Re: In-place updates and serializable transactions

On Wed, Nov 14, 2018 at 5:13 PM Joshua Yanovski
<joshua.yanovski@gmail.com> wrote:
>
> This is only a personal anecdote, but from my own experience with serializability, this sort of blind update isn't
oftencontended in realistic workloads.  The reason is that (again, IME), most blind writes are either insertions, or
"read-writesin disguise" (the client read an old value in a different transaction); in the latter case, the data in
questionare often logically "owned" by the client, and will therefore rarely be contended.  I think there are two major
exceptionsto this: transactions that perform certain kinds of monotonic updates (for instance, marking a row complete
ina worklist irrespective of whether it was already completed), and automatic bulk updates.  However, these were
exactlythe classes of transactions that we already ran under a lower isolation level than serializability, since they
havetightly constrained shapes and don't benefit much from the additional guarantees. 
>
> So, if this only affects transactions with blind updates, I doubt it will cause much pain in real workloads (even
thoughit might look bad in benchmarks which include a mix of blind writes and rmw operations).  Particularly if it only
happensif you explicitly opt into zheap storage. 
>
Thanks Joshua for sharing your input on this. I'm not aware of any
realistic workloads for serializable transactions. So, it is really
helpful.


--
Thanks & Regards,
Kuntal Ghosh
EnterpriseDB: http://www.enterprisedb.com