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 current server's timeline ID.
All the functions of this module will try to find the first valid WAL record
that is at or after the given in_lsn or
start_lsn and will emit error if no such record
is available. Similarly, the end_lsn must be
available, and if it falls in the middle of a record, the entire record must
be available.
Some functions, such as pg_logical_emit_message,
return the LSN after the record just
inserted. Therefore, if you pass that LSN as
in_lsn or start_lsn
to one of these functions, it will return the next
record.
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 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:
postgres=# SELECT * FROM pg_get_wal_record_info('0/1E826E98');
-[ RECORD 1 ]----+----------------------------------------------------
start_lsn | 0/1E826F20
end_lsn | 0/1E826F60
prev_lsn | 0/1E826C80
xid | 0
resource_manager | Heap2
record_type | PRUNE
record_length | 58
main_data_length | 8
fpi_length | 0
description | snapshotConflictHorizon 33748 nredirected 0 ndead 2
block_ref | blkref #0: rel 1663/5/60221 fork main blk 2
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. If start_lsn
or end_lsn are not yet available, the
function will raise an error. 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 |
pg_get_wal_records_info_till_end_of_wal(start_lsn pg_lsn)
returns setof record
This function is the same as pg_get_wal_records_info(),
except that it gets information of all the valid WAL records from
start_lsn till the end of WAL.
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.
If start_lsn
or end_lsn are not yet available, the
function will raise an error. 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
pg_get_wal_stats_till_end_of_wal(start_lsn pg_lsn, per_record boolean DEFAULT false)
returns setof record
This function is the same as pg_get_wal_stats(),
except that it gets statistics of all the valid WAL records from
start_lsn till end of WAL.
Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>