Re: Small run-time pruning doc fix

Thanks for looking at this.

On 2 November 2018 at 20:30, Peter Eisentraut
<peter(dot)eisentraut(at)2ndquadrant(dot)com> wrote:
> <para>
> - Execution-time partition pruning currently occurs for the
> + Execution-time partition pruning currently only occurs for the
> <literal>Append</literal> and <literal>MergeAppend</literal> node
> types.
> + It is not yet implemented for the <literal>ModifyTable</literal> node
> + type.
> </para>
>
> Isn't this implied by the preceding paragraph
>
> Currently, pruning of partitions during the planning of an UPDATE or
> DELETE command is implemented using the constraint exclusion method

That paragraph is talking about plan-time partition pruning. The
paragraph the patch changes is mentioning execution-time partition
pruning. In PG11 we have plan-time partition pruning for Append and
MergeAppend. UPDATE/DELETE does pruning, but it uses the constraint
exclusion code which does not work for HASH partitioned tables. In
PG11 we only have run-time partition pruning for Append. MergeAppend
support was only added in 5220bb7533 (PG12).

> Also, could there be other node types that could benefit from partition
> pruning that are not implemented yet? Not sure we want to mention all
> of them.

I'm unsure what might exist in the future, but there are currently
only 3 node types that take a list of subpaths. I think if we get the
support for ModifyTable later, then likely the paragraph edited in
this patch would disappear. In fact, the entire note would disappear
as adding run-time pruning for ModifyTable will require the planner to
use the new partition pruning code. There's a patch for that in [1],
so maybe it'll all be a little less confusing in PG12.

[1] https://commitfest.postgresql.org/20/1778/

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