On Thu, Jul 21, 2022 at 10:02:18PM -0400, Bruce Momjian wrote:
> On Sun, Feb 6, 2022 at 09:29:56PM -0500, Jonathan Katz wrote:
> > I agree with Bruce's point that we should have a new section (or
> > subsection). As I mentioned in my previous post, given HOT involves
> > indexing, I would suggest putting it there.
> >
> > I think that something that follows the general outline of Laurenz's post
> > would satisfy the user requirements. It explains at a high level what HOT
> > is, it's advantages, and how it works.
>
> Attached is a patch that adds a new HOT section to the storage chapter.
I wasn't happy with the last paragraph so I added some more details.
URL contents updated too:
https://momjian.us/tmp/pgsql/storage-hot.html
--
Bruce Momjian <br...@momjian.us> https://momjian.us
EDB https://enterprisedb.com
Indecision is a decision. Inaction is an action. Mark Batterson
diff --git a/doc/src/sgml/acronyms.sgml b/doc/src/sgml/acronyms.sgml
index 9ed148ab84..2df6559acc 100644
--- a/doc/src/sgml/acronyms.sgml
+++ b/doc/src/sgml/acronyms.sgml
@@ -299,9 +299,7 @@
<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>
diff --git a/doc/src/sgml/btree.sgml b/doc/src/sgml/btree.sgml
index a9200ee52e..6f608a14bf 100644
--- a/doc/src/sgml/btree.sgml
+++ b/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
— one for <emphasis>each and every</emphasis> index on the
diff --git a/doc/src/sgml/catalogs.sgml b/doc/src/sgml/catalogs.sgml
index a186e35f00..248dbc0e26 100644
--- a/doc/src/sgml/catalogs.sgml
+++ b/doc/src/sgml/catalogs.sgml
@@ -4381,7 +4381,7 @@ SCRAM-SHA-256$<replaceable><iteration count></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>
diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index e2d728e0c4..e5a84ed76d 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -4482,7 +4482,8 @@ 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
diff --git a/doc/src/sgml/indexam.sgml b/doc/src/sgml/indexam.sgml
index cf359fa9ff..4f83970c85 100644
--- a/doc/src/sgml/indexam.sgml
+++ b/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.
diff --git a/doc/src/sgml/indices.sgml b/doc/src/sgml/indices.sgml
index 023157d888..42e1e86c8a 100644
--- a/doc/src/sgml/indices.sgml
+++ b/doc/src/sgml/indices.sgml
@@ -749,7 +749,7 @@ 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>
diff --git a/doc/src/sgml/monitoring.sgml b/doc/src/sgml/monitoring.sgml
index 7dbbab6f5c..6408d28c5d 100644
--- a/doc/src/sgml/monitoring.sgml
+++ b/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>
diff --git a/doc/src/sgml/storage.sgml b/doc/src/sgml/storage.sgml
index f4b9f66589..1b947beafe 100644
--- a/doc/src/sgml/storage.sgml
+++ b/doc/src/sgml/storage.sgml
@@ -1075,4 +1075,67 @@ 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 cause additional rows to be added to tables.
+ This can also require new index entries for each updated row, and
+ removal of old versions of rows can be expensive.
+ </para>
+
+ <para>
+ Fortunately, there is an automatic system called heap-only tuples
+ (<acronym>HOT</acronym>) which minimizes the downsides of
+ <productname>PostgreSQL</productname> updates. This optimization is
+ possible when:
+
+ <itemizedlist>
+ <listitem>
+ <para>
+ The table's indexed columns are not modified by the update.
+ </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 the updated row.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ Old versions of the updated row can be removed during normal
+ operation, including <command>SELECT</command>s, instead of requiring
+ periodic vacuum operations.
+ </para>
+ </listitem>
+ </itemizedlist>
+ </para>
+
+ <para>
+ In summary, heap-only tuple updates can only happen if indexed
+ columns are not updated. You can increase the change of
+ <acronym>HOT</acronym> updates by using non-default table <link
+ linkend="sql-createtable"><literal>fillfactor</literal></link> settings.
+ 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.
+ </para>
+</sect1>
+
</chapter>
