Re: Overhead cost of Serializable Snapshot Isolation

On Oct11, 2011, at 22:55 , Simon Riggs wrote:
> Probably as a matter of policy all new features that effect semantics
> should have some kind of compatibility or off switch, if easily
> possible.

There's a huge downside to that, though. After a while, you end up
with a gazillion settings, each influencing behaviour in non-obvious,
subtle ways. Plus, every new code we add would have to be tested against
*all* combinations of these switches. Or, maybe, we'd punt and make
some features work only with "reasonable" settings. And by doing so
cause much frustration of the kind "I need to set X to Y to use feature
Z, but I can't because our app requires X to be set to Y2".

I've recently had to use Microsoft SQL Server for a project, and they
fell into *precisely* this trap. Nearly *everything* is a setting there,
like whether various things follow the ANSI standard (NULLS, CHAR types,
one setting for each), whether identifiers are double-quoted or put between
square brackets, whether loss of precision is an error, ...

And, some of their very own features depend on specific combination of
these settings. Sometimes on the values in effect when the object was
created, sometimes when it's used. For example, their flavour of
materialized views (called "indexed views") requires a bunch of options
to be set correctly to be able to create such an object. Some of these
must even be in effect to update the view's base tables, once the view
is created...

That experience has taught me that backwards compatibility, while very
important in a lot of cases, has the potential to do just as much harm
if overdone.

best regards,
Florian Pflug