Re: Recovery Features - Mailing list pgsql-hackers

On 7/5/2004 6:16 PM, Simon Riggs wrote:

> On Mon, 2004-07-05 at 22:30, Tom Lane wrote:
>> Simon Riggs <simon@2ndquadrant.com> writes:
>> > ...While recovering, it is very straightforward to simply ignore every
>> > record associated with one (or more) transactions. That gives us the
>> > ability to recover "all apart from txnid X".
>> 
>> Don't even *think* of going there.
> 
> Hmmm... thinking is important, as are differing viewpoints. I value
> yours and those of everyone else on this list, hence the post.
> 
>> What will happen when transaction Y comes along and wants to modify or
>> delete a row that was inserted by X?  There's no chance of staying
>> consistent.
> 
> I did point out this downside...a few sentences down.
> **This is awful because: transactions are isolated from each other, but
> they also provide changes of state that rely on previous committed
> transactions. If you change the past, you could well invalidate the
> future. If you blow away a transaction and a later one depends upon it,
> then you will have broken the recovery chain and will not be able to
> recover to present time.**
> 
> Theoretically, this is a disaster area.
> 
> Practically, Oracle10g provides similar-ish features...

IF ... the recovery process would be primary key based, and IF the 
database definitions would allow for balance type field handling (the 
log contains value deltas for balance fields instead of overwriting 
them), THEN this would be a direction I would be looking into.

But as things are, the whole recovery is ctid and binary block based. So 
you would now leave out the ctid based changes to several tuples because 
of belonging to said transaction. Later on, an original whole block 
appears in the WAL and overwrites ... so you get what ... partial 
transactions into the recoverd DB?

> 
> ...Nobody is shouting YES, so its a dodo...

No way!

> 
> Best regards, Simon Riggs
> 
> 
> ---------------------------(end of broadcast)---------------------------
> TIP 8: explain analyze is your friend


-- 
#======================================================================#
# It's easier to get forgiveness for being wrong than for being right. #
# Let's break this rule - forgive me.                                  #
#================================================== JanWieck@Yahoo.com #