Re: unclear behavior xmax/xmin

Just exploring the database and trying to understand the logic behind
things :)
So after a rollback why the xmax isnt changed back to 0 ? Seems like it
should be set again to 0.

Thanks a lot Andres !

‫בתאריך יום א׳, 18 בנוב׳ 2018 ב-22:24 מאת ‪Andres Freund‬‏ <‪
andres(at)anarazel(dot)de‬‏>:‬

> Hi,
>
> On 2018-11-18 19:13:28 +0200, Mariel Cherkassky wrote:
> > - I started transaction A and deleted one row but I didnt commit.
> > Afterwards I rolledback the transaction but the xmax of the row was set
> to
> > the txid of transaction A. I opened a new session and run VACUUM on the
> > table. However, that record wasnt deleted but the xmax was still set to
> > transaction`s A txid. Is that a bug ?
>
> No. A vacuum will not necessarily immeditely process a row/page if has
> recently been modified.
>
>
> > the vacuum process shouldnt delete
> > that row because it has an old xmax (there arent any younger
> transactions).
> > In addition, I'm still capable of seeing the record (because of the
> > rollback..) but my txid is bigger then the xmax of the record. ? Is that
> a
> > bug ?
>
> No, it's not. For visiblity checks it's not just the xid that counts,
> but also whether this xid committed.
>
> bool
> HeapTupleSatisfiesMVCC(HeapTuple htup, Snapshot snapshot,
> Buffer buffer)
> ...
> if (!(tuple->t_infomask & HEAP_XMAX_COMMITTED))
> {
> if
> (TransactionIdIsCurrentTransactionId(HeapTupleHeaderGetRawXmax(tuple)))
> {
> if (HeapTupleHeaderGetCmax(tuple) >= snapshot->curcid)
> return true; /* deleted after scan started */
> else
> return false; /* deleted before scan started */
> }
>
> if (XidInMVCCSnapshot(HeapTupleHeaderGetRawXmax(tuple), snapshot))
> return true;
>
> if (!TransactionIdDidCommit(HeapTupleHeaderGetRawXmax(tuple)))
> {
> /* it must have aborted or crashed */
> SetHintBits(tuple, buffer, HEAP_XMAX_INVALID,
> InvalidTransactionId);
> return true;
> }
>
> > -When I run an update basically its just like running an insert and
> delete
> > (the xmax of the old row is changed to the transaction`s txid and the new
> > row contains xmin same as txid of the current transaction and xmax is set
> > to 0). However, when I query the table (didnt commit yet) I dont see the
> > old record. Is there a way to see it ? Why is that ? I thought that I
> > should see the table with a non zero value in the xmax col.
>
> You can use the pageinspect extension to see such rows. We can't
> normally show such rows, because it'd violate transactional semantics.
>
> What are you trying to achieve? Just exploring the system?
>
> Greetings,
>
> Andres Freund
>