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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 Ringer http://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
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