diff options
Diffstat (limited to 'doc/src')
| -rw-r--r-- | doc/src/sgml/pgwalinspect.sgml | 205 |
1 files changed, 133 insertions, 72 deletions
diff --git a/doc/src/sgml/pgwalinspect.sgml b/doc/src/sgml/pgwalinspect.sgml index 9a0241a8d67..300ffac3744 100644 --- a/doc/src/sgml/pgwalinspect.sgml +++ b/doc/src/sgml/pgwalinspect.sgml @@ -18,29 +18,38 @@ <para> All the functions of this module will provide the WAL information using the - current server's timeline ID. - </para> - - <para> - All the functions of this module will try to find the first valid WAL record - that is at or after the given <replaceable>in_lsn</replaceable> or - <replaceable>start_lsn</replaceable> and will emit error if no such record - is available. Similarly, the <replaceable>end_lsn</replaceable> must be - available, and if it falls in the middle of a record, the entire record must - be available. + server's current timeline ID. </para> <note> <para> - 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 just - inserted. Therefore, if you pass that LSN as - <replaceable>in_lsn</replaceable> or <replaceable>start_lsn</replaceable> - to one of these functions, it will return the <emphasis>next</emphasis> - record. + 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 @@ -58,11 +67,9 @@ <listitem> <para> - Gets WAL record information of a given LSN. If the given LSN isn't - at the start of a WAL record, it gives the information of the next - available valid WAL record; or an error if no such record is found. - For example, usage of the function is as - follows: + 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('0/1E826E98'); -[ RECORD 1 ]----+---------------------------------------------------- @@ -79,6 +86,12 @@ description | snapshotConflictHorizon 33748 nredirected 0 ndead 2 block_ref | blkref #0: rel 1663/5/60221 fork main blk 2 </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> @@ -94,11 +107,7 @@ block_ref | blkref #0: rel 1663/5/60221 fork main blk 2 <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. If a future - <replaceable>end_lsn</replaceable> (i.e. ahead of the current LSN of - the server) is specified, it returns information until the end of WAL. - The function raises an error if <replaceable>start_lsn</replaceable> - is not available. For example, usage of the function is as follows: + 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 ]----+-------------------------------------------------------------- @@ -115,6 +124,99 @@ 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> + <term> + <function>pg_get_wal_block_info(start_lsn pg_lsn, end_lsn pg_lsn) 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('0/10E9D80', '0/10E9DC0') LIMIT 1; +-[ RECORD 1 ]-----+----------------------------------- +start_lsn | 0/10E9D80 +end_lsn | 0/10E9DC0 +prev_lsn | 0/10E9860 +block_id | 0 +reltablespace | 1663 +reldatabase | 1 +relfilenode | 2690 +relforknumber | 0 +relblocknumber | 5 +xid | 117 +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 14 +block_data | \x00005400020010001407000000000000 +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 block/relation was modified by each WAL record + during original execution + </para> + </tip> + <para> + The function raises an error if + <replaceable>start_lsn</replaceable> is not available. + </para> </listitem> </varlistentry> @@ -133,11 +235,8 @@ block_ref | <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>. If a - future <replaceable>end_lsn</replaceable> (i.e. ahead of the current - LSN of the server) is specified, it returns statistics until the end - of WAL. An error is raised if <replaceable>start_lsn</replaceable> is - not available. For example, usage of the function is as follows: + 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 LIMIT 1 AND @@ -154,47 +253,9 @@ combined_size | 875 combined_size_percentage | 2.8634072910530795 </screen> </para> - </listitem> - </varlistentry> - - <varlistentry> - <term> - <function>pg_get_wal_block_info(start_lsn pg_lsn, end_lsn pg_lsn) returns setof record</function> - </term> - - <listitem> <para> - Gets a copy of the block information stored in WAL records. This includes - copies of the block data (<literal>NULL</literal> if none) and full page - images as <type>bytea</type> values (after - applying decompression when necessary, or <literal>NULL</literal> if none) - and their information associated with all the valid WAL records between - <replaceable>start_lsn</replaceable> and - <replaceable>end_lsn</replaceable>. Returns one row per block registered - in a WAL record. If a future <replaceable>end_lsn</replaceable> (i.e. - ahead of the current LSN of the server) is specified, it returns - statistics until the end of WAL. An error is raised if - <replaceable>start_lsn</replaceable> is not available. For example, - usage of the function is as follows: -<screen> -postgres=# SELECT lsn, blockid, reltablespace, reldatabase, relfilenode, - relblocknumber, forkname, - substring(blockdata for 24) as block_trimmed, - substring(fpi for 24) as fpi_trimmed, fpilen, fpiinfo - FROM pg_get_wal_block_info('0/1871080', '0/1871440'); --[ RECORD 1 ]--+--------------------------------------------------- -lsn | 0/18712F8 -blockid | 0 -reltablespace | 1663 -reldatabase | 16384 -relfilenode | 16392 -relblocknumber | 0 -forkname | main -block_trimmed | \x02800128180164000000 -fpi_trimmed | \x0000000050108701000000002c00601f00200420e0020000 -fpilen | 204 -fpiinfo | {HAS_HOLE,APPLY} -</screen> + The function raises an error if + <replaceable>start_lsn</replaceable> is not available. </para> </listitem> </varlistentry> |