database - What would you log in a write-ahead log? -

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what dbmss implement multi-version timestamp ordering concurrency control include in write-ahead logs ? before , after images, or 1 of them ? timestamps ? else ?

documentation of postgres wal. postgres uses mvto type of mvcc. innodb uses mvrc.

here postgres log structure , pg_control structure, important recovery. timestamps not used not reliable, rather use monotonically increasing integer counter (transaction id).

so rollback related data stored in main data itself, not in wal.

main purpose of wal recover data incase of problems due power failure, os problems or hardware failure (obviously except serious disk failures). wal should pretty independent of that.

innodb log structure in innodb/include/log0log.h .


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