287 lines
10 KiB
Plaintext
287 lines
10 KiB
Plaintext
<!-- doc/src/sgml/pgwalinspect.sgml -->
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<sect1 id="pgwalinspect" xreflabel="pg_walinspect">
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<title>pg_walinspect — low-level WAL inspection</title>
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<indexterm zone="pgwalinspect">
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<primary>pg_walinspect</primary>
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</indexterm>
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<para>
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The <filename>pg_walinspect</filename> module provides SQL functions that
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allow you to inspect the contents of write-ahead log of
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a running <productname>PostgreSQL</productname> database cluster at a low
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level, which is useful for debugging, analytical, reporting or
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educational purposes. It is similar to <xref linkend="pgwaldump"/>, but
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accessible through SQL rather than a separate utility.
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</para>
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<para>
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All the functions of this module will provide the WAL information using the
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server's current timeline ID.
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</para>
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<note>
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<para>
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The <filename>pg_walinspect</filename> functions are often called
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using an LSN argument that specifies the location at which a known
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WAL record of interest <emphasis>begins</emphasis>. However, some
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functions, such as
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<function><link linkend="pg-logical-emit-message">pg_logical_emit_message</link></function>,
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return the LSN <emphasis>after</emphasis> the record that was just
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inserted.
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</para>
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</note>
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<tip>
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<para>
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All of the <filename>pg_walinspect</filename> functions that show
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information about records that fall within a certain LSN range are
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permissive about accepting <replaceable>end_lsn</replaceable>
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arguments that are after the server's current LSN. Using an
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<replaceable>end_lsn</replaceable> <quote>from the future</quote>
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will not raise an error.
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</para>
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<para>
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It may be convenient to provide the value
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<literal>FFFFFFFF/FFFFFFFF</literal> (the maximum valid
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<type>pg_lsn</type> value) as an <replaceable>end_lsn</replaceable>
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argument. This is equivalent to providing an
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<replaceable>end_lsn</replaceable> argument matching the server's
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current LSN.
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</para>
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</tip>
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<para>
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By default, use of these functions is restricted to superusers and members of
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the <literal>pg_read_server_files</literal> role. Access may be granted by
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superusers to others using <command>GRANT</command>.
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</para>
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<sect2 id="pgwalinspect-funcs">
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<title>General Functions</title>
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<variablelist>
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<varlistentry id="pgwalinspect-funcs-pg-get-wal-record-info">
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<term>
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<function>pg_get_wal_record_info(in_lsn pg_lsn) returns record</function>
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</term>
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<listitem>
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<para>
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Gets WAL record information about a record that is located at or
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after the <replaceable>in_lsn</replaceable> argument. For
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example:
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<screen>
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postgres=# SELECT * FROM pg_get_wal_record_info('0/E419E28');
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-[ RECORD 1 ]----+-------------------------------------------------
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start_lsn | 0/E419E28
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end_lsn | 0/E419E68
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prev_lsn | 0/E419D78
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xid | 0
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resource_manager | Heap2
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record_type | VACUUM
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record_length | 58
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main_data_length | 2
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fpi_length | 0
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description | nunused: 5, unused: [1, 2, 3, 4, 5]
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block_ref | blkref #0: rel 1663/16385/1249 fork main blk 364
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</screen>
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</para>
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<para>
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If <replaceable>in_lsn</replaceable> isn't at the start of a WAL
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record, information about the next valid WAL record is shown
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instead. If there is no next valid WAL record, the function
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raises an error.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry id="pgwalinspect-funcs-pg-get-wal-records-info">
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<term>
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<function>
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pg_get_wal_records_info(start_lsn pg_lsn, end_lsn pg_lsn)
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returns setof record
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</function>
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</term>
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<listitem>
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<para>
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Gets information of all the valid WAL records between
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<replaceable>start_lsn</replaceable> and <replaceable>end_lsn</replaceable>.
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Returns one row per WAL record. For example:
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<screen>
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postgres=# SELECT * FROM pg_get_wal_records_info('0/1E913618', '0/1E913740') LIMIT 1;
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-[ RECORD 1 ]----+--------------------------------------------------------------
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start_lsn | 0/1E913618
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end_lsn | 0/1E913650
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prev_lsn | 0/1E9135A0
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xid | 0
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resource_manager | Standby
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record_type | RUNNING_XACTS
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record_length | 50
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main_data_length | 24
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fpi_length | 0
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description | nextXid 33775 latestCompletedXid 33774 oldestRunningXid 33775
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block_ref |
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</screen>
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</para>
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<para>
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The function raises an error if
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<replaceable>start_lsn</replaceable> is not available.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry id="pgwalinspect-funcs-pg-get-wal-block-info">
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<term>
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<function>pg_get_wal_block_info(start_lsn pg_lsn, end_lsn pg_lsn, show_data boolean DEFAULT true) returns setof record</function>
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</term>
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<listitem>
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<para>
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Gets information about each block reference from all the valid
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WAL records between <replaceable>start_lsn</replaceable> and
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<replaceable>end_lsn</replaceable> with one or more block
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references. Returns one row per block reference per WAL record.
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For example:
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<screen>
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postgres=# SELECT * FROM pg_get_wal_block_info('0/1230278', '0/12302B8');
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-[ RECORD 1 ]-----+-----------------------------------
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start_lsn | 0/1230278
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end_lsn | 0/12302B8
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prev_lsn | 0/122FD40
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block_id | 0
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reltablespace | 1663
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reldatabase | 1
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relfilenode | 2658
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relforknumber | 0
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relblocknumber | 11
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xid | 341
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resource_manager | Btree
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record_type | INSERT_LEAF
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record_length | 64
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main_data_length | 2
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block_data_length | 16
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block_fpi_length | 0
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block_fpi_info |
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description | off: 46
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block_data | \x00002a00070010402630000070696400
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block_fpi_data |
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</screen>
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</para>
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<para>
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This example involves a WAL record that only contains one block
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reference, but many WAL records contain several block
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references. Rows output by
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<function>pg_get_wal_block_info</function> are guaranteed to
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have a unique combination of
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<replaceable>start_lsn</replaceable> and
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<replaceable>block_id</replaceable> values.
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</para>
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<para>
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Much of the information shown here matches the output that
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<function>pg_get_wal_records_info</function> would show, given
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the same arguments. However,
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<function>pg_get_wal_block_info</function> unnests the
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information from each WAL record into an expanded form by
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outputting one row per block reference, so certain details are
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tracked at the block reference level rather than at the
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whole-record level. This structure is useful with queries that
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track how individual blocks changed over time. Note that
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records with no block references (e.g.,
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<literal>COMMIT</literal> WAL records) will have no rows
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returned, so <function>pg_get_wal_block_info</function> may
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actually return <emphasis>fewer</emphasis> rows than
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<function>pg_get_wal_records_info</function>.
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</para>
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<para>
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The <structfield>reltablespace</structfield>,
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<structfield>reldatabase</structfield>, and
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<structfield>relfilenode</structfield> parameters reference
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<link linkend="catalog-pg-tablespace"><structname>pg_tablespace</structname></link>.<structfield>oid</structfield>,
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<link linkend="catalog-pg-database"><structname>pg_database</structname></link>.<structfield>oid</structfield>, and
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<link linkend="catalog-pg-class"><structname>pg_class</structname></link>.<structfield>relfilenode</structfield>
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respectively. The <structfield>relforknumber</structfield>
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field is the fork number within the relation for the block
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reference; see <filename>common/relpath.h</filename> for
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details.
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</para>
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<tip>
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<para>
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The <function>pg_filenode_relation</function> function (see
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<xref linkend="functions-admin-dblocation"/>) can help you to
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determine which relation was modified during original execution.
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</para>
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</tip>
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<para>
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It is possible for clients to avoid the overhead of
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materializing block data. This may make function execution
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significantly faster. When <replaceable>show_data</replaceable>
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is set to <literal>false</literal>, <structfield>block_data</structfield>
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and <structfield>block_fpi_data</structfield> values are omitted
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(that is, the <structfield>block_data</structfield> and
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<structfield>block_fpi_data</structfield> <literal>OUT</literal>
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arguments are <literal>NULL</literal> for all rows returned).
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Obviously, this optimization is only feasible with queries where
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block data isn't truly required.
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</para>
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<para>
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The function raises an error if
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<replaceable>start_lsn</replaceable> is not available.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry id="pgwalinspect-funcs-pg-get-wal-stats">
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<term>
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<function>
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pg_get_wal_stats(start_lsn pg_lsn, end_lsn pg_lsn, per_record boolean DEFAULT false)
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returns setof record
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</function>
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</term>
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<listitem>
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<para>
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Gets statistics of all the valid WAL records between
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<replaceable>start_lsn</replaceable> and
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<replaceable>end_lsn</replaceable>. By default, it returns one row per
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<replaceable>resource_manager</replaceable> type. When
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<replaceable>per_record</replaceable> is set to <literal>true</literal>,
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it returns one row per <replaceable>record_type</replaceable>.
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For example:
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<screen>
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postgres=# SELECT * FROM pg_get_wal_stats('0/1E847D00', '0/1E84F500')
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WHERE count > 0 AND
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"resource_manager/record_type" = 'Transaction'
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LIMIT 1;
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-[ RECORD 1 ]----------------+-------------------
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resource_manager/record_type | Transaction
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count | 2
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count_percentage | 8
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record_size | 875
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record_size_percentage | 41.23468426013195
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fpi_size | 0
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fpi_size_percentage | 0
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combined_size | 875
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combined_size_percentage | 2.8634072910530795
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</screen>
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</para>
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<para>
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The function raises an error if
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<replaceable>start_lsn</replaceable> is not available.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</sect2>
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<sect2 id="pgwalinspect-author">
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<title>Author</title>
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<para>
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Bharath Rupireddy <email>bharath.rupireddyforpostgres@gmail.com</email>
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</para>
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</sect2>
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</sect1>
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