From: | Robert Haas <robertmhaas(at)gmail(dot)com> |
---|---|
To: | Alvaro Herrera <alvherre(at)commandprompt(dot)com> |
Cc: | Andres Freund <andres(at)anarazel(dot)de>, pgsql-hackers(at)postgresql(dot)org, Albe Laurenz <laurenz(dot)albe(at)wien(dot)gv(dot)at>, "Kevin Grittner *EXTERN*" <Kevin(dot)Grittner(at)wicourts(dot)gov> |
Subject: | Re: Update on true serializable techniques in MVCC |
Date: | 2009-12-16 18:27:08 |
Message-ID: | 603c8f070912161027k6674a397n44ca9ceda03d402a@mail.gmail.com |
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Lists: | pgsql-hackers |
On Wed, Dec 16, 2009 at 1:25 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> On Wed, Dec 16, 2009 at 1:14 PM, Alvaro Herrera
> <alvherre(at)commandprompt(dot)com> wrote:
>> Robert Haas escribió:
>>> On Wed, Dec 16, 2009 at 10:29 AM, Andres Freund <andres(at)anarazel(dot)de> wrote:
>>> > On Wednesday 16 December 2009 16:24:42 Robert Haas wrote:
>>> >> > Inserts and deletes follow the same protocol, obtaining an exclusive
>>> >> > lock on the row after the one being inserted or deleted. The result
>>> >> > of this locking protocol is that a range scan prevents concurrent
>>> >> > inserts or delete within the range of the scan, and vice versa.
>>> >> >
>>> >> > That sounds like it should actually work.
>>> >>
>>> >> Only if you can guarantee that the database will access the rows using
>>> >> some particular index. If it gets to the data some other way it might
>>> >> accidentally circumvent the lock. That's kind of a killer in terms of
>>> >> making this work for PostgreSQL.
>>> > Isnt the whole topic only relevant for writing access? There you have to
>>> > access the index anyway.
>>>
>>> Yeah, I guess you have to insert the new tuple. I guess while you
>>> were at it you might check whether the next tuple is locked...
>>
>> So you'd have to disable HOT updates when true serializability was
>> active?
>
> I thought about that, but I don't think so. HOT only applies to
> updates, and predicate locking only applies to inserts. Unless I have
> my head in the sand?
Err, no, wait. Predicate locking can apply to updates, but since HOT
updates never update an indexed column, I think we might still be OK?
...Robert
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