F.52. pg_walinspect
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.
Note
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.
F.52.1. General Functions
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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_lsnandend_lsn. Returns one row per WAL record. Ifstart_lsnorend_lsnare 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 fromstart_lsntill 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_lsnandend_lsn. By default, it returns one row perresource_managertype. Whenper_recordis set totrue, it returns one row perrecord_type. Ifstart_lsnorend_lsnare 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 fromstart_lsntill end of WAL.
F.52.2. Author
Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>