What are the advantages and drawbacks of multiversion timestamp ordering in comparison with ordinary timestamp ordering?

Question

What will be an ideal response?

Answer 1

The algorithm allows more concurrency than single version timestamp ordering but incurs additional storage
costs.
Advantages:
The presence of multiple committed versions allows late read operations to succeed.
write operations are allowed to proceed immediately unless they will invalidate earlier reads (a write by a
transaction with timestamp Ti is rejected if a transaction with timestamp Tj has read a data item with write
timestamp Tk and Tk < Ti < Tj).
Drawbacks:
The algorithm requires storage for multiple versions of each committed objects and for information about the
read and write timestamps of each version to be used in carrying out the read and write rules. In the case that
a version is deleted, read operations will have to be rejected and transactions aborted.
Exercise 13.22shows that the algorithm can provide yet more concurrency, at the risk of cascading aborts, by
allowing read operations to proceed immediately. In this case, to ensure recoverability, requests to commit
must be delayed until any the completion (commitment or abortion) of any transaction whose tentative objects
have been observed.