Re: Overhead cost of Serializable Snapshot Isolation

On Tue, Oct 11, 2011 at 9:32 PM, Bruce Momjian <bruce(at)momjian(dot)us> wrote:
> Greg Sabino Mullane wrote:
> -- Start of PGP signed section.
>> > If the normal default_transaction_isolation = read committed and all
>> > transactions that require serializable are explicitly marked in the
>> > application then there is no way to turn off SSI without altering the
>> > application. That is not acceptable, since it causes changes in
>> > application behaviour and possibly also performance issues.
>>
>> Performance, perhaps. What application behavior changes? Less
>> serialization conflicts?

If you change default_transaction_isolation then the behaviour of the
application will change.

>> > We should provide a mechanism to allow people to upgrade to 9.1+
>> > without needing to change the meaning and/or performance of their
>> > apps.
>>
>> That ship has sailed.
>
> Simon seems to value backward-compatibility more than the average
> hackers poster.  The lack of complaints about 9.1 I think means that the
> hackers decision of _not_ providing a swich was the right one.

So its been out 1 month and you think that is sufficient time for us
to decide that there are no user complaints about SSI? I doubt it.
Longer term I have every confidence that it will be appreciated.

I'm keen to ensure people enjoy the possibility of upgrading to the
latest release. The continual need to retest applications mean that
very few users upgrade quickly or with anywhere near the frequency
with which we put out new releases. What is the point of rushing out
software that nobody can use? pg_upgrade doesn't change your
applications, so there isn't a fast path to upgrade in the way you
seem to think.

--
 Simon Riggs                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services