Re: WIP: Rework access method interface

Alexander Korotkov <a.korotkov@postgrespro.ru> writes:
> On Mon, Aug 10, 2015 at 1:12 PM, Petr Jelinek <petr@2ndquadrant.com> wrote:
>> I don't understand this, there is already AmRoutine in RelationData, why
>> the need for additional field for just amsupport?

> We need amsupport in load_relcache_init_file() which reads
> "pg_internal.init". I'm not sure this is correct place to call am_handler.
> It should work in the case of built-in AM. But if AM is defined in the
> extension then we wouldn't be able to do catalog lookup for am_handler on
> this stage of initialization.

This is an issue we'll have to face before there's much hope of having
index AMs as extensions: how would you locate any extension function
without catalog access?  Storing raw function pointers in pg_internal.init
is not an answer in an ASLR world.

I think we can dodge the issue so far as pg_internal.init is concerned by
decreeing that system catalogs can only have indexes with built-in AMs.
Calling a built-in function doesn't require catalog access, so there
should be no problem with re-calling the handler function by OID during
load_relcache_init_file().

We could also have problems with WAL replay, though I think the consensus
there is that extension AMs have to use generic WAL records that don't
require any index-specific replay code.