Supported Versions: Current (15)
Development Versions: devel

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

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