Re: WIP: Deferrable unique constraints

Another thought on the index AM API issues: after poking through the
code I realized that there is *nobody* paying any attention to the
existing bool result of aminsert() (ie, did we insert anything or not).
So I think that instead of adding a bool* parameter, we should repurpose
the function result, along the lines of this spec:

<para>
The method's boolean result value is significant only when
<literal>checkUnique</> is <literal>UNIQUE_CHECK_PARTIAL</>.
In this case a TRUE result means the new entry is known unique, whereas
FALSE means it might be non-unique (and a deferred uniqueness check must
be scheduled). For other cases a constant FALSE result is recommended.
</para>

<para>
For non-unique indexes, it is not required that <function>aminsert</>
do anything; it might for instance refuse to index NULLs.
</para>

The bool* parameter is fairly ugly in a couple of ways: it's not clear
when it's okay to pass a NULL pointer, and the compiler doesn't give
you a lot of help in being sure you've set the result in all code paths.
So I'd rather not use one if we don't have to.

regards, tom lane