Re: Multixid hindsight design

Simon Riggs <simon(at)2ndQuadrant(dot)com> writes:
> On 11 May 2015 at 22:20, Heikki Linnakangas <hlinnaka(at)iki(dot)fi> wrote:
>> So the lesson here is that having a permanent pg_multixact is not nice,
>> and we should get rid of it. Here's how to do that:

> An alternate proposal:

> 1. Store only the Locking xids in the Members SLRU
> 2. In the Offsets SLRU store: 1) the Updating Xid and 2) the offset to the
> Locking xids in the Members SLRU.

> This means the Offsets SLRU will be around twice the size it was before BUT
> since we reduce the size of each Members array by one, there is a balanced
> saving there, so this change is disk-space-neutral.

> That way if we need to make Offsets SLRU persistent it won't bloat.
> We then leave the Members SLRU as non-persistent, just as it was <9.3

I don't think you can do that, because it supposes that locking XIDs need
not be remembered across a crash. Don't prepared transactions break that
assumption?

regards, tom lane