Re: [HACKERS] READ COMMITTED isolevel is implemented ...

Hiroshi Inoue wrote:
> 
> > Subject: [HACKERS] READ COMMITTED isolevel is implemented ...
> >
> > and this is now the DEFAULT isolevel.
> >
> 
> It's different from current(v6.4.2).

First, I think that DEFAULT isolevel must be configure-able.

> The way will be provided to upgrade user's current code ?

Even SERIALIZABLE isolevel in MVCC is different from
one in locking systems. There is only one way to don't
change anything in applications - use table level locking.
Should we provide ability to turn MVCC off?

> > Processing updates in READ COMMITTED isolevel is much
> > complex than in SERIALIZABLE one, because of if transaction T1
> > notices that tuple to be updated/deleted/selected_for_update
> > is changed by concurrent transaction T2 then T1 has to check
> > does new version of tuple satisfy T1 plan qual or not.
> 
> How about   UPDATE t set x = x + 1 where .... ?
> 
> The values of x used for x = x + 1 are at the time when statement
> started ?
> It seems that this case also requires re-execution.

x + 1 is in target list of execution plan. And so when child plan
is executed, new value of x is used to evaluate target list
expressions. Executor uses tuple from child plan as new version
of tuple.

Vadim