Postgres Pro Standard : Documentation: 16: F.53. pg_walinspect — low-level WAL inspection : Postgres Professional

F.53. pg_walinspect — low-level WAL inspection
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F.53. pg_walinspect — low-level WAL inspection #

F.53.1. General Functions
F.53.2. Author

The pg_walinspect module provides SQL functions that allow you to inspect the contents of write-ahead log of a running Postgres Pro database cluster at a low level, which is useful for debugging, analytical, reporting or educational purposes. It is similar to pg_waldump, but accessible through SQL rather than a separate utility.

All the functions of this module will provide the WAL information using the server's current timeline ID.

Note

The pg_walinspect functions are often called using an LSN argument that specifies the location at which a known WAL record of interest begins. However, some functions, such as pg_logical_emit_message, return the LSN after the record that was just inserted.

Tip

All of the pg_walinspect functions that show information about records that fall within a certain LSN range are permissive about accepting end_lsn arguments that are after the server's current LSN. Using an end_lsn “from the future” will not raise an error.

It may be convenient to provide the value FFFFFFFF/FFFFFFFF (the maximum valid pg_lsn value) as an end_lsn argument. This is equivalent to providing an end_lsn argument matching the server's current LSN.

By default, use of these functions is restricted to superusers and members of the pg_read_server_files role. Access may be granted by superusers to others using GRANT.

F.53.1. General Functions #

pg_get_wal_record_info(in_lsn pg_lsn) returns record #

Gets WAL record information about a record that is located at or after the in_lsn argument. For example:

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

If in_lsn 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.

pg_get_wal_records_info(start_lsn pg_lsn, end_lsn pg_lsn) returns setof record #

Gets information of all the valid WAL records between start_lsn and end_lsn. Returns one row per WAL record. For example:

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        |

The function raises an error if start_lsn is not available.

pg_get_wal_block_info(start_lsn pg_lsn, end_lsn pg_lsn, show_data boolean DEFAULT true) returns setof record #

Gets information about each block reference from all the valid WAL records between start_lsn and end_lsn with one or more block references. Returns one row per block reference per WAL record. For example:

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    |

This example involves a WAL record that only contains one block reference, but many WAL records contain several block references. Rows output by pg_get_wal_block_info are guaranteed to have a unique combination of start_lsn and block_id values.

Much of the information shown here matches the output that pg_get_wal_records_info would show, given the same arguments. However, pg_get_wal_block_info 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., COMMIT WAL records) will have no rows returned, so pg_get_wal_block_info may actually return fewer rows than pg_get_wal_records_info.

The reltablespace, reldatabase, and relfilenode parameters reference pg_tablespace.oid, pg_database.oid, and pg_class.relfilenode respectively. The relforknumber field is the fork number within the relation for the block reference; see common/relpath.h for details.

Tip

The pg_filenode_relation function (see Table 9.98) can help you to determine which relation was modified during original execution.

It is possible for clients to avoid the overhead of materializing block data. This may make function execution significantly faster. When show_data is set to false, block_data and block_fpi_data values are omitted (that is, the block_data and block_fpi_data OUT arguments are NULL for all rows returned). Obviously, this optimization is only feasible with queries where block data isn't truly required.

The function raises an error if start_lsn is not available.

pg_get_wal_stats(start_lsn pg_lsn, end_lsn pg_lsn, per_record boolean DEFAULT false) returns setof record #

Gets statistics of all the valid WAL records between start_lsn and end_lsn. By default, it returns one row per resource_manager type. When per_record is set to true, it returns one row per record_type. For example:

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

The function raises an error if start_lsn is not available.

F.53.2. Author #

Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>

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