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Re: Serializable Isolation without blocking

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Kevin Grittner <Kevin(dot)Grittner(at)wicourts(dot)gov>
Cc: gsstark(at)mit(dot)edu, nicolas(dot)barbier(at)gmail(dot)com, pgsql-hackers(at)postgresql(dot)org, laurenz(dot)albe(at)wien(dot)gv(dot)at
Subject: Re: Serializable Isolation without blocking
Date: 2009-12-31 17:50:20
Message-ID: 603c8f070912310950h2c6ee6f2g5ac31a5b71ee5e11@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On Thu, Dec 31, 2009 at 12:10 PM, Kevin Grittner
<Kevin(dot)Grittner(at)wicourts(dot)gov> wrote:
> Greg Stark  wrote:
>
>> Hm, this raises the issue that you'll have to figure out what
>> should happen if two different transactions are using different
>> isolation modes. Currently our two isolation modes only control
>> behaviour within your transaction so they co-exist perfectly fine.
>>
>> ISTM you would need to have transactions in read-comitted and
>> snapshot isolation modes recording what sets of records they read
>> in order to be able to guarantee serializable transactions to make
>> any guarantees as well.
>
> No, there is no requirement that serializable transactions serialize
> with weaker modes.  The Cahill thesis addresses this point directly.
> Unless you can point out some flaw in his proofs, this is not an
> issue.
>
>> [criticisms of hypothetical implementation techniques]
>
> There are no such proposals on the table, and the hypothetical
> techniques you mention seem unlikely to be ones I would use.  The one
> and only issue I have on the table at the moment is to create a new
> lock method for SIREAD locks.  I'd welcome any comments on that.  If
> I get to the point of feeling comfortable with that, I'll move
> forward to other issues.

Kevin,

I think I understand why you're trying to break this down into
manageable pieces, but I don't think it's really possible to have this
conversation in isolation.  If your question is "Could it ever be
acceptable to add a new lock mode?" then I answer "yes".  And I am
pretty confident that this will also be the consensus view.  But if
your question is "Does it make sense to handle SIREAD locks as a new
lock mode?" then I answer "I don't know, because I haven't seen the
whole design yet".  You just can't answer a question like this in
isolation.

It seems to me that the hard part of this problem is to describe the
general mechanism by which conflicts will be detected, with specific
references to the types of data structures that will be used to hold
that information.

...Robert

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