doc: add section about heap-only tuples (HOT)

Reported-by: Jonathan S. Katz

Discussion: https://postgr.es/m/c59ffbd5-96ac-a5a5-a401-14f627ca1405@postgresql.org

Backpatch-through: 11

doc/src/sgml/acronyms.sgml

@@ -299,9 +299,7 @@
     <term><acronym>HOT</acronym></term>
     <listitem>
      <para>
-      <ulink
+      <link linkend="storage-hot">Heap-Only Tuples</link>
-      url="https://git.postgresql.org/gitweb/?p=postgresql.git;a=blob;f=src/backend/access/heap/README.HOT;hb=HEAD">Heap-Only
-      Tuples</ulink>
      </para>
     </listitem>
    </varlistentry>

doc/src/sgml/btree.sgml

@@ -639,7 +639,8 @@ options(<replaceable>relopts</replaceable> <type>local_relopts *</type>) returns
    accumulate and adversely affect query latency and throughput.  This
    typically occurs with <command>UPDATE</command>-heavy workloads
    where most individual updates cannot apply the
-   <acronym>HOT</acronym> optimization.  Changing the value of only
+   <link linkend="storage-hot"><acronym>HOT</acronym> optimization.</link>
+   Changing the value of only
    one column covered by one index during an <command>UPDATE</command>
    <emphasis>always</emphasis> necessitates a new set of index tuples
    &mdash; one for <emphasis>each and every</emphasis> index on the

doc/src/sgml/catalogs.sgml

@@ -4381,7 +4381,7 @@ SCRAM-SHA-256$<replaceable>&lt;iteration count&gt;</replaceable>:<replaceable>&l
       <para>
        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
       </para></entry>
      </row>

doc/src/sgml/config.sgml

@@ -4491,7 +4491,8 @@ ANY <replaceable class="parameter">num_sync</replaceable> ( <replaceable class="
       <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

doc/src/sgml/indexam.sgml

@@ -45,7 +45,8 @@
    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.

doc/src/sgml/indices.sgml

@@ -103,7 +103,9 @@ CREATE INDEX test1_id_index ON test1 (id);
 
   <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>
@@ -749,7 +751,7 @@ CREATE INDEX people_names ON people ((first_name || ' ' || last_name));
   <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>

doc/src/sgml/monitoring.sgml

@@ -4426,7 +4426,7 @@ SELECT pid, wait_event_type, wait_event FROM pg_stat_activity WHERE wait_event i
        <structfield>n_tup_upd</structfield> <type>bigint</type>
       </para>
       <para>
-       Number of rows updated (includes HOT updated rows)
+       Number of rows updated (includes <link linkend="storage-hot">HOT updated rows</link>)
       </para></entry>
      </row>
 

doc/src/sgml/ref/create_table.sgml

@@ -1435,7 +1435,9 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
       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.

doc/src/sgml/storage.sgml

@@ -1075,4 +1075,74 @@ data. Empty in ordinary tables.</entry>
  </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="monitoring-pg-stat-all-tables-view">pg_stat_all_tables</link>
+  allows monitoring of the occurrence of HOT and non-HOT updates.
+ </para>
+</sect1>
+
 </chapter>