Re: wCTE: why not finish sub-updates at the end, not the beginning?

On 2011-02-25 4:58 PM, Tom Lane wrote:
> Specifically, I'm imagining getting rid of the patch's additions to
> InitPlan and ExecutePlan that find all the modifying sub-queries and
> force them to be cycled to completion before the main plan runs.
> Just run the main plan and let it pull tuples from the CTEs as needed.
> Then, in ExecutorEnd, cycle any unfinished ModifyTable nodes to
> completion before shutting down the plan.  (In the event of an error,
> we'd never get to ExecutorEnd, but it doesn't matter since whatever
> updates we did apply are nullified anyhow.)

This idea has actually been discussed before when we talked about 
optimizing wCTEs, but IIRC you said that doing this in ExecutorEnd is a 
bit ugly.  But if you can write this idea down in a way that makes you 
happy with the implementation, I think it's a huge benefit and we should 
definitely do it.

> This has a number of immediate and future implementation benefits:

> 3. The code could be significantly simpler.  Instead of that rather
> complex and fragile logic in InitPlan to try to locate all the
> ModifyTable nodes and their CTEScan parents, we could just have
> ModifyTable nodes add themselves to a list in the EState during
> ExecInitNode.  Then ExecutorEnd just traverses that list.

Sounds good to me.

> However, the real reason for doing it isn't any of those, but rather
> to establish the principle that the executions of the modifying
> sub-queries are interleaved not sequential.  We're never going to be
> able to do any significant optimization of such queries if we have to
> preserve the behavior that the sub-queries execute sequentially.
> And I think it's inevitable that users will manage to build such an
> assumption into their queries if the first release with the feature
> behaves that way.

Yeah, you might be right.


Regards,
Marko Tiikkaja