F.38. pg_walinspect — low-level WAL inspection
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## F.38. pg_walinspect — low-level WAL inspection #

The `pg_walinspect` module provides SQL functions that allow you to inspect the contents of write-ahead log of a running PostgreSQL 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.38.1. 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 9.97) 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.38.2. Author #

Bharath Rupireddy `<bharath.rupireddyforpostgres@gmail.com>`

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