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

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Reported-by: Jonathan S. Katz

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

Backpatch-through: 11
This commit is contained in:
Bruce Momjian 2022-08-12 15:05:13 -04:00
parent 50e088d6f2
commit 115464bb5b
9 changed files with 86 additions and 11 deletions
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4
doc/src/sgml/acronyms.sgml
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@ -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>

3
doc/src/sgml/btree.sgml
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@ -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

2
doc/src/sgml/catalogs.sgml
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@ -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>

3
doc/src/sgml/config.sgml
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@ -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

3
doc/src/sgml/indexam.sgml
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@ -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.

6
doc/src/sgml/indices.sgml
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@ -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>

2
doc/src/sgml/monitoring.sgml
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@ -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>

4
doc/src/sgml/ref/create_table.sgml
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@ -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.

70
doc/src/sgml/storage.sgml
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@ -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>