On 3 December 2014 at 09:29, David Rowley <dgrowleyml@gmail.com> wrote:
> *** Method 3: Marking scans as possibly skippable during planning and
> removing redundant join nodes at executor startup (Simon's method)
>
> Pros:
> 1. The plan can be executed as normal if there are any foreign key triggers
> pending.
> 2. Does not require extra code in all join types (see cons #2 above)
> 3. Does not suffer from extra node visiting overhead (see cons #3 above)
>
> Cons:
> 1. Executor must modify the plan.
> 2. Planner may have generated a plan which is not optimal for modification
> by the executor (e.g. Sort nodes for merge join, or index scans for
> pre-sorted input won't become seqscans which may be more efficient as
> ordering may not be required after removing a merge join)
>
> With each of the methods listed above, someone has had a problem with, and
> from the feedback given I've made changes based and ended up with the next
> revision of the patch.
>
> Tom has now pointed out that he does not like the executor modifying the
> plan, which I agree with to an extent as it I really do hate the extra
> useless nodes that I'm unable to remove from the plan.
I guess we need an Option node. Tom and I discussed that about an aeon ago.
The Option node has a plan for each situation. At execution time, we
make the test specified in the plan and then select the appropriate
subplan.
That way we can see what is happening in the plan and the executor
doesn't need to edit anything.
-- Simon Riggs http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training & Services