PostgreSQL DBA (17)-XLOG Record data internal structure

XLOG Record data for DML

The above sections are described one by one, and the relevant data is viewed by using the hexdump tool.

1 、 XLogRecordBlockHeader

Uint8 id

[xdb@localhost pg_wal] $hexdump-C 000000010000000100000042-s 80-n 10000005000 |. | 00000051

The block reference ID is 0x00, that is, Block 0.

Uint8 fork_flags

[xdb@localhost pg_wal] $hexdump-C 000000010000000100000042-s 81-n 100000051 20 | 00000052

The value is 0x20, and the highest 4 bits are used for tagging, namely BKPBLOCK_HAS_DATA

Uint16 data_length

[xdb@localhost pg_wal] $hexdump-C 000000010000000100000042-s 82-n 200000052 1e00 |.. | 00000054

Payload bytes = 0x001E, the decimal value is 30.

And then there's RelFileNode.

RelFileNode

Tablespace/database/relation, all of which are Oid type (unsigned int)

1.tablespace

[xdb@localhost pg_wal] $hexdump-C 000000010000000100000042-s 84-n 400000054 7f 06 0000 | | 00000058

Value is 0x0000067F, decimal value is 1663

Tablespace is default

Testdb=# select * from pg_tablespace where oid=1663; spcname | spcowner | spcacl | spcoptions-+-pg_default | 10 | (1 row)

2.database

[xdb@localhost pg_wal] $hexdump-C 000000010000000100000042-s 88-n 400000058 12 400000 |. @.. | 0000005c

Value is 0x00004012, decimal value is 16402, and database is testdb

Testdb=# select * from pg_database where oid=16402 Datname | datdba | encoding | datcollate | datctype | datistemplate | datallowconn | datconnlimit | datlastsysoid | datfrozenxid | datminmxid | dattablespace | datacl-+- -+-testdb | 10 | 6 | C | C | f | t |-1 | 13284 | 13284 | 1 | 1663 | (1 row)

3.relation

[xdb@localhost pg_wal] $hexdump-C 000000010000000100000042-s 92-n 40000005c 56 42 0000 | VB.. | 00000060

Value is 0x00004256, decimal value is 16982

Testdb=# select oid,relfilenode,relname from pg_class where relfilenode = 16982; oid | relfilenode | relname-+-16982 | 16982 | t_jfxx (1 row)

The corresponding relationship is t_jfxx

BlockNumber

[xdb@localhost pg_wal] $hexdump-C 000000010000000100000042-s 96-n 400000060 85 000000 |. | 00000064

The value is 0x00000085, and the decimal value is 133, which is the corresponding block number.

2 、 XLogRecordDataHeaderShort

Next is XLogRecordDataHeaderShort/Long, because the data is less than 256B, using the XLogRecordDataHeaderShort structure

Unit8 id

[xdb@localhost pg_wal] $hexdump-C 000000010000000100000042-s 100-n 100000064 ff |. | 00000065

The value is 0xFF-- > XLR_BLOCK_ID_DATA_SHORT 255

Uint8 data_length

[xdb@localhost pg_wal] $hexdump-C 000000010000000100000042-s 101-n 10000006503 |. | 00000066

The value of 0x03 is 3 bytes, which refers to the size of main data, and 3 bytes is the size of xl_heap_insert structure.

3 、 block data

XLogRecordDataHeaderShort is followed by block data, which consists of two parts:

1.xl_heap_header

2.Tuple data

Xl_heap_header

1.uint16 t_infomask2

[xdb@localhost pg_wal] $hexdump-C 000000010000000100000042-s 102-n 200000066 03 00 |.. | 00000068

The t _ infomask2 value is 0x03 and the binary value is 00000000 00000011

2.uint16 t_infomask

[xdb@localhost pg_wal] $hexdump-C 000000010000000100000042-s 104-n 200000068 02 08 |.. | 0000006a

The t _ infomask value is 0x0802 and the binary value is 00001000 00000010

3.uint8 t_hoff

[xdb@localhost pg_wal] $hexdump-C 000000010000000100000042-s 106-n 10000006a 18 |. | 0000006b

T _ hoff value (offset) is 0x18, decimal value is 24

Tuple data

The size of XLOG Record is 0x4F, or 79B, minus header data XLogRecord (24B) + XLogRecordBlockHeader (20B) + XLogRecordDataHeaderShort (2B) + xl_heap_header (5B) + main data (3B), leaving 25B

[xdb@localhost pg_wal] $hexdump-C 000000010000000100000042-s 107-n 250000006b 0000d 32 30 39 31 39 0f 32 30 31 33 30 37 0000 | 20919.201307..0000007b 000000000000 03 b3 40 | .@ | 000000844, main data

This is the xl_heap_insert structure.

Uint16 OffsetNumber

[xdb@localhost pg_wal] $hexdump-C 000000010000000100000042-s 132-n 200000084 26 00 | &. | 00000086

The offset of the inserted tuple is 0x0026, and the decimal is 38

Uint8 flags

[xdb@localhost pg_wal] $hexdump-C 000000010000000100000042-s 134-n 10000008600 |. | 00000087

The flag bit is 0x00

II. Reference materials

WAL Internals Of PostgreSQL

PostgreSQL Source Code interpretation (109)-WAL#5 (related data structures)

PostgreSQL DBA (16)-WAL segment file internal structure

Summary on the space occupied by the structure