Re: Why copy_relation_data only use wal whenWALarchivingis enabled

On Wed, 2007-10-17 at 17:36 +0100, Heikki Linnakangas wrote:
> Simon Riggs wrote:
> > On Wed, 2007-10-17 at 15:02 +0100, Heikki Linnakangas wrote:
> >> Simon Riggs wrote:
> >>> If you've got a better problem statement it would be good to get that
> >>> right first before we discuss solutions.
> >> Reusing a relfilenode of a deleted relation, before next checkpoint
> >> following the commit of the deleting transaction, for an operation that
> >> doesn't WAL log the contents of the new relation, leads to data loss on
> >> recovery.
> >
> > OK, thanks.
> >
> > I wasn't aware we reused refilenode ids. The code in GetNewOid() doesn't
> > look deterministic to me, or at least isn't meant to be.
> > GetNewObjectId() should be cycling around, so although the oid index
> > scan using SnapshotDirty won't see committed deleted rows that shouldn't
> > matter for 2^32 oids. So what gives?
>
> I don't think you still quite understand what's happening.

Clearly. It's not a problem to admit that.

> GetNewOid()
> is not interesting here, look at GetNewRelFileNode() instead. And
> neither are snapshots or MVCC visibility rules.

Which calls GetNewOid() in all cases, AFAICS.

How does the reuse you say is happening come about? Seems like the bug
is in the reuse, not in how we cope with potential reuse.

--
Simon Riggs
2ndQuadrant http://www.2ndQuadrant.com