[PATCH] WIP Add ALWAYS DEFERRED option for constraints

Attached are patches to add an ALWAYS DEFERRED option to CONSTRAINTs and
CONSTRAINT TRIGGERs, meaning: SET CONSTRAINTS .. IMMEDIATE will not make
immediate any constraint/trigger that is declared as ALWAYS DEFERRED.

I.e., the opposite of NOT DEFERRED. Perhaps I should make this NOT
IMMEDIATE? Making it NOT IMMEDIATE has the benefit of not having to
change the precedence of ALWAYS to avoid a shift/reduce conflict... It
may also be more in keeping with NOT DEFERRED.

Motivation:

- I have trigger procedures that must run at the end of the transaction
(after the last statement prior to COMMIT sent by the client/user),
which I make DEFERRABLE, INITIALLY DEFERRED CONSTRAINT TRIGGERs out
of, but SET CONSTRAINTS can be used to foil my triggers. I have
written SQL code to detect that constraint triggers have fired too
soon, but I'd rather not need it.

- Symmetry. If we can have NOT DEFERRABLE constraints, why not also
NOT IMMEDIABLE? :) Naturally "immediable" is not a word, but you
get the point.

- To learn my way around PostgreSQL source code in preparation for
other contributions.

Anyways, this patch is NOT passing tests at the moment, and I'm not sure
why. I'm sure I can figure it out, but first I need to understand the
failures. E.g., I see this sort of difference:

\d testschema.test_index1
Index "testschema.test_index1"
Column | Type | Definition
--------+--------+------------
id | bigint | id
-btree, for table "testschema.test_default_tab"
+f, for table "testschema.btree", predicate (test_default_tab)

which means, I think, that I've screwed up in src/bin/psql/describe.c,
don't it's not obvious to me yet how.

Some questions for experienced PostgreSQL developers:

Q0: Is this sort of patch welcomed?

Q1: Should new columns for pg_catalog.pg_constraint go at the end, or may
they be added in the middle?

Q2: Can I add new columns to information_schema tables, or are there
standards-compliance issues with that?

Q3: Is the C-style for PG documented somewhere? (sorry if I missed this)

Q4: Any ideas what I'm doing wrong in this patch series?

Nico
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