From fef2bcdcba0888c95ddf2a7535179c3b9a6a2f0e Mon Sep 17 00:00:00 2001
From: Peter Eisentraut <peter_e@gmx.net>
Date: Fri, 17 Mar 2017 09:49:10 -0400
Subject: pageinspect: Add page_checksum function

Author: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Reviewed-by: Ashutosh Sharma <ashu.coek88@gmail.com>
---
 doc/src/sgml/pageinspect.sgml | 47 +++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 45 insertions(+), 2 deletions(-)

(limited to 'doc/src')

diff --git a/doc/src/sgml/pageinspect.sgml b/doc/src/sgml/pageinspect.sgml
index 5e6712f9cde..9f41bb21eb9 100644
--- a/doc/src/sgml/pageinspect.sgml
+++ b/doc/src/sgml/pageinspect.sgml
@@ -73,12 +73,55 @@
 test=# SELECT * FROM page_header(get_raw_page('pg_class', 0));
     lsn    | checksum | flags  | lower | upper | special | pagesize | version | prune_xid
 -----------+----------+--------+-------+-------+---------+----------+---------+-----------
- 0/24A1B50 |        1 |      1 |   232 |   368 |    8192 |     8192 |       4 |         0
+ 0/24A1B50 |        0 |      1 |   232 |   368 |    8192 |     8192 |       4 |         0
 </screen>
       The returned columns correspond to the fields in the
       <structname>PageHeaderData</> struct.
       See <filename>src/include/storage/bufpage.h</> for details.
-    </para>
+     </para>
+
+     <para>
+      The <structfield>checksum</structfield> field is the checksum stored in
+      the page, which might be incorrect if the page is somehow corrupted.  If
+      data checksums are not enabled for this instance, then the value stored
+      is meaningless.
+     </para>
+    </listitem>
+   </varlistentry>
+
+   <varlistentry>
+    <term>
+     <function>page_checksum(page bytea, blkno int4) returns smallint</function>
+     <indexterm>
+      <primary>page_checksum</primary>
+     </indexterm>
+    </term>
+
+    <listitem>
+     <para>
+      <function>page_checksum</function> computes the checksum for the page, as if
+      it was located at the given block.
+     </para>
+
+     <para>
+      A page image obtained with <function>get_raw_page</function> should be
+      passed as argument.  For example:
+<screen>
+test=# SELECT page_checksum(get_raw_page('pg_class', 0), 0);
+ page_checksum
+---------------
+         13443
+</screen>
+      Note that the checksum depends on the block number, so matching block
+      numbers should be passed (except when doing esoteric debugging).
+     </para>
+
+     <para>
+      The checksum computed with this function can be compared with
+      the <structfield>checksum</structfield> result field of the
+      function <function>page_header</function>.  If data checksums are
+      enabled for this instance, then the two values should be equal.
+     </para>
     </listitem>
    </varlistentry>
 
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