Re: [HACKERS] CTE inlining

On 30/04/17 16:28, Tom Lane wrote:
> Craig Ringer <craig.ringer@2ndquadrant.com> writes:
>> - as you noted, it is hard to decide when it's worth inlining vs
>> materializing for CTE terms referenced more than once.
> [ raised eyebrow... ]  Please explain why the answer isn't trivially
> "never".
>
> There's already a pretty large hill to climb here in the way of
> breaking peoples' expectations about CTEs being optimization
> fences.  Breaking the documented semantics about CTEs being
> single-evaluation seems to me to be an absolute non-starter.
>
>             regards, tom lane
>
>
Could not each CTE be only evaluated once, but restricted (as far as is 
practicable) to the rows actually needed by the body of the SELECT?


Cheers,
Gavin