On 22/11/12 13:08, Craig Ringer wrote:
On 11/22/2012 03:30 AM, Gavin Flower wrote:
On 22/11/12 04:56, Heikki Linnakangas wrote:
On 21.11.2012 17:42, Gavin Flower wrote:
On 22/11/12 04:32, Andres Freund wrote:
On 2012-11-21 10:21:16 -0500, Andrew Dunstan wrote:
I wasn't talking about removing it. My point was that if the
optimization
fence around CTEs is removed a lot of people will need to rework
apps
where
they have used them for that purpose. And I continue to think that
spelling
it "OFFSET 0" is horribly obscure.
+1
FWIW, I'm happy with "OFFSET 0". Granted, it's pretty obscure, but
that's what we've historically recommended, and it's pretty ugly to
have to specify a fence like that in the first place. Whenever you
have to resort to it, you ought have a comment in the query
explaining why you need to force the planner like that, anyway.
WITH foo AS (SELECT ...) (barrier=on|off)?
9.3 introduces the syntax, defaulting to on
9.4 switches the default to off.
WITH foo AS (SELECT ...) (fence=on|off)?
WITH foo AS (SELECT ...) (optimisation_fence=on|off)?
If we are to invent a new syntax for this, can we please come up
with something that's more widely applicable than just the WITH
syntax. Something that you could use to replace OFFSET 0 in a
subquery, too.
- Heikki
WITH FENCE foo AS (SELECT ...)
default?
That doesn't bind tightly enough to a specific CTE term. Consider:
WITH
FENCE foo AS (SELECT ...),
bar AS (SELECT ...)
SELECT * FROM bar;
Are we fencing just foo? Or all expressions?
--
Craig Ringerhttp://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services
Are we fencing or fooing??? :-)
Hmm...
How about:
(a) If we have lots of WITH SELECTS which mostly have one specific type
of fencing, then we could specify the common fence value after the WITH
and the exceptions after the AS:
WITH FENCE
foo AS (SELECT ...),
bar AS NOT FENCE (SELECT ...).
baz AS (SELECT ...)
SELECT * FROM bar;
alternatively:
WITH NOT FENCE
foo AS FENCE (SELECT ...),
bar AS (SELECT ...).
baz AS FENCE (SELECT ...)
SELECT * FROM bar;
(b) If we retain that FENCE is the default, then it would be simpler
just to just allow a FENCE clause after the AS keyword.
WITH
foo AS (SELECT ...),
bar AS NOT FENCE (SELECT ...).
baz AS (SELECT ...)
SELECT * FROM bar;
Obviously even for (a), we have to have one value of the FENCE clause as
the default. Either make the default FENCE, as now - or NOT FENCE if
that is seen to be a better default, especially if that is easier for
people coming from Oracle.
I suspect most people are blissfully unaware of CTE's being fenced, or
at least not really sure what it means. So I suspect NOT FENCE would be
the better default.
Alternative spellings might be better such as:
FENCED / NOT FENCED
or
FENCED / UNFENCED
Cheers,
Gavin
