Re: Should we add GUCs to allow partition pruning to be disabled?

On Wed, Apr 18, 2018 at 12:07:18PM +1200, David Rowley wrote:
> In PG10 the planner's partition pruning could be disabled by changing
> the constraint_exclusion GUC to off. This is still the case for PG11,
> but only for UPDATE and DELETE queries. There is currently no way to
> disable partition pruning for SELECT.
>
> Should we allow this?
>
> To make this a bit more complex, we now also have run-time pruning
> which can allow further partition pruning to be performed during
> execution. I imagine if we're going to add a GUC for plan-time
> pruning then we should also have one for run-time pruning. These could
> also perhaps share the same GUC, so it seems there are some sub
> choices to make here:
>
> 1. Add a single enable_ GUC which allows both plan-time and run-time
> pruning to be disabled.
> 2. Add two new enable_ GUCs, one for plan-time and one for run-time pruning.
> 3. No new GUCs / Do nothing.

Maybe this is divergent from the details of the implementation; but, from a
user's perspective: why not continue to use constraint_exclusion?

I would suggest to add zero new GUCs:

0. constraint_exclusion={off,partition,on,PLANNER*,EXECUTOR*}

I tentatively assume that "constraint_exclusion=partition" would disable PG11
"pruning", and that the new default setting would be "executor".

* Caveat: there may be a better name than planner/executor..
planner_prune? execute_filter?

Justin