Re: Multiple Xids in PGPROC?

Alvaro Herrera wrote:
> Hackers,
> 
> I've whacked the subtrans patch enough so that the simple tests (i.e.
> non concurrent) for tuple visibility work.  I can create a table and
> populate it in subtransactions, rollback or commit them selectively and
> get the desired effect at the end.  Naturally, catalog entries also
> behave [somewhat] sanely.  Oh, I made pg_subtrans work too.  (Though
> whether it's relatively bug-free is yet to be proven.)
> 
> I'm now looking at changing the concurrent visibility rules, i.e.
> utils/time/tqual.c.  It seems to me one of the most important parts is
> making TransactionIdIsInProgress() behave, that is, yield true for every
> committed and in-progress subtransaction of a current transaction tree.
> (Not only the topmost Xid, which is what it currently does.)
> 
> So, the big question is, how do we do this?  The most obvious way (to
> me) is to keep the whole array inside the PGPROC struct.  This would be
> nice because it would only need little modification to
> access/transam/varsup.c.  The main downside is that it potentially
> requires a lot of shared memory.  Can we afford that?
> 
> I am already keeping the list of committed Xids in a backend-local list,
> but of course this is not visible to other backends.

I remember going through this.  Other backends will use pg_subtrans to
know what transactions are in progress. They have to do the standard
lookups to find the status of the parent transaction.  The backend-local
list of xids is needed so the commit can clean up those subtransaction
xids so that later transactions don't have to use pg_subtrans.

Does this help?

Sorry I haven't gotten your patches in yet.  Tom is working on some
other back patches.  Also, do you have a plan to handle some of the more
complex issues like locking in subtransactions?

-- 
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