Re: Idea for nested transactions / savepoints

Tom Lane wrote:
> 
> Bruce Momjian <pgman@candle.pha.pa.us> writes:
> >> The complaints about WAL size amount to "we don't have the disk space
> >> to keep track of this, for long-running transactions".  If it doesn't
> >> fit on disk, how likely is it that it will fit in memory?
> 
> > Sure, we can put on the disk if that is better.
> 
> I think you missed my point.  Unless something can be done to make the
> log info a lot smaller than it is now, keeping it all around until
> transaction end is just not pleasant.  Waving your hands and saying
> that we'll keep it in a different place doesn't affect the fundamental
> problem: if the transaction runs a long time, the log is too darn big.

Keeping it in a different place does have other benefits - you can
discard 
each subtransaction after it is committed/aborted regardless of what WAL 
log does, so the chap who did a "begin transaction" 8 hours ago does not
get 
subtransactions kept as well, thus postponing the problem a lot.

> There probably are things we can do --- for example, I bet an UNDO
> log kept in this way wouldn't need to include page images.

Not keeping something that does not need to be kept is always a good
idea 
when preserving space is important.

> But it's that sort of consideration that will make or break UNDO, 
> not where we store the info.

But "how long do we need to keep the info" _is_ an important
consideration.

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Hannu