| From: | Gavin Flower <GavinFlower(at)archidevsys(dot)co(dot)nz> |
|---|---|
| To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Craig Ringer <craig(dot)ringer(at)2ndquadrant(dot)com> |
| Cc: | Ilya Shkuratov <motr(dot)ilya(at)ya(dot)ru>, PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org>, Mario Becroft <mb(at)true(dot)group> |
| Subject: | Re: CTE inlining |
| Date: | 2017-05-04 10:02:06 |
| Message-ID: | 2dab0187-42fe-3b43-eed9-921aca564ec2@archidevsys.co.nz |
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| Lists: | pgsql-hackers |
On 30/04/17 16:28, Tom Lane wrote:
> Craig Ringer <craig(dot)ringer(at)2ndquadrant(dot)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
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