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pageinspect: Add page_checksum function
Author: Tomas Vondra <tomas.vondra@2ndquadrant.com> Reviewed-by: Ashutosh Sharma <ashu.coek88@gmail.com>
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‎contrib/pageinspect/expected/page.out

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@@ -49,6 +49,12 @@ SELECT pagesize, version FROM page_header(get_raw_page('test1', 0));
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8192 | 4
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(1 row)
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SELECT page_checksum(get_raw_page('test1', 0), 0) IS NOT NULL AS silly_checksum_test;
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silly_checksum_test
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---------------------
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t
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(1 row)
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SELECT tuple_data_split('test1'::regclass, t_data, t_infomask, t_infomask2, t_bits)
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FROM heap_page_items(get_raw_page('test1', 0));
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tuple_data_split

‎contrib/pageinspect/pageinspect--1.5--1.6.sql

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@@ -75,3 +75,11 @@ CREATE FUNCTION hash_metapage_info(IN page bytea,
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OUT mapp int8[])
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AS 'MODULE_PATHNAME', 'hash_metapage_info'
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LANGUAGE C STRICT PARALLEL SAFE;
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--
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-- page_checksum()
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--
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CREATE FUNCTION page_checksum(IN page bytea, IN blkno int4)
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RETURNS smallint
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AS 'MODULE_PATHNAME', 'page_checksum'
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LANGUAGE C STRICT PARALLEL SAFE;

‎contrib/pageinspect/rawpage.c

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@@ -24,6 +24,7 @@
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#include "funcapi.h"
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#include "miscadmin.h"
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#include "storage/bufmgr.h"
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#include "storage/checksum.h"
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#include "utils/builtins.h"
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#include "utils/pg_lsn.h"
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#include "utils/rel.h"
@@ -280,3 +281,39 @@ page_header(PG_FUNCTION_ARGS)
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PG_RETURN_DATUM(result);
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}
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/*
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* page_checksum
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*
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* Compute checksum of a raw page
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*/
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PG_FUNCTION_INFO_V1(page_checksum);
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Datum
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page_checksum(PG_FUNCTION_ARGS)
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{
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bytea *raw_page = PG_GETARG_BYTEA_P(0);
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uint32 blkno = PG_GETARG_INT32(1);
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int raw_page_size;
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PageHeader page;
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if (!superuser())
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ereport(ERROR,
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(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
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(errmsg("must be superuser to use raw page functions"))));
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raw_page_size = VARSIZE(raw_page) - VARHDRSZ;
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/*
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* Check that the supplied page is of the right size.
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*/
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if (raw_page_size != BLCKSZ)
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ereport(ERROR,
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(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
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errmsg("incorrect size of input page (%d bytes)", raw_page_size)));
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page = (PageHeader) VARDATA(raw_page);
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PG_RETURN_INT16(pg_checksum_page((char *)page, blkno));
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}

‎contrib/pageinspect/sql/page.sql

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@@ -24,6 +24,8 @@ SELECT get_raw_page('test1', 0) = get_raw_page('test1', 'main', 0);
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SELECT pagesize, version FROM page_header(get_raw_page('test1', 0));
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SELECT page_checksum(get_raw_page('test1', 0), 0) IS NOT NULL AS silly_checksum_test;
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SELECT tuple_data_split('test1'::regclass, t_data, t_infomask, t_infomask2, t_bits)
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FROM heap_page_items(get_raw_page('test1', 0));
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‎doc/src/sgml/pageinspect.sgml

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