Re: Adding REPACK [concurrently]

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

> 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);

When posting version 12 of the patch [1] I raised a concern that the the MVCC
safety is too expensive when it comes to logical decoding. Therefore, I
abandoned the concept for now, and v13 [2] uses plain heap_insert(). Once we
implement the MVCC safety, we simply rewrite the tuple like v12 did - that's
the simplest way to preserve fields like xmin, cmin, ...

[1] https://www.postgresql.org/message-id/178741.1743514291%40localhost
[2] https://www.postgresql.org/message-id/97795.1744363522%40localhost

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