Re: Poor performance using CTE - Mailing list pgsql-performance

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