Re: [Firebird-devel] Feature request & discussion (Reply to Planning the post v3 development)

[Leyne, Sean]

Vlad,

is your scenario even possible?

wouldn't the engine report an update collision when update version3 was 
attempted?

(i am not near a firebird install to test for myself)

Sean

-------- Original Message --------
From:Vlad Khorsun <hv...@users.sourceforge.net>
Sent:Sat, 10 May 2014 18:21:23 -0400
To:For discussion among Firebird Developers 
<firebird-devel@lists.sourceforge.net>
Subject:Re: [Firebird-devel] Feature request & discussion (Reply to Planning 
the post v3 development)

> 10.05.2014 15:30, Vlad Khorsun wrote:
>>      Here you wrong: if blob contents was not changed by update, there will 
>> be no new blob.
>
>   Yes, and in this case blob cannot be garbage collected because next record 
> version
> refers to it. No need to analyze whole versions chain (collect staying list) 
> - two these
> versions are enough for decision making. (Actually, there must be three 
> versions for
> analyze, but one of them may be missed if going version is on the end of 
> chain.)

    It is wrong for "lost update" scenario, which is allowed for read-committed 
transactions:

att1: start tx1, insert version1 (blob1), commit tx1

att1: start tx2 (read-committed), read version1 (blob1)

att2: start tx4, update version2 (blob2), commit tx4

att1: update version3 (blob1), commit tx2

    Here we have chain of versions:

version3 (tx2, blob1) -> version2 (tx4, blob2) -> version1 (tx1, blob1)


Regards,
Vlad

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