Re: Allow FDW extensions to support MERGE command via CustomScan

Hi,

Just some thoughts on documentation part.

> +</programlisting>
> +
> +     Postgres doesn't support <command>MERGE</command> on foreign tables,
> +     see <function>ExecMerge</function>. Still, extensions may provide
> +     custom scan nodes to support <command>MERGE</command> on foreign
> +     tables. If your extension provides such custom scan node, this
> +     function should return true.
> +    </para>

What's the point about mentioning the ExecMerge function? I didn't 
find any relevant documentation about why it not support foreign 
tables, maybe its because it should not?

I didn't understand what is a "custom scan node" on the fdw context at 
first place (I don't know if it is an already know word on this 
context), but from what I've understood so far, to a fdw extension 
support MERGE it should implements on PlanForeignModify right? If 
that's the case maybe it's worth updating the PlanForeignModify 
documentation as well? It only mention insert, update, or delete 
operations. Also, I don't know if would be good to link the 
PlanForeignModify on this part of the documentation, WYT?

--
Matheus Alcantara
EDB: https://www.enterprisedb.com