From: | "Joshua D(dot) Drake" <jd(at)commandprompt(dot)com> |
---|---|
To: | Hannu Krosing <hannu(at)skype(dot)net> |
Cc: | Chad Wagner <chad(dot)wagner(at)gmail(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, RPK <rohitprakash123(at)indiatimes(dot)com>, pgsql-hackers(at)postgresql(dot)org |
Subject: | Re: New feature request: FlashBack Query |
Date: | 2007-02-18 22:27:49 |
Message-ID: | 45D8D2E5.2000802@commandprompt.com |
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Lists: | pgsql-hackers |
Hannu Krosing wrote:
> Ühel kenal päeval, L, 2007-02-17 kell 22:49, kirjutas Chad Wagner:
>
>>
>> However, they don't have vacuum, we do.
>>
>> Right, and I think that is more or less because Oracle doesn't need
>> it. Vacuum's main purpose (correct me if I am wrong) is to
>> recover/mark rows that are no longer used, and Oracle essentially
>> reuses the space immediately.
>>
>> Obviously with Oracle if you bloat out a table and delete a ton of
>> rows then you have to rebuild the table, but that is more or less the
>> same problem that PostgreSQL has and where vacuum full comes into
>> play.
>>
>> The only benefit with the Oracle model is that you can achieve
>> flashback, which is a very rarely used feature in my book.
>
> We can have flashbacks up to the last vacuum. It is just not exposed.
> Don't vacuum, and you have the whole history. (Actually you can't go for
> more than 2G transactions, or you get trx id rollover).
>
> To get a flashback query, you "just" have to construct a snapshot from
> that time and you are done. We don't store transaction times anywere, so
> the flashback has to be by transaction id, but there is very little
> extra work involved. We just don't have syntax for saying "SELECT ... AS
> SEEN BY TRANSACTION XXX"
Well this is certainly interesting. What do we think it would take to
enable the functionality?
Joshua D. Drake
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