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