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docs: update guidelines on when to use GIN and GiST indexes

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Report by Tomas Vondra

Backpatch through 9.5
This commit is contained in:
Bruce Momjian 2015-10-05 13:38:36 -04:00
parent d62359144d
commit 7d88b3d154
1 changed files with 30 additions and 72 deletions
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doc/src/sgml/textsearch.sgml
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@ -3192,7 +3192,7 @@ SELECT plainto_tsquery('supernovae stars');
</sect1> </sect1>
<sect1 id="textsearch-indexes"> <sect1 id="textsearch-indexes">
<title>GiST and GIN Index Types</title> <title>GIN and GiST Index Types</title>
<indexterm zone="textsearch-indexes"> <indexterm zone="textsearch-indexes">
<primary>text search</primary> <primary>text search</primary>
@ -3208,6 +3208,26 @@ SELECT plainto_tsquery('supernovae stars');
<variablelist> <variablelist>
<varlistentry>
<term>
<indexterm zone="textsearch-indexes">
<primary>index</primary>
<secondary>GIN</secondary>
<tertiary>text search</tertiary>
</indexterm>
<literal>CREATE INDEX <replaceable>name</replaceable> ON <replaceable>table</replaceable> USING GIN (<replaceable>column</replaceable>);</literal>
</term>
<listitem>
<para>
Creates a GIN (Generalized Inverted Index)-based index.
The <replaceable>column</replaceable> must be of <type>tsvector</> type.
</para>
</listitem>
</varlistentry>
<varlistentry> <varlistentry>
<term> <term>
@ -3229,37 +3249,22 @@ SELECT plainto_tsquery('supernovae stars');
</listitem> </listitem>
</varlistentry> </varlistentry>
<varlistentry>
<term>
<indexterm zone="textsearch-indexes">
<primary>index</primary>
<secondary>GIN</secondary>
<tertiary>text search</tertiary>
</indexterm>
<literal>CREATE INDEX <replaceable>name</replaceable> ON <replaceable>table</replaceable> USING GIN (<replaceable>column</replaceable>);</literal>
</term>
<listitem>
<para>
Creates a GIN (Generalized Inverted Index)-based index.
The <replaceable>column</replaceable> must be of <type>tsvector</> type.
</para>
</listitem>
</varlistentry>
</variablelist> </variablelist>
</para> </para>
<para> <para>
There are substantial performance differences between the two index types, GIN indexes are the preferred text search index type. As inverted
so it is important to understand their characteristics. indexes, they contain an index entry for each word (lexeme), with a
compressed list of matching locations. Multi-word searches can find
the first match, then use the index to remove rows that are lacking
additional words. GIN indexes store only the words (lexemes) of
<type>tsvector</> values, and not their weight labels. Thus a table
row recheck is needed when using a query that involves weights.
</para> </para>
<para> <para>
A GiST index is <firstterm>lossy</firstterm>, meaning that the index A GiST index is <firstterm>lossy</firstterm>, meaning that the index
may produce false matches, and it is necessary might produce false matches, and it is necessary
to check the actual table row to eliminate such false matches. to check the actual table row to eliminate such false matches.
(<productname>PostgreSQL</productname> does this automatically when needed.) (<productname>PostgreSQL</productname> does this automatically when needed.)
GiST indexes are lossy because each document is represented in the GiST indexes are lossy because each document is represented in the
@ -3280,53 +3285,6 @@ SELECT plainto_tsquery('supernovae stars');
recommended. recommended.
</para> </para>
<para>
GIN indexes are not lossy for standard queries, but their performance
depends logarithmically on the number of unique words.
(However, GIN indexes store only the words (lexemes) of <type>tsvector</>
values, and not their weight labels. Thus a table row recheck is needed
when using a query that involves weights.)
</para>
<para>
In choosing which index type to use, GiST or GIN, consider these
performance differences:
<itemizedlist spacing="compact" mark="bullet">
<listitem>
<para>
GIN index lookups are about three times faster than GiST
</para>
</listitem>
<listitem>
<para>
GIN indexes take about three times longer to build than GiST
</para>
</listitem>
<listitem>
<para>
GIN indexes are moderately slower to update than GiST indexes, but
about 10 times slower if fast-update support was disabled
(see <xref linkend="gin-fast-update"> for details)
</para>
</listitem>
<listitem>
<para>
GIN indexes are two-to-three times larger than GiST indexes
</para>
</listitem>
</itemizedlist>
</para>
<para>
As a rule of thumb, <acronym>GIN</acronym> indexes are best for static data
because lookups are faster. For dynamic data, GiST indexes are
faster to update. Specifically, <acronym>GiST</acronym> indexes are very
good for dynamic data and fast if the number of unique words (lexemes) is
under 100,000, while <acronym>GIN</acronym> indexes will handle 100,000+
lexemes better but are slower to update.
</para>
<para> <para>
Note that <acronym>GIN</acronym> index build time can often be improved Note that <acronym>GIN</acronym> index build time can often be improved
by increasing <xref linkend="guc-maintenance-work-mem">, while by increasing <xref linkend="guc-maintenance-work-mem">, while
@ -3335,7 +3293,7 @@ SELECT plainto_tsquery('supernovae stars');
</para> </para>
<para> <para>
Partitioning of big collections and the proper use of GiST and GIN indexes Partitioning of big collections and the proper use of GIN and GiST indexes
allows the implementation of very fast searches with online update. allows the implementation of very fast searches with online update.
Partitioning can be done at the database level using table inheritance, Partitioning can be done at the database level using table inheritance,
or by distributing documents over or by distributing documents over