nikosv wrote:

That's pretty much correct, although the isolation levels are defined in
terms of anomalies that will be visible, rather than locking. So when
you choose READ COMMITTED isolation you are saying that you are willing
to accept more anomalies than you would see at a stricter isolation
level, such as REAPEATABLE READ say, on the assumption that you will
get better concurrency in return. One does not usually ask for a
relaxed isolation level in order to actually see anomalies so the fact
that MVCC does a better job of concealing them than a particular
isolation level expects is acceptable behaviour.

Absolutely. This is only feature of Oracle that I've ever been jealous
of, and it's long overdue. Programmers pretty much assume MVCC and
have done for 10-15 years. That how this thread started. Once the
Ingres 10 implementation matures we'll wonder how we did without it. :-)

--
Roy

UK Ingres User Association Conference 2011 will be on Tuesday June 7 2011.
Register at http://www.regonline.co.uk/ukiua2011

nikosv wrote:

Yep. When did the first completed SQL standard appear? Circa 1986?
commercial Ingres was 7 years old by then, and Oracle not much younger.

--
Roy

UK Ingres User Association Conference 2011 will be on Tuesday June 7 2011.
Register at http://www.regonline.co.uk/ukiua2011

On Mar 13, 8:39*pm, Roy Hann <specia... (AT) processed (DOT) almost.meat> wrote:
yes like in the sense that the vendor can implement the underlying
locking scheme of his liking to match the anomaly described by the
isolation level

nikosv wrote:

Or not even use locking at all. Anything that produces the right effect
will do. And whatever is used can be changed, provided it continues to
produce the right effect. For example Adrian Hudnott presented a novel
MVCC algorithm at last years UK IUA conference which ought to permit
even better performance than conventional MVCC.

You mentioned abstraction earlier. The isolation level concept is
nicely abstract and makes no prescriptions about how it must be
implemented (which it couldn't, being very late to the party).
It seems to me that it's an example of standardization at it's best. Too
often the ANSI/ISO standard goes off on fantastical speculations
unconstrained by where the products and vendors really are (or want to
be). In this case they had to think harder and they came up with a
good answer.

--
Roy

UK Ingres User Association Conference 2011 will be on Tuesday June 7 2011.
Register at http://www.regonline.co.uk/ukiua2011