Re: Adding REPACK [concurrently]

Mihail Nikalayeu <mihailnikalayeu(at)gmail(dot)com> wrote:

> > Not sure I understand in all details, but I don't think SnapshotSelf is the
> > correct snapshot. Note that HeapTupleSatisfiesSelf() does not use its
> > 'snapshot' argument at all. Instead, it considers the set of running
> > transactions as it is at the time the function is called.
>
> Yes, and it is almost the same behavior when a typical MVCC snapshot
> encounters a tuple created by its own transaction.
>
> So, how it works in the non MVCC-safe case (current patch behaviour):
>
> 1) we have a whole initial table snapshot with all the xmin = repack XID
> 2) appling transaction sees ALL the self-alive (no xmax) tuples in it
> because all tuples created\deleted by transaction itself
> 3) each update/delete during the replay selects the last existing
> tuple version, updates it xmax and inserts a new one
> 4) so, there is no any real MVCC involved - just find the latest
> version and create a new version
> 5) and it works correctly because all ordering issues were resolved by
> locking mechanisms on the original table or by reordering buffer

ok

> How it maps to MVCC-safe case (SnapshotSelf):
>
> 1) we have a whole initial table snapshot with all xmin copied from
> the original table. All such xmin are committed.
> 2) appling transaction sees ALL the self-alive (no xmax) tuple in it
> because its xmin\xmax is committed and SnapshotSelf is happy with it

How does HeapTupleSatisfiesSelf() recognize the status of any XID w/o using a
snapshot? Do you mean by checking the commit log (TransactionIdDidCommit) ?

> 3) each update/delete during the replay selects the last existing
> tuple version, updates it xmax=original xid and inserts a new one
> keeping with xmin=orignal xid
> 4) --//--
> 5) --//--

> > However, at the time we're replaying the UPDATE in the new table, the tuple
> > may have been already deleted from the old table, and the deleting transaction
> > may already have committed. In such a case, HeapTupleSatisfiesSelf() will
> > conclude the old version invisible and the we'll fail to replay the UPDATE.
>
> No, it will see it - because its xmax will be empty in the repacked
> version of the table.

You're right, it'll be empty in the new table.

--
Antonin Houska
Web: https://www.cybertec-postgresql.com