Add support for NOT ENFORCED in foreign key constraints.

Typically, when a foreign key (FK) constraint is created on a table,
action and check triggers are added to maintain data integrity.  With
this patch, if a constraint is marked as NOT ENFORCED, integrity
checks are no longer required, making these triggers unnecessary.
Consequently, when creating a NOT ENFORCED FK constraint, triggers
will not be created, and the constraint will be marked as NOT VALID.
Similarly, if an existing FK constraint is changed to NOT ENFORCED,
the associated triggers will be dropped, and the constraint will also
be marked as NOT VALID. Conversely, if a NOT ENFORCED FK constraint is
changed to ENFORCED, the necessary triggers will be created, and the
will be changed to VALID by performing necessary validation.

----
NOTE: In this patch, the tryAttachPartitionForeignKey() function will
not merge the constraint if the enforcibility differs. This will be
addressed in the next patch.
----

Author: amulsul

doc/src/sgml/catalogs.sgml

@@ -2620,7 +2620,7 @@ SCRAM-SHA-256$<replaceable>&lt;iteration count&gt;</replaceable>:<replaceable>&l
       </para>
       <para>
        Is the constraint enforced?
-       Currently, can be false only for CHECK constraints
+       Currently, can be false only for foreign keys and CHECK constraints
       </para></entry>
      </row>
 

doc/src/sgml/ref/alter_table.sgml

@@ -58,7 +58,7 @@ ALTER TABLE [ IF EXISTS ] <replaceable class="parameter">name</replaceable>
     ALTER [ COLUMN ] <replaceable class="parameter">column_name</replaceable> SET COMPRESSION <replaceable class="parameter">compression_method</replaceable>
     ADD <replaceable class="parameter">table_constraint</replaceable> [ NOT VALID ]
     ADD <replaceable class="parameter">table_constraint_using_index</replaceable>
-    ALTER CONSTRAINT <replaceable class="parameter">constraint_name</replaceable> [ DEFERRABLE | NOT DEFERRABLE ] [ INITIALLY DEFERRED | INITIALLY IMMEDIATE ]
+    ALTER CONSTRAINT <replaceable class="parameter">constraint_name</replaceable> [ DEFERRABLE | NOT DEFERRABLE ] [ INITIALLY DEFERRED | INITIALLY IMMEDIATE ] [ ENFORCED | NOT ENFORCED ]
     ALTER CONSTRAINT <replaceable class="parameter">constraint_name</replaceable> [ INHERIT | NO INHERIT ]
     VALIDATE CONSTRAINT <replaceable class="parameter">constraint_name</replaceable>
     DROP CONSTRAINT [ IF EXISTS ]  <replaceable class="parameter">constraint_name</replaceable> [ RESTRICT | CASCADE ]
@@ -589,7 +589,8 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
       This form validates a foreign key or check constraint that was
       previously created as <literal>NOT VALID</literal>, by scanning the
       table to ensure there are no rows for which the constraint is not
-      satisfied.  Nothing happens if the constraint is already marked valid.
+      satisfied.  If the constraint is not enforced, an error is thrown.
+      Nothing happens if the constraint is already marked valid.
       (See <xref linkend="sql-altertable-notes"/> below for an explanation
       of the usefulness of this command.)
      </para>

doc/src/sgml/ref/create_table.sgml

@@ -1409,7 +1409,7 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
      </para>
 
      <para>
-      This is currently only supported for <literal>CHECK</literal>
+      This is currently only supported for foreign key and <literal>CHECK</literal>
       constraints.
      </para>
     </listitem>

src/backend/catalog/pg_constraint.c

@@ -100,8 +100,9 @@ CreateConstraintEntry(const char *constraintName,
 	ObjectAddresses *addrs_auto;
 	ObjectAddresses *addrs_normal;
 
-	/* Only CHECK constraint can be not enforced */
-	Assert(isEnforced || constraintType == CONSTRAINT_CHECK);
+	/* Only CHECK or FOREIGN KEY constraint can be not enforced */
+	Assert(isEnforced || constraintType == CONSTRAINT_CHECK ||
+		   constraintType == CONSTRAINT_FOREIGN);
 	/* NOT ENFORCED constraint must be NOT VALID */
 	Assert(isEnforced || !isValidated);
 

src/backend/catalog/sql_features.txt

@@ -281,7 +281,7 @@ F461	Named character sets			NO
 F471	Scalar subquery values			YES	
 F481	Expanded NULL predicate			YES	
 F491	Constraint management			YES	
-F492	Optional table constraint enforcement			NO	check constraints only
+F492	Optional table constraint enforcement			YES	except not-null constraints
 F501	Features and conformance views			YES	
 F501	Features and conformance views	01	SQL_FEATURES view	YES	
 F501	Features and conformance views	02	SQL_SIZING view	YES