doc: Clarify CHECKPOINT handling of unlogged buffers

The CHECKPOINT reference page still described checkpoints as flushing
all data files, which could be misleading as it depends on the value
of FLUSH_UNLOGGED option.  Update the description to make it clearer
that only data files of permanent relations are flushed by default.

Author: Chao Li <lic@highgo.com>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: https://postgr.es/m/4855807D-F1CA-44E6-9B58-406691832848@gmail.com

diff --git a/doc/src/sgml/ref/checkpoint.sgml b/doc/src/sgml/ref/checkpoint.sgml
index cd981cf2cab9fabad80129c28c92ff777a18aca9..08dbe62c612f09a07a6160da4bd9f751ad5b2acd 100644 (file)
--- a/doc/src/sgml/ref/checkpoint.sgml
+++ b/doc/src/sgml/ref/checkpoint.sgml
@@ -35,8 +35,10 @@ CHECKPOINT [ ( option [, ...] ) ]
 
   <para>
    A checkpoint is a point in the write-ahead log sequence at which
-   all data files have been updated to reflect the information in the
-   log.  All data files will be flushed to disk.  Refer to
+   all data files for permanent relations have been updated to reflect the
+   information in the log.  All data for permanent relations files will be
+   flushed to disk, dirty buffers of unlogged relations are not flushed unless
+   <literal>FLUSH_UNLOGGED</literal> is specified.  Refer to
    <xref linkend="wal-configuration"/> for more details about what happens
    during a checkpoint.
   </para>