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