Re: CTE optimization fence on the todo list?

Andrew Dunstan <andrew@dunslane.net> writes:
> On 05/01/2015 07:24 PM, Josh Berkus wrote:
>>> (A possible compromise position would be to offer a new GUC to
>>> enable/disable the optimization globally; that would add only a reasonably
>>> small amount of control code, and people who were afraid of the change
>>> breaking their apps would probably want a global disable anyway.)

> This could be a very bad, almost impossible to catch, behaviour break. 
> Even if we add the GUC, we're probably going to be imposing very 
> significant code audit costs on some users.

On what grounds do you claim it'd be a behavior break?  It's possible
that the subquery flattening would result in less-desirable plans not
more-desirable ones, but the results should still be correct.
        regards, tom lane