit</function> implementations
|
|
is where the challenge of implementing well-performing
|
|
<acronym>GiST</acronym> indexes lies.
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><function>same</function></term>
|
|
<listitem>
|
|
<para>
|
|
Returns true if two index entries are identical, false otherwise.
|
|
(An <quote>index entry</quote> is a value of the index's storage type,
|
|
not necessarily the original indexed column's type.)
|
|
</para>
|
|
|
|
<para>
|
|
The <acronym>SQL</acronym> declaration of the function must look like this:
|
|
|
|
<programlisting>
|
|
CREATE OR REPLACE FUNCTION my_same(storage_type, storage_type, internal)
|
|
RETURNS internal
|
|
AS 'MODULE_PATHNAME'
|
|
LANGUAGE C STRICT;
|
|
</programlisting>
|
|
|
|
And the matching code in the C module could then follow this skeleton:
|
|
|
|
<programlisting>
|
|
PG_FUNCTION_INFO_V1(my_same);
|
|
|
|
Datum
|
|
my_same(PG_FUNCTION_ARGS)
|
|
{
|
|
prefix_range *v1 = PG_GETARG_PREFIX_RANGE_P(0);
|
|
prefix_range *v2 = PG_GETARG_PREFIX_RANGE_P(1);
|
|
bool *result = (bool *) PG_GETARG_POINTER(2);
|
|
|
|
*result = my_eq(v1, v2);
|
|
PG_RETURN_POINTER(result);
|
|
}
|
|
</programlisting>
|
|
|
|
For historical reasons, the <function>same</function> function doesn't
|
|
just return a Boolean result; instead it has to store the flag
|
|
at the location indicated by the third argument. The return
|
|
value per se is ignored, though it's conventional to pass back the
|
|
address of that argument.
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><function>distance</function></term>
|
|
<listitem>
|
|
<para>
|
|
Given an index entry <literal>p</literal> and a query value <literal>q</literal>,
|
|
this function determines the index entry's
|
|
<quote>distance</quote> from the query value. This function must be
|
|
supplied if the operator class contains any ordering operators.
|
|
A query using the ordering operator will be implemented by returning
|
|
index entries with the smallest <quote>distance</quote> values first,
|
|
so the results must be consistent with the operator's semantics.
|
|
For a leaf index entry the result just represents the distance to
|
|
the index entry; for an internal tree node, the result must be the
|
|
smallest distance that any child entry could have.
|
|
</para>
|
|
|
|
<para>
|
|
The <acronym>SQL</acronym> declaration of the function must look like this:
|
|
|
|
<programlisting>
|
|
CREATE OR REPLACE FUNCTION my_distance(internal, data_type, smallint, oid, internal)
|
|
RETURNS float8
|
|
AS 'MODULE_PATHNAME'
|
|
LANGUAGE C STRICT;
|
|
</programlisting>
|
|
|
|
And the matching code in the C module could then follow this skeleton:
|
|
|
|
<programlisting>
|
|
PG_FUNCTION_INFO_V1(my_distance);
|
|
|
|
Datum
|
|
my_distance(PG_FUNCTION_ARGS)
|
|
{
|
|
GISTENTRY *entry = (GISTENTRY *) PG_GETARG_POINTER(0);
|
|
data_type *query = PG_GETARG_DATA_TYPE_P(1);
|
|
StrategyNumber strategy = (StrategyNumber) PG_GETARG_UINT16(2);
|
|
/* Oid subtype = PG_GETARG_OID(3); */
|
|
/* bool *recheck = (bool *) PG_GETARG_POINTER(4); */
|
|
data_type *key = DatumGetDataType(entry->key);
|
|
double retval;
|
|
|
|
/*
|
|
* determine return value as a function of strategy, key and query.
|
|
*/
|
|
|
|
PG_RETURN_FLOAT8(retval);
|
|
}
|
|
</programlisting>
|
|
|
|
The arguments to the <function>distance</function> function are identical to
|
|
the arguments of the <function>consistent</function> function.
|
|
</para>
|
|
|
|
<para>
|
|
Some approximation is allowed when determining the distance, so long
|
|
as the result is never greater than the entry's actual distance. Thus,
|
|
for example, distance to a bounding box is usually sufficient in
|
|
geometric applications. For an internal tree node, the distance
|
|
returned must not be greater than the distance to any of the child
|
|
nodes. If the returned distance is not exact, the function must set
|
|
<literal>*recheck</literal> to true. (This is not necessary for internal tree
|
|
nodes; for them, the calculation is always assumed to be inexact.) In
|
|
this case the executor will calculate the accurate distance after
|
|
fetching the tuple from the heap, and reorder the tuples if necessary.
|
|
</para>
|
|
|
|
<para>
|
|
If the distance function returns <literal>*recheck = true</literal> for any
|
|
leaf node, the original ordering operator's return type must
|
|
be <type>float8</type> or <type>float4</type>, and the distance function's
|
|
result values must be comparable to those of the original ordering
|
|
operator, since the executor will sort using both distance function
|
|
results and recalculated ordering-operator results. Otherwise, the
|
|
distance function's result values can be any finite <type>float8</type>
|
|
values, so long as the relative order of the result values matches the
|
|
order returned by the ordering operator. (Infinity and minus infinity
|
|
are used internally to handle cases such as nulls, so it is not
|
|
recommended that <function>distance</function> functions return these values.)
|
|
</para>
|
|
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><function>fetch</function></term>
|
|
<listitem>
|
|
<para>
|
|
Converts the compressed index representation of a data item into the
|
|
original data type, for index-only scans. The returned data must be an
|
|
exact, non-lossy copy of the originally indexed value.
|
|
</para>
|
|
|
|
<para>
|
|
The <acronym>SQL</acronym> declaration of the function must look like this:
|
|
|
|
<programlisting>
|
|
CREATE OR REPLACE FUNCTION my_fetch(internal)
|
|
RETURNS internal
|
|
AS 'MODULE_PATHNAME'
|
|
LANGUAGE C STRICT;
|
|
</programlisting>
|
|
|
|
The argument is a pointer to a <structname>GISTENTRY</structname> struct. On
|
|
entry, its <structfield>key</structfield> field contains a non-NULL leaf datum in
|
|
compressed form. The return value is another <structname>GISTENTRY</structname>
|
|
struct, whose <structfield>key</structfield> field contains the same datum in its
|
|
original, uncompressed form. If the opclass's compress function does
|
|
nothing for leaf entries, the <function>fetch</function> method can return the
|
|
argument as-is. Or, if the opclass does not have a compress function,
|
|
the <function>fetch</function> method can be omitted as well, since it would
|
|
necessarily be a no-op.
|
|
</para>
|
|
|
|
<para>
|
|
The matching code in the C module could then follow this skeleton:
|
|
|
|
<programlisting>
|
|
PG_FUNCTION_INFO_V1(my_fetch);
|
|
|
|
Datum
|
|
my_fetch(PG_FUNCTION_ARGS)
|
|
{
|
|
GISTENTRY *entry = (GISTENTRY *) PG_GETARG_POINTER(0);
|
|
input_data_type *in = DatumGetPointer(entry->key);
|
|
fetched_data_type *fetched_data;
|
|
GISTENTRY *retval;
|
|
|
|
retval = palloc(sizeof(GISTENTRY));
|
|
fetched_data = palloc(sizeof(fetched_data_type));
|
|
|
|
/*
|
|
* Convert 'fetched_data' into the a Datum of the original datatype.
|
|
*/
|
|
|
|
/* fill *retval from fetched_data. */
|
|
gistentryinit(*retval, PointerGetDatum(converted_datum),
|
|
entry->rel, entry->page, entry->offset, FALSE);
|
|
|
|
PG_RETURN_POINTER(retval);
|
|
}
|
|
</programlisting>
|
|
</para>
|
|
|
|
<para>
|
|
If the compress method is lossy for leaf entries, the operator class
|
|
cannot support index-only scans, and must not define
|
|
a <function>fetch</function> function.
|
|
</para>
|
|
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><function>options</function></term>
|
|
<listitem>
|
|
<para>
|
|
Allows definition of user-visible parameters that control operator
|
|
class behavior.
|
|
</para>
|
|
|
|
<para>
|
|
The <acronym>SQL</acronym> declaration of the function must look like this:
|
|
|
|
<programlisting>
|
|
CREATE OR REPLACE FUNCTION my_options(internal)
|
|
RETURNS void
|
|
AS 'MODULE_PATHNAME'
|
|
LANGUAGE C STRICT;
|
|
</programlisting>
|
|
</para>
|
|
|
|
<para>
|
|
The function is passed a pointer to a <structname>local_relopts</structname>
|
|
struct, which needs to be filled with a set of operator class
|
|
specific options. The options can be accessed from other support
|
|
functions using the <literal>PG_HAS_OPCLASS_OPTIONS()</literal> and
|
|
<literal>PG_GET_OPCLASS_OPTIONS()</literal> macros.
|
|
</para>
|
|
|
|
<para>
|
|
An example implementation of my_options() and parameters use
|
|
from other support functions are given below:
|
|
|
|
<programlisting>
|
|
typedef enum MyEnumType
|
|
{
|
|
MY_ENUM_ON,
|
|
MY_ENUM_OFF,
|
|
MY_ENUM_AUTO
|
|
} MyEnumType;
|
|
|
|
typedef struct
|
|
{
|
|
int32 vl_len_; /* varlena header (do not touch directly!) */
|
|
int int_param; /* integer parameter */
|
|
double real_param; /* real parameter */
|
|
MyEnumType enum_param; /* enum parameter */
|
|
int str_param; /* string parameter */
|
|
} MyOptionsStruct;
|
|
|
|
/* String representation of enum values */
|
|
static relopt_enum_elt_def myEnumValues[] =
|
|
{
|
|
{"on", MY_ENUM_ON},
|
|
{"off", MY_ENUM_OFF},
|
|
{"auto", MY_ENUM_AUTO},
|
|
{(const char *) NULL} /* list terminator */
|
|
};
|
|
|
|
static char *str_param_default = "default";
|
|
|
|
/*
|
|
* Sample validator: checks that string is not longer than 8 bytes.
|
|
*/
|
|
static void
|
|
validate_my_string_relopt(const char *value)
|
|
{
|
|
if (strlen(value) > 8)
|
|
ereport(ERROR,
|
|
(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
|
|
errmsg("str_param must be at most 8 bytes")));
|
|
}
|
|
|
|
/*
|
|
* Sample filler: switches characters to lower case.
|
|
*/
|
|
static Size
|
|
fill_my_string_relopt(const char *value, void *ptr)
|
|
{
|
|
char *tmp = str_tolower(value, strlen(value), DEFAULT_COLLATION_OID);
|
|
int len = strlen(tmp);
|
|
|
|
if (ptr)
|
|
strcpy((char *) ptr, tmp);
|
|
|
|
pfree(tmp);
|
|
return len + 1;
|
|
}
|
|
|
|
PG_FUNCTION_INFO_V1(my_options);
|
|
|
|
Datum
|
|
my_options(PG_FUNCTION_ARGS)
|
|
{
|
|
local_relopts *relopts = (local_relopts *) PG_GETARG_POINTER(0);
|
|
|
|
init_local_reloptions(relopts, sizeof(MyOptionsStruct));
|
|
add_local_int_reloption(relopts, "int_param", "integer parameter",
|
|
100, 0, 1000000,
|
|
offsetof(MyOptionsStruct, int_param));
|
|
add_local_real_reloption(relopts, "real_param", "real parameter",
|
|
1.0, 0.0, 1000000.0,
|
|
offsetof(MyOptionsStruct, real_param));
|
|
add_local_enum_reloption(relopts, "enum_param", "enum parameter",
|
|
myEnumValues, MY_ENUM_ON,
|
|
"Valid values are: \"on\", \"off\" and \"auto\".",
|
|
offsetof(MyOptionsStruct, enum_param));
|
|
add_local_string_reloption(relopts, "str_param", "string parameter",
|
|
str_param_default,
|
|
&validate_my_string_relopt,
|
|
&fill_my_string_relopt,
|
|
offsetof(MyOptionsStruct, str_param));
|
|
|
|
PG_RETURN_VOID();
|
|
}
|
|
|
|
PG_FUNCTION_INFO_V1(my_compress);
|
|
|
|
Datum
|
|
my_compress(PG_FUNCTION_ARGS)
|
|
{
|
|
int int_param = 100;
|
|
double real_param = 1.0;
|
|
MyEnumType enum_param = MY_ENUM_ON;
|
|
char *str_param = str_param_default;
|
|
|
|
/*
|
|
* Normally, when opclass contains 'options' method, then options are always
|
|
* passed to support functions. However, if you add 'options' method to
|
|
* existing opclass, previously defined indexes have no options, so the
|
|
* check is required.
|
|
*/
|
|
if (PG_HAS_OPCLASS_OPTIONS())
|
|
{
|
|
MyOptionsStruct *options = (MyOptionsStruct *) PG_GET_OPCLASS_OPTIONS();
|
|
|
|
int_param = options->int_param;
|
|
real_param = options->real_param;
|
|
enum_param = options->enum_param;
|
|
str_param = GET_STRING_RELOPTION(options, str_param);
|
|
}
|
|
|
|
/* the rest implementation of support function */
|
|
}
|
|
|
|
</programlisting>
|
|
</para>
|
|
|
|
<para>
|
|
Since the representation of the key in <acronym>GiST</acronym> is
|
|
flexible, it may depend on user-specified parameters. For instance,
|
|
the length of key signature may be specified. See
|
|
<literal>gtsvector_options()</literal> for example.
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><function>sortsupport</function></term>
|
|
<listitem>
|
|
<para>
|
|
Returns a comparator function to sort data in a way that preserves
|
|
locality. It is used by <command>CREATE INDEX</command> and
|
|
<command>REINDEX</command> commands. The quality of the created index
|
|
depends on how well the sort order determined by the comparator function
|
|
preserves locality of the inputs.
|
|
</para>
|
|
<para>
|
|
The <function>sortsupport</function> method is optional. If it is not
|
|
provided, <command>CREATE INDEX</command> builds the index by inserting
|
|
each tuple to the tree using the <function>penalty</function> and
|
|
<function>picksplit</function> functions, which is much slower.
|
|
</para>
|
|
|
|
<para>
|
|
The <acronym>SQL</acronym> declaration of the function must look like
|
|
this:
|
|
|
|
<programlisting>
|
|
CREATE OR REPLACE FUNCTION my_sortsupport(internal)
|
|
RETURNS void
|
|
AS 'MODULE_PATHNAME'
|
|
LANGUAGE C STRICT;
|
|
</programlisting>
|
|
|
|
The argument is a pointer to a <structname>SortSupport</structname>
|
|
struct. At a minimum, the function must fill in its comparator field.
|
|
The comparator takes three arguments: two Datums to compare, and
|
|
a pointer to the <structname>SortSupport</structname> struct. The
|
|
Datums are the two indexed values in the format that they are stored
|
|
in the index; that is, in the format returned by the
|
|
<function>compress</function> method. The full API is defined in
|
|
<filename>src/include/utils/sortsupport.h</filename>.
|
|
</para>
|
|
|
|
<para>
|
|
The matching code in the C module could then follow this skeleton:
|
|
|
|
<programlisting>
|
|
PG_FUNCTION_INFO_V1(my_sortsupport);
|
|
|
|
static int
|
|
my_fastcmp(Datum x, Datum y, SortSupport ssup)
|
|
{
|
|
/* establish order between x and y by computing some sorting value z */
|
|
|
|
int z1 = ComputeSpatialCode(x);
|
|
int z2 = ComputeSpatialCode(y);
|
|
|
|
return z1 == z2 ? 0 : z1 > z2 ? 1 : -1;
|
|
}
|
|
|
|
Datum
|
|
my_sortsupport(PG_FUNCTION_ARGS)
|
|
{
|
|
SortSupport ssup = (SortSupport) PG_GETARG_POINTER(0);
|
|
|
|
ssup->comparator = my_fastcmp;
|
|
PG_RETURN_VOID();
|
|
}
|
|
</programlisting>
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
</variablelist>
|
|
|
|
<para>
|
|
All the GiST support methods are normally called in short-lived memory
|
|
contexts; that is, <varname>CurrentMemoryContext</varname> will get reset after
|
|
each tuple is processed. It is therefore not very important to worry about
|
|
pfree'ing everything you palloc. However, in some cases it's useful for a
|
|
support method to cache data across repeated calls. To do that, allocate
|
|
the longer-lived data in <literal>fcinfo->flinfo->fn_mcxt</literal>, and
|
|
keep a pointer to it in <literal>fcinfo->flinfo->fn_extra</literal>. Such
|
|
data will survive for the life of the index operation (e.g., a single GiST
|
|
index scan, index build, or index tuple insertion). Be careful to pfree
|
|
the previous value when replacing a <literal>fn_extra</literal> value, or the leak
|
|
will accumulate for the duration of the operation.
|
|
</para>
|
|
|
|
</sect1>
|
|
|
|
<sect1 id="gist-implementation">
|
|
<title>Implementation</title>
|
|
|
|
<sect2 id="gist-buffering-build">
|
|
<title>GiST Index Build Methods</title>
|
|
|
|
<para>
|
|
The simplest way to build a GiST index is just to insert all the entries,
|
|
one by one. This tends to be slow for large indexes, because if the
|
|
index tuples are scattered across the index and the index is large enough
|
|
to not fit in cache, a lot of random I/O will be
|
|
needed. <productname>PostgreSQL</productname> supports two alternative
|
|
methods for initial build of a GiST index: <firstterm>sorted</firstterm>
|
|
and <firstterm>buffered</firstterm> modes.
|
|
</para>
|
|
|
|
<para>
|
|
The sorted method is only available if each of the opclasses used by the
|
|
index provides a <function>sortsupport</function> function, as described
|
|
in <xref linkend="gist-extensibility"/>. If they do, this method is
|
|
usually the best, so it is used by default.
|
|
</para>
|
|
|
|
<para>
|
|
The buffered method works by not inserting tuples directly into the index
|
|
right away. It can dramatically reduce the amount of random I/O needed
|
|
for non-ordered data sets. For well-ordered data sets the benefit is
|
|
smaller or non-existent, because only a small number of pages receive new
|
|
tuples at a time, and those pages fit in cache even if the index as a
|
|
whole does not.
|
|
</para>
|
|
|
|
<para>
|
|
The buffered method needs to call the <function>penalty</function>
|
|
function more often than the simple method does, which consumes some
|
|
extra CPU resources. Also, the buffers need temporary disk space, up to
|
|
the size of the resulting index. Buffering can also influence the quality
|
|
of the resulting index, in both positive and negative directions. That
|
|
influence depends on various factors, like the distribution of the input
|
|
data and the operator class implementation.
|
|
</para>
|
|
|
|
<para>
|
|
If sorting is not possible, then by default a GiST index build switches
|
|
to the buffering method when the index size reaches
|
|
<xref linkend="guc-effective-cache-size"/>. Buffering can be manually
|
|
forced or prevented by the <literal>buffering</literal> parameter to the
|
|
CREATE INDEX command. The default behavior is good for most cases, but
|
|
turning buffering off might speed up the build somewhat if the input data
|
|
is ordered.
|
|
</para>
|
|
|
|
</sect2>
|
|
</sect1>
|
|
|
|
<sect1 id="gist-examples">
|
|
<title>Examples</title>
|
|
|
|
<para>
|
|
The <productname>PostgreSQL</productname> source distribution includes
|
|
several examples of index methods implemented using
|
|
<acronym>GiST</acronym>. The core system currently provides text search
|
|
support (indexing for <type>tsvector</type> and <type>tsquery</type>) as well as
|
|
R-Tree equivalent functionality for some of the built-in geometric data types
|
|
(see <filename>src/backend/access/gist/gistproc.c</filename>). The following
|
|
<filename>contrib</filename> modules also contain <acronym>GiST</acronym>
|
|
operator classes:
|
|
|
|
<variablelist>
|
|
<varlistentry>
|
|
<term><filename>btree_gist</filename></term>
|
|
<listitem>
|
|
<para>B-tree equivalent functionality for several data types</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><filename>cube</filename></term>
|
|
<listitem>
|
|
<para>Indexing for multidimensional cubes</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><filename>hstore</filename></term>
|
|
<listitem>
|
|
<para>Module for storing (key, value) pairs</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><filename>intarray</filename></term>
|
|
<listitem>
|
|
<para>RD-Tree for one-dimensional array of int4 values</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><filename>ltree</filename></term>
|
|
<listitem>
|
|
<para>Indexing for tree-like structures</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><filename>pg_trgm</filename></term>
|
|
<listitem>
|
|
<para>Text similarity using trigram matching</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><filename>seg</filename></term>
|
|
<listitem>
|
|
<para>Indexing for <quote>float ranges</quote></para>
|
|
</listitem>
|
|
</varlistentry>
|
|
</variablelist>
|
|
</para>
|
|
|
|
</sect1>
|
|
|
|
</chapter>
|