Multi-version Concurrency Control (cont)
Removing old versions:
- Vj and Vk are versions of same item
- WTS(Vj) and WTS(Vk) precede TS(Ti) for all Ti
- remove version with smaller WTS(Vx) value
When to make this check?
- every time a new version of a data item is added?
- periodically, with fast access to blocks of data
PostgreSQL uses the latter (vacuum).
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