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pageinspect: Add page_checksum function

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Author: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Reviewed-by: Ashutosh Sharma <ashu.coek88@gmail.com>
This commit is contained in:
Peter Eisentraut
2017-03-17 09:49:10 -04:00
parent 64ae420b27
commit fef2bcdcba
5 changed files with 98 additions and 2 deletions
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47
doc/src/sgml/pageinspect.sgml
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@ -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>