On 5/4/17 7:56 PM, Andrew Dunstan wrote:



On 05/04/2017 01:52 PM, Joe Conway wrote:
On 05/04/2017 10:33 AM, Alvaro Herrera wrote:
I'm not sure what your point is.  We know that for some cases the
optimization barrier semantics are useful, which is why the proposal is
to add a keyword to install one explicitely:

          with materialized r as
          (
             select json_populate_record(null::mytype, myjson) as x
             from mytable
          )
          select (x).*
          from r;

this would preserve the current semantics.
I haven't been able to follow this incredibly long thread, so please
excuse me if way off base, but are we talking about that a CTE would be
silently be rewritten as an inline expression potentially unless it is
decorated with some new syntax?

I would find that very disconcerting myself. For example, would this CTE
potentially get rewritten with multiple evaluation as follows?

DROP SEQUENCE IF EXISTS foo_seq;
CREATE SEQUENCE foo_seq;

WITH a(f1) AS (SELECT nextval('foo_seq'))
SELECT a.f1, a.f1 FROM a;
  f1 | ?column?
----+----------
   1 |        1
(1 row)

ALTER SEQUENCE foo_seq RESTART;
SELECT nextval('foo_seq'), nextval('foo_seq');
  nextval | ?column?
---------+----------
        1 |        2
(1 row)




I think that would be a change in semantics, which we should definitely
not be getting. Avoiding a change in semantics might be an interesting
exercise, but we have lots of clever coders ...


nextval is a volatile function, and in my understanding the plan was not to inline CTEs with volatile functions (or CTEs doing writes etc.). That is, we'd guarantee the same results as we get now.

regards

--
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services


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