Hi all
I have read some code on transaction part.
When the new transaction starts, it record the snapshot of database containing the 
current transaction id,etc. So depending on the snapshot
, the transaction decide which tuple is visible.
But transaction could also be implemented by lock. so I am not sure how 
the transaction is implemented, by MVCC or Lock, or by both?
In my option, when tuple is processed in readonly mode(select), MVCC is enough. but 
when tuple is changed, lock is used. I am not sure
whether the explanation is correct.
Thanks for any comments.
Best regards.

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