Add ALTER TABLE ... ALTER CONSTRAINT ... SET [NO] INHERIT

This allows to redefine an existing non-inheritable constraint to be
inheritable, which allows to straighten up situations with NO INHERIT
constraints so that thay can become normal constraints without having to
re-verify existing data.  For existing inheritance children this may
require creating additional constraints, if they don't exist already.

It also allows to do the opposite, if only for symmetry.

Author: Suraj Kharage <suraj.kharage@enterprisedb.com>
Reviewed-by: jian he <jian.universality@gmail.com>
Discussion: https://postgr.es/m/CAF1DzPVfOW6Kk=7SSh7LbneQDJWh=PbJrEC_Wkzc24tHOyQWGg@mail.gmail.com

Author: alvherre

doc/src/sgml/ref/alter_table.sgml

@@ -59,6 +59,7 @@ ALTER TABLE [ IF EXISTS ] <replaceable class="parameter">name</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> SET [ INHERIT | NO INHERIT ]
     VALIDATE CONSTRAINT <replaceable class="parameter">constraint_name</replaceable>
     DROP CONSTRAINT [ IF EXISTS ]  <replaceable class="parameter">constraint_name</replaceable> [ RESTRICT | CASCADE ]
     DISABLE TRIGGER [ <replaceable class="parameter">trigger_name</replaceable> | ALL | USER ]
@@ -556,11 +557,31 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
     <listitem>
      <para>
       This form alters the attributes of a constraint that was previously
-      created. Currently only foreign key constraints may be altered.
+      created. Currently only foreign key constraints may be altered in
+      this fashion, but see below.
      </para>
     </listitem>
    </varlistentry>
 
+   <varlistentry id="sql-altertable-desc-alter-constraint-inherit">
+    <term><literal>ALTER CONSTRAINT ... SET INHERIT</literal></term>
+    <term><literal>ALTER CONSTRAINT ... SET NO INHERIT</literal></term>
+    <listitem>
+     <para>
+      These forms modify a inheritable constraint so that it becomes not
+      inheritable, or vice-versa. Only not-null constraints may be altered
+      in this fashion at present.
+      In addition to changing the inheritability status of the constraint,
+      in the case where a non-inheritable constraint is being marked
+      inheritable, if the table has children, an equivalent constraint
+      will be added to them. If marking an inheritable constraint as
+      non-inheritable on a table with children, then the corresponding
+      constraint on children will be marked as no longer inherited,
+      but not removed.
+      </para>
+     </listitem>
+    </varlistentry>
+
    <varlistentry id="sql-altertable-desc-validate-constraint">
     <term><literal>VALIDATE CONSTRAINT</literal></term>
     <listitem>

src/backend/commands/tablecmds.c

@@ -389,9 +389,10 @@ static void AlterIndexNamespaces(Relation classRel, Relation rel,
 static void AlterSeqNamespaces(Relation classRel, Relation rel,
 							   Oid oldNspOid, Oid newNspOid, ObjectAddresses *objsMoved,
 							   LOCKMODE lockmode);
-static ObjectAddress ATExecAlterConstraint(Relation rel, ATAlterConstraint *cmdcon,
+static ObjectAddress ATExecAlterConstraint(List **wqueue, Relation rel,
+										   ATAlterConstraint *cmdcon,
 										   bool recurse, LOCKMODE lockmode);
-static bool ATExecAlterConstraintInternal(ATAlterConstraint *cmdcon, Relation conrel,
+static bool ATExecAlterConstraintInternal(List **wqueue, ATAlterConstraint *cmdcon, Relation conrel,
 										  Relation tgrel, Relation rel, HeapTuple contuple,
 										  bool recurse, List **otherrelids, LOCKMODE lockmode);
 static void AlterConstrTriggerDeferrability(Oid conoid, Relation tgrel, Relation rel,
@@ -5437,8 +5438,8 @@ ATExecCmd(List **wqueue, AlteredTableInfo *tab,
 											   lockmode);
 			break;
 		case AT_AlterConstraint:	/* ALTER CONSTRAINT */
-			address = ATExecAlterConstraint(rel, castNode(ATAlterConstraint,
-														  cmd->def),
+			address = ATExecAlterConstraint(wqueue, rel,
+											castNode(ATAlterConstraint, cmd->def),
 											cmd->recurse, lockmode);
 			break;
 		case AT_ValidateConstraint: /* VALIDATE CONSTRAINT */
@@ -11813,14 +11814,14 @@ GetForeignKeyCheckTriggers(Relation trigrel,
  *
  * Update the attributes of a constraint.
  *
- * Currently only works for Foreign Key constraints.
+ * Currently only works for Foreign Key and not null constraints.
  *
  * If the constraint is modified, returns its address; otherwise, return
  * InvalidObjectAddress.
  */
 static ObjectAddress
-ATExecAlterConstraint(Relation rel, ATAlterConstraint *cmdcon, bool recurse,
-					  LOCKMODE lockmode)
+ATExecAlterConstraint(List **wqueue, Relation rel, ATAlterConstraint *cmdcon,
+					  bool recurse, LOCKMODE lockmode)
 {
 	Relation	conrel;
 	Relation	tgrel;
@@ -11871,11 +11872,26 @@ ATExecAlterConstraint(Relation rel, ATAlterConstraint *cmdcon, bool recurse,
 						cmdcon->conname, RelationGetRelationName(rel))));
 
 	currcon = (Form_pg_constraint) GETSTRUCT(contuple);
-	if (currcon->contype != CONSTRAINT_FOREIGN)
+	if (cmdcon->alterDeferrability && currcon->contype != CONSTRAINT_FOREIGN)
 		ereport(ERROR,
 				(errcode(ERRCODE_WRONG_OBJECT_TYPE),
 				 errmsg("constraint \"%s\" of relation \"%s\" is not a foreign key constraint",
 						cmdcon->conname, RelationGetRelationName(rel))));
+	if (cmdcon->alterInheritability &&
+		currcon->contype != CONSTRAINT_NOTNULL)
+		ereport(ERROR,
+				errcode(ERRCODE_WRONG_OBJECT_TYPE),
+				errmsg("constraint \"%s\" of relation \"%s\" is not a not-null constraint",
+					   cmdcon->conname, RelationGetRelationName(rel)));
+
+	/* Refuse to modify inheritability of inherited constraints */
+	if (cmdcon->alterInheritability &&
+		cmdcon->noinherit && currcon->coninhcount > 0)
+		ereport(ERROR,
+				errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+				errmsg("cannot alter inherited constraint \"%s\" on relation \"%s\"",
+					   NameStr(currcon->conname),
+					   RelationGetRelationName(rel)));
 
 	/*
 	 * If it's not the topmost constraint, raise an error.
@@ -11926,8 +11942,8 @@ ATExecAlterConstraint(Relation rel, ATAlterConstraint *cmdcon, bool recurse,
 	/*
 	 * Do the actual catalog work, and recurse if necessary.
 	 */
-	if (ATExecAlterConstraintInternal(cmdcon, conrel, tgrel, rel, contuple,
-									  recurse, &otherrelids, lockmode))
+	if (ATExecAlterConstraintInternal(wqueue, cmdcon, conrel, tgrel, rel,
+									  contuple, recurse, &otherrelids, lockmode))
 		ObjectAddressSet(address, ConstraintRelationId, currcon->oid);
 
 	/*
@@ -11958,9 +11974,10 @@ ATExecAlterConstraint(Relation rel, ATAlterConstraint *cmdcon, bool recurse,
  * but existing releases don't do that.)
  */
 static bool
-ATExecAlterConstraintInternal(ATAlterConstraint *cmdcon, Relation conrel,
-							  Relation tgrel, Relation rel, HeapTuple contuple,
-							  bool recurse, List **otherrelids, LOCKMODE lockmode)
+ATExecAlterConstraintInternal(List **wqueue, ATAlterConstraint *cmdcon,
+							  Relation conrel, Relation tgrel, Relation rel,
+							  HeapTuple contuple, bool recurse,
+							  List **otherrelids, LOCKMODE lockmode)
 {
 	Form_pg_constraint currcon;
 	Oid			refrelid = InvalidOid;
@@ -12040,14 +12057,82 @@ ATExecAlterConstraintInternal(ATAlterConstraint *cmdcon, Relation conrel,
 			Relation	childrel;
 
 			childrel = table_open(childcon->conrelid, lockmode);
-			ATExecAlterConstraintInternal(cmdcon, conrel, tgrel, childrel, childtup,
-										  recurse, otherrelids, lockmode);
+			ATExecAlterConstraintInternal(wqueue, cmdcon, conrel, tgrel, childrel,
+										  childtup, recurse, otherrelids, lockmode);
 			table_close(childrel, NoLock);
 		}
 
 		systable_endscan(pscan);
 	}
 
+	/*
+	 * Update the catalog for inheritability.  No work if the constraint is
+	 * already in the requested state.
+	 */
+	if (cmdcon->alterInheritability &&
+		(cmdcon->noinherit != currcon->connoinherit))
+	{
+		AttrNumber	colNum;
+		char	   *colName;
+		List	   *children;
+		HeapTuple	copyTuple;
+		Form_pg_constraint copy_con;
+
+		/* The current implementation only works for NOT NULL constraints */
+		Assert(currcon->contype == CONSTRAINT_NOTNULL);
+
+		copyTuple = heap_copytuple(contuple);
+		copy_con = (Form_pg_constraint) GETSTRUCT(copyTuple);
+		copy_con->connoinherit = cmdcon->noinherit;
+
+		CatalogTupleUpdate(conrel, &copyTuple->t_self, copyTuple);
+		CommandCounterIncrement();
+		heap_freetuple(copyTuple);
+		changed = true;
+
+		/* Fetch the column number and name */
+		colNum = extractNotNullColumn(contuple);
+		colName = get_attname(currcon->conrelid, colNum, false);
+
+		/*
+		 * Propagate the change to children.  For SET NO INHERIT, we don't
+		 * recursively affect children, just the immediate level.
+		 */
+		children = find_inheritance_children(RelationGetRelid(rel),
+											 lockmode);
+		foreach_oid(childoid, children)
+		{
+			ObjectAddress addr;
+
+			if (cmdcon->noinherit)
+			{
+				HeapTuple	childtup;
+				Form_pg_constraint childcon;
+
+				childtup = findNotNullConstraint(childoid, colName);
+				if (!childtup)
+					elog(ERROR, "cache lookup failed for not-null constraint on column \"%s\" of relation %u",
+						 colName, childoid);
+				childcon = (Form_pg_constraint) GETSTRUCT(childtup);
+				Assert(childcon->coninhcount > 0);
+				childcon->coninhcount--;
+				childcon->conislocal = true;
+				CatalogTupleUpdate(conrel, &childtup->t_self, childtup);
+				heap_freetuple(childtup);
+			}
+			else
+			{
+				Relation	childrel = table_open(childoid, NoLock);
+
+				addr = ATExecSetNotNull(wqueue, childrel, NameStr(currcon->conname),
+										colName, true, true, lockmode);
+				if (OidIsValid(addr.objectId))
+					CommandCounterIncrement();
+				table_close(childrel, NoLock);
+			}
+		}
+	}
+
 	return changed;
 }
 

src/backend/parser/gram.y

@@ -2669,6 +2669,34 @@ alter_table_cmd:
 									NULL, NULL, NULL, yyscanner);
 					$$ = (Node *) n;
 				}
+			/* ALTER TABLE <name> ALTER CONSTRAINT SET INHERIT */
+			| ALTER CONSTRAINT name SET INHERIT
+				{
+					AlterTableCmd *n = makeNode(AlterTableCmd);
+					ATAlterConstraint *c = makeNode(ATAlterConstraint);
+
+					n->subtype = AT_AlterConstraint;
+					n->def = (Node *) c;
+					c->conname = $3;
+					c->alterInheritability = true;
+					c->noinherit = false;
+
+					$$ = (Node *) n;
+				}
+			/* ALTER TABLE <name> ALTER CONSTRAINT SET NO INHERIT */
+			| ALTER CONSTRAINT name SET NO INHERIT
+				{
+					AlterTableCmd *n = makeNode(AlterTableCmd);
+					ATAlterConstraint *c = makeNode(ATAlterConstraint);
+
+					n->subtype = AT_AlterConstraint;
+					n->def = (Node *) c;
+					c->conname = $3;
+					c->alterInheritability = true;
+					c->noinherit = true;
+
+					$$ = (Node *) n;
+				}
 			/* ALTER TABLE <name> VALIDATE CONSTRAINT ... */
 			| VALIDATE CONSTRAINT name
 				{

src/include/nodes/parsenodes.h

@@ -2493,6 +2493,8 @@ typedef struct ATAlterConstraint
 	bool		alterDeferrability; /* changing deferrability properties? */
 	bool		deferrable;		/* DEFERRABLE? */
 	bool		initdeferred;	/* INITIALLY DEFERRED? */
+	bool		alterInheritability;	/* changing inheritability properties */
+	bool		noinherit;
 } ATAlterConstraint;
 
 /* Ad-hoc node for AT_ReplicaIdentity */