Re: Theoretical Basis for SELECT FOR UPDATE

On 30 Sep 2005 08:32:12 -0700, "vc" <boston103_at_hotmail.com> wrote:

>Tony Andrews wrote:

[snip]

>> It means that after any executable statement that changes the data, the
>> data is is a valid state - i.e. each executable statement IS a
>> transaction. The TTM therefore requires it to be possible to combine
>> multiple operations into a single statement. i.e. instead of:
>>
>> update account set bal=bal+10 where ac_no=123;
>> update account set bal=bal-10 where ac_no=456;
>> commit;
>>
>> TTM would have something more like:
>>
>> update account set bal=bal+10 where ac_no=123,
>> update account set bal=bal-10 where ac_no=456;
>>
>> (Note the comma rather than semi-colon on line 1).

>But that's just a syntactical difference because S1;S2;commit; is
>another way of saying S,S; (or vice versa).

     No, it is not. Imagine a statement in between the two updates that, say, prints data from the transaction in progress. Then, imagine the transaction being rolled back. There is a potential for loss of consistency.

>> The important difference is that in the TTM version there is no time
>> when even this transaction can see data that is in an inconsistent
>> state.

>Time is immaterial. How do two atomic writes with a pause in between,
>however small, can avoid inconsistency slipping in ?

     Disallow the access between the two statements, and how can you lose constistency? Allow it, and the barndoor is open.

>> That is my understanding of the TTM position, I hope I have it right.

     I believe you do.

Sincerely,

Gene Wirchenko Received on Fri Sep 30 2005 - 20:39:32 CEST