On 05/04/2017 10:56 AM, 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?
>> WITH a(f1) AS (SELECT nextval('foo_seq'))
>> SELECT a.f1, a.f1 FROM a;
>>  f1 | ?column?
>> ----+----------
>>   1 |        1
>> (1 row)
>> 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 ...

Well I think my point is that I always have understood CTEs to be
executed precisely once producing a temporary result set that is then
referenced elsewhere. I don't think that property of CTEs should change.
Somewhere else in the thread someone mentioned predicate push down --
that makes sense and maybe some clever coder can come up with a patch
that does that, but I would not be in favor of CTEs being inlined and
therefore evaluated multiple times.


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