Re: Nested transactions and tuple header info

Tom Lane wrote:
> Bruce Momjian <pgman@candle.pha.pa.us> writes:
> > Tom Lane wrote:
> >> This is exactly the same argument as not being able to overwrite cmin.
> 
> > Basically the phantom xid's are a shorthand for saying the tuple was
> > created by xid1 and deleted by xid2, both part of the same main
> > transaction.
> 
> > A cursor looking at the rows has to recognize the xid is a phantom (via
> > pg_subtrans) and look up the creation xid.
> 
> You still don't see the point.  Consider
> 
>     BEGIN;
>     DECLARE CURSOR c1 FOR SELECT * FROM a ...;
>     INSERT INTO a VALUES(...);    -- call this row x
>     DECLARE CURSOR c2 FOR SELECT * FROM a ...;
>     BEGIN;
>     DELETE FROM a WHERE ...;    -- assume this deletes row x
>     ROLLBACK;
>     FETCH FROM c1;            -- must NOT see row x
>     FETCH FROM c2;            -- must see row x
> 
> AFAICS your proposal does not support this.  The two cursors' snapshots
> will differ only in the recorded current-cid for the outer transaction.
> If the subtrans has overwritten xmin/cmin, there is no way to make that
> decision correctly.

I do not overwrite cmin or cmax.  If xid=1 creates a row:
xmin=1cmin=1

and xid=2 goes to expire it, we get:
xmin=3 (phantom for xmin=1, xmax=2)cmin=1cmax=1

and we set a phantom bit on the tuple.  When we see it later and need to
know the xmin or xmax, we look it up in local memory. (Maybe we don't
even need a hash, just a List because I can't imagine more than a few of
these phantoms being used.)  Other backends see the tuple with a xmin as
"in progress" so they don't need to look any further.

Then, on commit, we decide if a tuple created by xid=1 and expired by
xid=2 should appear created or not, and mark the phantom commit status
accordingly.  

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