F.37. pg_walinspect
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## F.37. 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 or analytical or 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.37.1. General Functions

`pg_get_wal_record_info(in_lsn pg_lsn, start_lsn OUT pg_lsn, end_lsn OUT pg_lsn, prev_lsn OUT pg_lsn, xid OUT xid, resource_manager OUT text, record_type OUT text, record_length OUT int4, main_data_length OUT int4, fpi_length OUT int4, description OUT text, block_ref OUT text)`

  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.

`pg_get_wal_records_info(start_lsn pg_lsn, end_lsn pg_lsn, start_lsn OUT pg_lsn, end_lsn OUT pg_lsn, prev_lsn OUT pg_lsn, xid OUT xid, resource_manager OUT text, record_type OUT text, record_length OUT int4, main_data_length OUT int4, fpi_length OUT int4, description OUT text, block_ref OUT text) 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, usage of the function is as follows:

      postgres=# select start_lsn, end_lsn, prev_lsn, xid, resource_manager, record_type, record_length, main_data_length, fpi_length, description from pg_get_wal_records_info('0/14F9A30', '0/15011D7');
       start_lsn |  end_lsn  | prev_lsn  | xid | resource_manager | record_type  | record_length | main_data_length | fpi_length |     description
      -----------+-----------+-----------+-----+------------------+--------------+---------------+------------------+------------+---------------------
       0/14FA118 | 0/14FB4B0 | 0/14F9958 | 725 | Btree            | INSERT_LEAF  |          5013 |                2 |       4960 | off 246
       0/14FB4B0 | 0/14FD050 | 0/14FA118 | 725 | Btree            | INSERT_LEAF  |          7045 |                2 |       6992 | off 130
       0/14FD050 | 0/14FD0A8 | 0/14FB4B0 | 725 | Heap2            | MULTI_INSERT |            85 |                6 |          0 | 1 tuples flags 0x02
       0/14FD0A8 | 0/14FD0F0 | 0/14FD050 | 725 | Btree            | INSERT_LEAF  |            72 |                2 |          0 | off 155
       0/14FD0F0 | 0/14FD138 | 0/14FD0A8 | 725 | Btree            | INSERT_LEAF  |            72 |                2 |          0 | off 134
       0/14FD138 | 0/14FD210 | 0/14FD0F0 | 725 | Heap             | INSERT       |           211 |                3 |          0 | off 11 flags 0x00
       0/14FD210 | 0/14FD250 | 0/14FD138 | 725 | Btree            | INSERT_LEAF  |            64 |                2 |          0 | off 246
       0/14FD250 | 0/14FF260 | 0/14FD210 | 725 | Btree            | INSERT_LEAF  |          8181 |                2 |       8128 | off 47
       0/14FF260 | 0/14FF2B8 | 0/14FD250 | 725 | Heap2            | MULTI_INSERT |            85 |                6 |          0 | 1 tuples flags 0x02
       0/14FF2B8 | 0/14FF300 | 0/14FF260 | 725 | Btree            | INSERT_LEAF  |            72 |                2 |          0 | off 155
       0/14FF300 | 0/15008D8 | 0/14FF2B8 | 725 | Btree            | INSERT_LEAF  |          5565 |                2 |       5512 | off 106
       0/15008D8 | 0/1500C48 | 0/14FF300 | 725 | Heap             | INSERT       |           874 |                3 |        820 | off 2 flags 0x01
      (12 rows)

`pg_get_wal_records_info_till_end_of_wal(start_lsn pg_lsn, start_lsn OUT pg_lsn, end_lsn OUT pg_lsn, prev_lsn OUT pg_lsn, xid OUT xid, resource_manager OUT text, record_type OUT text, record_length OUT int4, main_data_length OUT int4, fpi_length OUT int4, description OUT text, block_ref OUT text) returns setof record`

  This function is 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, "resource_manager/record_type" OUT text, count OUT int8, count_percentage OUT float8, record_length OUT int8, record_length_percentage OUT float8, fpi_length OUT int8, fpi_length_percentage OUT float8, combined_size OUT int8, combined_size_percentage OUT float8) 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, usage of the function is as follows:

      postgres=# select * from pg_get_wal_stats('0/12FBA30', '0/15011D7') where count > 0;
       resource_manager/record_type | count | count_percentage | record_size | record_size_percentage | fpi_size | fpi_size_percentage | combined_size | combined_size_percentage
      ------------------------------+-------+------------------+-------------+------------------------+----------+---------------------+---------------+--------------------------
       XLOG                         |    10 |       0.10871929 |         796 |            0.052369177 |      352 |         0.061031006 |          1148 |              0.054751817
       Transaction                  |   187 |        2.0330508 |       62773 |              4.1298623 |        0 |                   0 |         62773 |                2.9938467
       Storage                      |    13 |       0.14133507 |         546 |            0.035921574 |        0 |                   0 |           546 |                0.0260405
       Database                     |     2 |      0.021743858 |          84 |            0.005526396 |        0 |                   0 |            84 |             0.0040062307
       Standby                      |   218 |        2.3700805 |       15908 |              1.0465941 |        0 |                   0 |         15908 |               0.75870377
       Heap2                        |  1897 |        20.624048 |      383916 |              25.257998 |   364472 |           63.193447 |        748388 |                35.693035
       Heap                         |  1318 |        14.329202 |      621390 |               40.88151 |   139660 |           24.214746 |        761050 |                 36.29693
       Btree                        |  5553 |         60.37182 |      434565 |              28.590216 |    72272 |           12.530776 |        506837 |                 24.17269
      (8 rows)

  With `per_record` passed as `true`:

      postgres=# select * from pg_get_wal_stats('0/14AFC30', '0/15011D7', true) where count > 0;
       resource_manager/record_type | count | count_percentage | record_size | record_size_percentage | fpi_size | fpi_size_percentage | combined_size | combined_size_percentage
      ------------------------------+-------+------------------+-------------+------------------------+----------+---------------------+---------------+--------------------------
       XLOG/CHECKPOINT_SHUTDOWN     |     1 |       0.32894737 |         114 |             0.22891566 |        0 |                   0 |           114 |               0.03534489
       XLOG/CHECKPOINT_ONLINE       |     4 |        1.3157895 |         456 |             0.91566265 |        0 |                   0 |           456 |               0.14137957
       XLOG/NEXTOID                 |     1 |       0.32894737 |          30 |            0.060240965 |        0 |                   0 |            30 |              0.009301287
       Transaction/COMMIT           |     9 |        2.9605262 |        1173 |              2.3554218 |        0 |                   0 |          1173 |               0.36368033
       Storage/CREATE               |     1 |       0.32894737 |          42 |            0.084337346 |        0 |                   0 |            42 |             0.0130218025
       Database/CREATE_FILE_COPY    |     2 |       0.65789473 |          84 |             0.16867469 |        0 |                   0 |            84 |              0.026043605
       Standby/RUNNING_XACTS        |     6 |        1.9736842 |         316 |              0.6345382 |        0 |                   0 |           316 |               0.09797356
       Standby/INVALIDATIONS        |    45 |        14.802631 |        4018 |               8.068274 |        0 |                   0 |          4018 |                1.2457525
       Heap2/PRUNE                  |     4 |        1.3157895 |         270 |              0.5421687 |        0 |                   0 |           270 |               0.08371159
       Heap2/FREEZE_PAGE            |    27 |         8.881579 |       20904 |              41.975903 |        0 |                   0 |         20904 |                 6.481137
       Heap2/VISIBLE                |    29 |         9.539474 |        1756 |              3.5261045 |    73728 |           27.032736 |         75484 |                23.403278
       Heap2/MULTI_INSERT           |    13 |        4.2763157 |        1049 |              2.1064258 |    12216 |            4.479057 |         13265 |                 4.112719
       Heap/INSERT                  |    19 |             6.25 |        2324 |              4.6666665 |    43884 |           16.090284 |         46208 |                14.326463
       Heap/UPDATE                  |     7 |        2.3026316 |         511 |              1.0261045 |    54340 |           19.924028 |         54851 |                17.006165
       Heap/HOT_UPDATE              |    11 |         3.618421 |        1134 |              2.2771084 |      468 |          0.17159452 |          1602 |               0.49668875
       Heap/LOCK                    |     8 |         2.631579 |         432 |              0.8674699 |        0 |                   0 |           432 |               0.13393854
       Heap/INPLACE                 |    45 |        14.802631 |        9123 |              18.319277 |    16076 |            5.894345 |         25199 |                7.8127713
       Heap/UPDATE+INIT             |     1 |       0.32894737 |         817 |              1.6405623 |        0 |                   0 |           817 |               0.25330505
       Btree/INSERT_LEAF            |    70 |        23.026316 |        5183 |              10.407631 |    72024 |           26.407955 |         77207 |                23.937483
       Btree/DEDUP                  |     1 |       0.32894737 |          64 |             0.12851405 |        0 |                   0 |            64 |              0.019842746
      (20 rows)

`pg_get_wal_stats_till_end_of_wal(start_lsn pg_lsn, per_record boolean DEFAULT false, "resource_manager/record_type" OUT text, count OUT int8, count_percentage OUT float8, record_length OUT int8, record_length_percentage OUT float8, fpi_length OUT int8, fpi_length_percentage OUT float8, combined_size OUT int8, combined_size_percentage OUT float8) returns setof record`

  This function is same as `pg_get_wal_stats()` except that it gets statistics of all the valid WAL records from `start_lsn` till end of WAL.

### F.37.2. Author

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

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