The document discusses PostgreSQL database write-ahead logging (WAL) and how it relates to database tables. It explains that WAL segments contain redo logs that are replayed to update table pages in shared memory and synchronize their write positions (LSN). As transactions commit, their redo logs are replayed to bring table pages up to the latest LSN, ensuring data consistency.

1. TABLE_Adatabase cluster
WAL segments
LSN_0
REDO point
shared buffer pool
© Copyright 2015 Suzuki Hironobu All Rights Reserved.
See also http://www.interdb.jp/pg/pgsql09.html

2. TABLE_Adatabase cluster
WAL segments
TABLE_A
LSN_0
LSN_0
(1)
REDO point
shared buffer pool
Load TABLE_A's page
© Copyright 2015 Suzuki Hironobu All Rights Reserved.
See also http://www.interdb.jp/pg/pgsql09.html

3. TABLE_A
A
database cluster
WAL segments
A
TABLE_A
LSN_1
LSN_0
LSN_0 LSN_0
(1) (2)
COMMIT
REDO point
shared buffer pool
Replay the first XLOG record
because TABLE_A's pd_lsn (LSN_0) < XLOG record's LSN(LSN_1)
© Copyright 2015 Suzuki Hironobu All Rights Reserved.
See also http://www.interdb.jp/pg/pgsql09.html

4. TABLE_A
A
database cluster
WAL segments
A
TABLE_A
LSN_1
LSN_0
LSN_0 LSN_0 LSN_1
(1) (2)
COMMIT
REDO point
shared buffer pool
Update TABLE_A's pd_lsn from LSN_0 to LSN_1
© Copyright 2015 Suzuki Hironobu All Rights Reserved.
See also http://www.interdb.jp/pg/pgsql09.html

5. TABLE_A
A B A
database cluster
WAL segments
A B
TABLE_A
LSN_1 LSN_2
LSN_0
LSN_0 LSN_0 LSN_1 LSN_1 LSN_2
(1) (2) (3)
COMMITCOMMIT
REDO point
shared buffer pool
Replay the second XLOG record
And Update TABLE_A's pd_lsn
© Copyright 2015 Suzuki Hironobu All Rights Reserved.
See also http://www.interdb.jp/pg/pgsql09.html