<term><acronym>HOT</acronym></term>
<listitem>
<para>
- <ulink
- url="https://git.postgresql.org/gitweb/?p=postgresql.git;a=blob;f=src/backend/access/heap/README.HOT;hb=HEAD">Heap-Only
- Tuples</ulink>
+ <link linkend="storage-hot">Heap-Only Tuples</link>
</para>
</listitem>
</varlistentry>
<entry>
If true, queries must not use the index until the <structfield>xmin</structfield>
of this <structname>pg_index</structname> row is below their <symbol>TransactionXmin</symbol>
- event horizon, because the table may contain broken HOT chains with
+ event horizon, because the table may contain broken <link
+ linkend="storage-hot">HOT chains</link> with
incompatible rows that they can see
</entry>
</row>
<listitem>
<para>
Specifies the number of transactions by which <command>VACUUM</command> and
- <acronym>HOT</acronym> updates will defer cleanup of dead row versions. The
+ <link linkend="storage-hot"><acronym>HOT</acronym> updates</link>
+ will defer cleanup of dead row versions. The
default is zero transactions, meaning that dead row versions can be
removed as soon as possible, that is, as soon as they are no longer
visible to any open transaction. You may wish to set this to a
extant versions of the same logical row; to an index, each tuple is
an independent object that needs its own index entry. Thus, an
update of a row always creates all-new index entries for the row, even if
- the key values did not change. (HOT tuples are an exception to this
+ the key values did not change. (<link linkend="storage-hot">HOT
+ tuples</link> are an exception to this
statement; but indexes do not deal with those, either.) Index entries for
dead tuples are reclaimed (by vacuuming) when the dead tuples themselves
are reclaimed.
<para>
After an index is created, the system has to keep it synchronized with the
- table. This adds overhead to data manipulation operations.
+ table. This adds overhead to data manipulation operations. Indexes can
+ also prevent the creation of <link linkend="storage-hot">heap-only
+ tuples</link>.
Therefore indexes that are seldom or never used in queries
should be removed.
</para>
<para>
Index expressions are relatively expensive to maintain, because the
derived expression(s) must be computed for each row insertion
- and non-HOT update. However, the index expressions are
+ and <link linkend="storage-hot">non-HOT update.</link> However, the index expressions are
<emphasis>not</emphasis> recomputed during an indexed search, since they are
already stored in the index. In both examples above, the system
sees the query as just <literal>WHERE indexedcolumn = 'constant'</literal>
<row>
<entry><structfield>n_tup_upd</structfield></entry>
<entry><type>bigint</type></entry>
- <entry>Number of rows updated (includes HOT updated rows)</entry>
+ <entry>Number of rows updated (includes <link
+ linkend="storage-hot">HOT updated rows</link> updated rows)</entry>
</row>
<row>
<entry><structfield>n_tup_del</structfield></entry>
to the indicated percentage; the remaining space on each page is
reserved for updating rows on that page. This gives <command>UPDATE</command>
a chance to place the updated copy of a row on the same page as the
- original, which is more efficient than placing it on a different page.
+ original, which is more efficient than placing it on a different
+ page, and makes <link linkend="storage-hot">heap-only tuple
+ updates</link> more likely.
For a table whose entries are never updated, complete packing is the
best choice, but in heavily updated tables smaller fillfactors are
appropriate. This parameter cannot be set for TOAST tables.
</sect2>
</sect1>
+<sect1 id="storage-hot">
+
+ <title>Heap-Only Tuples (<acronym>HOT</acronym>)</title>
+
+ <para>
+ To allow for high concurrency, <productname>PostgreSQL</productname>
+ uses <link linkend="mvcc-intro">multiversion concurrency
+ control</link> (<acronym>MVCC</acronym>) to store rows. However,
+ <acronym>MVCC</acronym> has some downsides for update queries.
+ Specifically, updates require new versions of rows to be added to
+ tables. This can also require new index entries for each updated row,
+ and removal of old versions of rows and their index entries can be
+ expensive.
+ </para>
+
+ <para>
+ To help reduce the overhead of updates,
+ <productname>PostgreSQL</productname> has an optimization called
+ heap-only tuples (<acronym>HOT</acronym>). This optimization is
+ possible when:
+
+ <itemizedlist>
+ <listitem>
+ <para>
+ The update does not modify any columns referenced by the table's
+ indexes, including expression and partial indexes.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ There is sufficient free space on the page containing the old row
+ for the updated row.
+ </para>
+ </listitem>
+ </itemizedlist>
+
+ In such cases, heap-only tuples provide two optimizations:
+
+ <itemizedlist>
+ <listitem>
+ <para>
+ New index entries are not needed to represent updated rows.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ Old versions of updated rows can be completely removed during normal
+ operation, including <command>SELECT</command>s, instead of requiring
+ periodic vacuum operations. (This is possible because indexes
+ do not reference their <link linkend="storage-page-layout">page
+ item identifiers</link>.)
+ </para>
+ </listitem>
+ </itemizedlist>
+ </para>
+
+ <para>
+ In summary, heap-only tuple updates can only be created
+ if columns used by indexes are not updated. You can
+ increase the likelihood of sufficient page space for
+ <acronym>HOT</acronym> updates by decreasing a table's <link
+ linkend="sql-createtable"><literal>fillfactor</literal></link>.
+ If you don't, <acronym>HOT</acronym> updates will still happen because
+ new rows will naturally migrate to new pages and existing pages with
+ sufficient free space for new row versions. The system view <link
+ linkend="pg-stat-all-tables-view">pg_stat_all_tables</link>
+ allows monitoring of the occurrence of HOT and non-HOT updates.
+ </para>
+</sect1>
+
</chapter>
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