Re: WG: [HACKERS] MVCC works in serialized mode!

Zeugswetter Andreas IZ5 wrote:
>
> >>
> >> As to transaction blocks:
> >> Actually most clients never use commit work, and remember
>
> >In Oracle, COMMIT is implicit if DB connection is closed
> >in "normal" way - by using PQfinish in our case.
>
> Yes, but that is not really what we would want a client to do.
> (Not give a commit for 5 h)

I would say - it's up to client -:)
And remember - this is standard behaviour and so this mode
should be implemented in any case.

> >> that postgresql will never free it's memory before commit. This
> will
> >> hog up
> >> memory for otherwise lightweight clients.
> >
> >This is the bug, but I believe that it's fixed for
> >most cases ~1year ago by this
> >
> >#ifdef TBL_FREE_CMD_MEMORY
> > EndPortalAllocMode();
> > StartPortalAllocMode(DefaultAllocMode, 0);
> >#endif
> >
> >in xact.c
>
> Yes, if you do a commit the memory will be freed alright. I meant a client,
^^^^^^^^^^^
No! This part of code run after _each_ statement...

> >> I also think that the begin work; commit work; statement block is
> >> somewhat part of the postgresql philosophy. We do not stand alone
> >> with this approach (Informix).
> >
> >First, BEGIN/END is used in standard for grouping queries
> >(in PL etc), not for transaction control, and so currently
> >using of BEGIN/END is ambiguous.
>
> Sorry, I am not so good with my wording, I meant to say transaction block
> not statement block. I have been crying to take the begin/end syntax out of
> the PostgreSQL SQL syntax for very long now. The SQL statements should be:
> begin work;
> commit work;
> not begin and end which is unfortunately still allowed.

I'm not sure but it seems that COMMIT without WORK is allowed
by standard (at least, in some levels).

> >Second, Informix is using locking => chained transaction
> >mode is way to lock-escalation.
>
> I don't understand ? Informix has no lock escalation (the term meaning the
> promotion of a row to a page and then Table lock, if soandso many locks for
> a table are already held) and does not use chained mode.

I'm sure that Informix has nice lock manager,
but in SERIALIZABLE+chained mode Informix will hold lock on selected
rows untill COMMIT/ABORT: this increases possibility of
lock escalation and remember that read/write concurrent access
to the same row causes blocking (not give a commit for 5h
after simple select... -:))

Un-chained mode seems more suitable for locking systems.
BTW, is there ability to switch to chained mode in Informix?
SyBase has it.

> >> Actually the other DB's start the transaction with the first
> >> modifying
> >> statement, in my opinion a rather weird approach.
> >> I have seen programs that do a dummy update, just to simulate a
> >> begin work, what a waste.
> >
> >I believe that this is not how Oracle works.
> >I assumed that any DML statement will start transaction.
>
> Not in DB/2.

But in standard. And I read that DDL statements also start
transaction (if there is no active one).
Ability to run DDL & DML statements in the same
transaction is implementation dependent.

Vadim