pgsql: Extend ALTER TABLE to allow Foreign Keys to be added without ini

Extend ALTER TABLE to allow Foreign Keys to be added without initial validation.
FK constraints that are marked NOT VALID may later be VALIDATED, which uses an
ShareUpdateExclusiveLock on constraint table and RowShareLock on referenced
table. Significantly reduces lock strength and duration when adding FKs.
New state visible from psql.

Simon Riggs, with reviews from Marko Tiikkaja and Robert Haas

Branch
------
master

Details
-------
http://git.postgresql.org/gitweb?p=postgresql.git;a=commitdiff;h=722bf7017bbe796decc79c1fde03e7a83dae9ada

Modified Files
--------------
doc/src/sgml/ref/alter_table.sgml | 22 ++++++-
src/backend/catalog/heap.c | 1 +
src/backend/catalog/index.c | 1 +
src/backend/catalog/pg_constraint.c | 2 +
src/backend/commands/tablecmds.c | 105 +++++++++++++++++++++++++++--
src/backend/commands/trigger.c | 1 +
src/backend/commands/typecmds.c | 1 +
src/backend/parser/gram.y | 28 +++++++-
src/backend/utils/adt/ri_triggers.c | 7 ++-
src/bin/psql/describe.c | 9 ++-
src/include/catalog/pg_constraint.h | 40 ++++++-----
src/include/nodes/parsenodes.h | 2 +
src/include/parser/kwlist.h | 1 +
src/test/regress/expected/alter_table.out | 12 +++
src/test/regress/sql/alter_table.sql | 15 ++++
15 files changed, 215 insertions(+), 32 deletions(-)