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diff --git a/doc/src/sgml/pgwalinspect.sgml b/doc/src/sgml/pgwalinspect.sgml new file mode 100644 index 0000000..3a8121c --- /dev/null +++ b/doc/src/sgml/pgwalinspect.sgml @@ -0,0 +1,286 @@ +<!-- doc/src/sgml/pgwalinspect.sgml --> + +<sect1 id="pgwalinspect" xreflabel="pg_walinspect"> + <title>pg_walinspect — low-level WAL inspection</title> + + <indexterm zone="pgwalinspect"> + <primary>pg_walinspect</primary> + </indexterm> + + <para> + The <filename>pg_walinspect</filename> module provides SQL functions that + allow you to inspect the contents of write-ahead log of + a running <productname>PostgreSQL</productname> database cluster at a low + level, which is useful for debugging, analytical, reporting or + educational purposes. It is similar to <xref linkend="pgwaldump"/>, but + accessible through SQL rather than a separate utility. + </para> + + <para> + All the functions of this module will provide the WAL information using the + server's current timeline ID. + </para> + + <note> + <para> + The <filename>pg_walinspect</filename> functions are often called + using an LSN argument that specifies the location at which a known + WAL record of interest <emphasis>begins</emphasis>. However, some + functions, such as + <function><link linkend="pg-logical-emit-message">pg_logical_emit_message</link></function>, + return the LSN <emphasis>after</emphasis> the record that was just + inserted. + </para> + </note> + <tip> + <para> + All of the <filename>pg_walinspect</filename> functions that show + information about records that fall within a certain LSN range are + permissive about accepting <replaceable>end_lsn</replaceable> + arguments that are after the server's current LSN. Using an + <replaceable>end_lsn</replaceable> <quote>from the future</quote> + will not raise an error. + </para> + <para> + It may be convenient to provide the value + <literal>FFFFFFFF/FFFFFFFF</literal> (the maximum valid + <type>pg_lsn</type> value) as an <replaceable>end_lsn</replaceable> + argument. This is equivalent to providing an + <replaceable>end_lsn</replaceable> argument matching the server's + current LSN. + </para> + </tip> + <para> + By default, use of these functions is restricted to superusers and members of + the <literal>pg_read_server_files</literal> role. Access may be granted by + superusers to others using <command>GRANT</command>. + </para> + + <sect2 id="pgwalinspect-funcs"> + <title>General Functions</title> + + <variablelist> + <varlistentry id="pgwalinspect-funcs-pg-get-wal-record-info"> + <term> + <function>pg_get_wal_record_info(in_lsn pg_lsn) returns record</function> + </term> + + <listitem> + <para> + Gets WAL record information about a record that is located at or + after the <replaceable>in_lsn</replaceable> argument. For + example: +<screen> +postgres=# SELECT * FROM pg_get_wal_record_info('0/E419E28'); +-[ RECORD 1 ]----+------------------------------------------------- +start_lsn | 0/E419E28 +end_lsn | 0/E419E68 +prev_lsn | 0/E419D78 +xid | 0 +resource_manager | Heap2 +record_type | VACUUM +record_length | 58 +main_data_length | 2 +fpi_length | 0 +description | nunused: 5, unused: [1, 2, 3, 4, 5] +block_ref | blkref #0: rel 1663/16385/1249 fork main blk 364 +</screen> + </para> + <para> + If <replaceable>in_lsn</replaceable> isn't at the start of a WAL + record, information about the next valid WAL record is shown + instead. If there is no next valid WAL record, the function + raises an error. + </para> + </listitem> + </varlistentry> + + <varlistentry id="pgwalinspect-funcs-pg-get-wal-records-info"> + <term> + <function> + pg_get_wal_records_info(start_lsn pg_lsn, end_lsn pg_lsn) + returns setof record + </function> + </term> + + <listitem> + <para> + Gets information of all the valid WAL records between + <replaceable>start_lsn</replaceable> and <replaceable>end_lsn</replaceable>. + Returns one row per WAL record. For example: +<screen> +postgres=# SELECT * FROM pg_get_wal_records_info('0/1E913618', '0/1E913740') LIMIT 1; +-[ RECORD 1 ]----+-------------------------------------------------------------- +start_lsn | 0/1E913618 +end_lsn | 0/1E913650 +prev_lsn | 0/1E9135A0 +xid | 0 +resource_manager | Standby +record_type | RUNNING_XACTS +record_length | 50 +main_data_length | 24 +fpi_length | 0 +description | nextXid 33775 latestCompletedXid 33774 oldestRunningXid 33775 +block_ref | +</screen> + </para> + <para> + The function raises an error if + <replaceable>start_lsn</replaceable> is not available. + </para> + </listitem> + </varlistentry> + + <varlistentry id="pgwalinspect-funcs-pg-get-wal-block-info"> + <term> + <function>pg_get_wal_block_info(start_lsn pg_lsn, end_lsn pg_lsn, show_data boolean DEFAULT true) returns setof record</function> + </term> + + <listitem> + <para> + Gets information about each block reference from all the valid + WAL records between <replaceable>start_lsn</replaceable> and + <replaceable>end_lsn</replaceable> with one or more block + references. Returns one row per block reference per WAL record. + For example: +<screen> +postgres=# SELECT * FROM pg_get_wal_block_info('0/1230278', '0/12302B8'); +-[ RECORD 1 ]-----+----------------------------------- +start_lsn | 0/1230278 +end_lsn | 0/12302B8 +prev_lsn | 0/122FD40 +block_id | 0 +reltablespace | 1663 +reldatabase | 1 +relfilenode | 2658 +relforknumber | 0 +relblocknumber | 11 +xid | 341 +resource_manager | Btree +record_type | INSERT_LEAF +record_length | 64 +main_data_length | 2 +block_data_length | 16 +block_fpi_length | 0 +block_fpi_info | +description | off: 46 +block_data | \x00002a00070010402630000070696400 +block_fpi_data | +</screen> + </para> + <para> + This example involves a WAL record that only contains one block + reference, but many WAL records contain several block + references. Rows output by + <function>pg_get_wal_block_info</function> are guaranteed to + have a unique combination of + <replaceable>start_lsn</replaceable> and + <replaceable>block_id</replaceable> values. + </para> + <para> + Much of the information shown here matches the output that + <function>pg_get_wal_records_info</function> would show, given + the same arguments. However, + <function>pg_get_wal_block_info</function> unnests the + information from each WAL record into an expanded form by + outputting one row per block reference, so certain details are + tracked at the block reference level rather than at the + whole-record level. This structure is useful with queries that + track how individual blocks changed over time. Note that + records with no block references (e.g., + <literal>COMMIT</literal> WAL records) will have no rows + returned, so <function>pg_get_wal_block_info</function> may + actually return <emphasis>fewer</emphasis> rows than + <function>pg_get_wal_records_info</function>. + </para> + <para> + The <structfield>reltablespace</structfield>, + <structfield>reldatabase</structfield>, and + <structfield>relfilenode</structfield> parameters reference + <link linkend="catalog-pg-tablespace"><structname>pg_tablespace</structname></link>.<structfield>oid</structfield>, + <link linkend="catalog-pg-database"><structname>pg_database</structname></link>.<structfield>oid</structfield>, and + <link linkend="catalog-pg-class"><structname>pg_class</structname></link>.<structfield>relfilenode</structfield> + respectively. The <structfield>relforknumber</structfield> + field is the fork number within the relation for the block + reference; see <filename>common/relpath.h</filename> for + details. + </para> + <tip> + <para> + The <function>pg_filenode_relation</function> function (see + <xref linkend="functions-admin-dblocation"/>) can help you to + determine which relation was modified during original execution. + </para> + </tip> + <para> + It is possible for clients to avoid the overhead of + materializing block data. This may make function execution + significantly faster. When <replaceable>show_data</replaceable> + is set to <literal>false</literal>, <structfield>block_data</structfield> + and <structfield>block_fpi_data</structfield> values are omitted + (that is, the <structfield>block_data</structfield> and + <structfield>block_fpi_data</structfield> <literal>OUT</literal> + arguments are <literal>NULL</literal> for all rows returned). + Obviously, this optimization is only feasible with queries where + block data isn't truly required. + </para> + <para> + The function raises an error if + <replaceable>start_lsn</replaceable> is not available. + </para> + </listitem> + </varlistentry> + + <varlistentry id="pgwalinspect-funcs-pg-get-wal-stats"> + <term> + <function> + pg_get_wal_stats(start_lsn pg_lsn, end_lsn pg_lsn, per_record boolean DEFAULT false) + returns setof record + </function> + </term> + + <listitem> + <para> + Gets statistics of all the valid WAL records between + <replaceable>start_lsn</replaceable> and + <replaceable>end_lsn</replaceable>. By default, it returns one row per + <replaceable>resource_manager</replaceable> type. When + <replaceable>per_record</replaceable> is set to <literal>true</literal>, + it returns one row per <replaceable>record_type</replaceable>. + For example: +<screen> +postgres=# SELECT * FROM pg_get_wal_stats('0/1E847D00', '0/1E84F500') + WHERE count > 0 AND + "resource_manager/record_type" = 'Transaction' + LIMIT 1; +-[ RECORD 1 ]----------------+------------------- +resource_manager/record_type | Transaction +count | 2 +count_percentage | 8 +record_size | 875 +record_size_percentage | 41.23468426013195 +fpi_size | 0 +fpi_size_percentage | 0 +combined_size | 875 +combined_size_percentage | 2.8634072910530795 +</screen> + </para> + <para> + The function raises an error if + <replaceable>start_lsn</replaceable> is not available. + </para> + </listitem> + </varlistentry> + + </variablelist> + </sect2> + + <sect2 id="pgwalinspect-author"> + <title>Author</title> + + <para> + Bharath Rupireddy <email>bharath.rupireddyforpostgres@gmail.com</email> + </para> + </sect2> + +</sect1> |