Re: Adding REPACK [concurrently]

Hello everyone!

Alvaro Herrera <alvherre@alvh.no-ip.org>:
> Please note that Antonin already implemented this.  See his patches
> here:
> https://www.postgresql.org/message-id/77690.1725610115%40antos
> I proposed to leave this part out initially, which is why it hasn't been
> reposted.  We can review and discuss after the initial patches are in.

I think it is worth pushing it at least in the same release cycle.

> But you're welcome to review that part specifically if you're so
> inclined, and offer feedback on it.  (I suggest to rewind back your
> checked-out tree to branch master at the time that patch was posted, for
> easy application.  We can deal with a rebase later.)

I have rebased that on top of v18 (attached).

Also, I think I found an issue (or lost something during rebase): we
must preserve xmin,cmin during initial copy
to make sure that data is going to be visible by snapshots of
concurrent changes later:

static void
reform_and_rewrite_tuple(......)
.....
      /*It is also crucial to stamp the new record with the exact same
xid and cid,
      * because the tuple must be visible to the snapshot of the
applied concurrent
      * change later.
      */
      CommandId      cid = HeapTupleHeaderGetRawCommandId(tuple->t_data);
      TransactionId   xid = HeapTupleHeaderGetXmin(tuple->t_data);

      heap_insert(NewHeap, copiedTuple, xid, cid, HEAP_INSERT_NO_LOGICAL, NULL);

I'll try to polish that part a little bit.

> Because having an MVCC-safe mode has drawbacks, IMO we should make it
> optional.
Do you mean some option for the command? Like REPACK (CONCURRENTLY, SAFE)?

Best regards,
Mikhail.