Re: Overhead cost of Serializable Snapshot Isolation

From: "Kevin Grittner" <Kevin(dot)Grittner(at)wicourts(dot)gov>
To: "Greg Sabino Mullane" <greg(at)endpoint(dot)com>, <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Overhead cost of Serializable Snapshot Isolation
Date: 2011-10-11 20:52:36
Message-ID: 4E9466440200002500041E28@gw.wicourts.gov
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Greg Sabino Mullane <greg(at)endpoint(dot)com> wrote:

> Kevin Grittner:
>
>> Did these transactions write anything? If not, were they
>> declared to be READ ONLY? If they were, in fact, only reading,
>> it would be interesting to see what the performance looks like if
>> the recommendation to use the READ ONLY attribute is followed.
>
> Yes, I'll definitely look into that, but the great majority of
> the things done in this case are read/write.

But it is precisely *because* those were fully cached read-only
transactions that the numbers came out so bad. As Robert pointed
out, in other loads the difference in time per transaction could be
lost in the noise.

Now, I know SSI won't be good fit for all applications, but you
might not want to write it off on performance grounds for an
application where "the great majority of the things done ... are
read/write" based on a test which ran only read-only transactions
without declaring them READ ONLY.

-Kevin

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