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doc: improve wal_level docs for the 'minimal' level 

Reported-by: David G. Johnston

Discussion: https://postgr.es/m/CAKFQuwZ24UcfkoyLLSW3PMGQATomOcw1nuYFRuMev-NoOF+mYw@mail.gmail.com

Author: David G. Johnston

Backpatch-through: 14, partial to 13
This commit is contained in:
Bruce Momjian 2022-08-12 10:30:01 -04:00
parent e2a02a30c4
commit 4d807bbc4b
1 changed files with 14 additions and 12 deletions
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26
doc/src/sgml/config.sgml
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@ -2771,9 +2771,10 @@ include_dir 'conf.d'
levels. This parameter can only be set at server start.
</para>
<para>
In <literal>minimal</literal> level, no information is logged for
permanent relations for the remainder of a transaction that creates or
rewrites them. This can make operations much faster (see
The <literal>minimal</literal> level generates the least WAL
volume. It logs no row information for permanent relations
in transactions that create or
rewrite them. This can make operations much faster (see
<xref linkend="populate-pitr"/>). Operations that initiate this
optimization include:
<simplelist>
@ -2785,19 +2786,20 @@ include_dir 'conf.d'
<member><command>REINDEX</command></member>
<member><command>TRUNCATE</command></member>
</simplelist>
But minimal WAL does not contain enough information to reconstruct the
data from a base backup and the WAL logs, so <literal>replica</literal> or
higher must be used to enable WAL archiving
(<xref linkend="guc-archive-mode"/>) and streaming replication.
However, minimal WAL does not contain sufficient information for
point-in-time recovery, so <literal>replica</literal> or
higher must be used to enable continuous archiving
(<xref linkend="guc-archive-mode"/>) and streaming binary replication.
In fact, the server will not even start in this mode if
<varname>max_wal_senders</varname> is non-zero.
Note that changing <varname>wal_level</varname> to
<literal>minimal</literal> makes any base backups taken before
unavailable for archive recovery and standby server, which may
lead to data loss.
<literal>minimal</literal> makes previous base backups unusable
for point-in-time recovery and standby servers.
</para>
<para>
In <literal>logical</literal> level, the same information is logged as
with <literal>replica</literal>, plus information needed to allow
extracting logical change sets from the WAL. Using a level of
with <literal>replica</literal>, plus information needed to
extract logical change sets from the WAL. Using a level of
<literal>logical</literal> will increase the WAL volume, particularly if many
tables are configured for <literal>REPLICA IDENTITY FULL</literal> and
many <command>UPDATE</command> and <command>DELETE</command> statements are