Re: pgsql: Fix a couple of bugs in MultiXactId freezing

On 2013-12-03 13:11:13 -0500, Tom Lane wrote:
> Andres Freund <andres(at)2ndquadrant(dot)com> writes:
> > Any idea how to cheat our way out of that one given the current way
> > heap_freeze_tuple() works (running on both primary and standby)? My only
> > idea was to MultiXactIdWait() if !InRecovery but that's extremly grotty.
> > We can't even realistically create a new multixact with fewer members
> > with the current format of xl_heap_freeze.
>
> Maybe we should just bite the bullet and change the WAL format for
> heap_freeze (inventing an all-new record type, not repurposing the old
> one, and allowing WAL replay to continue to accept the old one). The
> implication for users would be that they'd have to update slave servers
> before the master when installing the update; which is unpleasant, but
> better than living with a known data corruption case.

I wondered about that myself. How would you suggest the format to look
like?
ISTM per tuple we'd need:

* OffsetNumber off
* uint16 infomask
* TransactionId xmin
* TransactionId xmax

I don't see why we'd need infomask2, but so far being overly skimpy in
that place hasn't shown itself to be the greatest idea?

Greetings,

Andres Freund

--
Andres Freund http://www.2ndQuadrant.com/
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