Improve ALTER DOMAIN / DROP CONSTRAINT with nonexistent constraint

ALTER DOMAIN / DROP CONSTRAINT on a nonexistent constraint name did
not report any error.  Now it reports an error.  The IF EXISTS option
was added to get the usual behavior of ignoring nonexistent objects to
drop.

doc/src/sgml/ref/alter_domain.sgml

@@ -30,7 +30,7 @@ ALTER DOMAIN <replaceable class="PARAMETER">name</replaceable>
 ALTER DOMAIN <replaceable class="PARAMETER">name</replaceable>
     ADD <replaceable class="PARAMETER">domain_constraint</replaceable> [ NOT VALID ]
 ALTER DOMAIN <replaceable class="PARAMETER">name</replaceable>
-    DROP CONSTRAINT <replaceable class="PARAMETER">constraint_name</replaceable> [ RESTRICT | CASCADE ]
+    DROP CONSTRAINT [ IF EXISTS ] <replaceable class="PARAMETER">constraint_name</replaceable> [ RESTRICT | CASCADE ]
 ALTER DOMAIN <replaceable class="PARAMETER">name</replaceable>
     VALIDATE CONSTRAINT <replaceable class="PARAMETER">constraint_name</replaceable>
 ALTER DOMAIN <replaceable class="PARAMETER">name</replaceable>
@@ -92,10 +92,12 @@ ALTER DOMAIN <replaceable class="PARAMETER">name</replaceable>
    </varlistentry>
 
    <varlistentry>
-    <term>DROP CONSTRAINT</term>
+    <term>DROP CONSTRAINT [ IF EXISTS ]</term>
     <listitem>
      <para>
       This form drops constraints on a domain.
+      If <literal>IF EXISTS</literal> is specified and the constraint
+      does not exist, no error is thrown. In this case a notice is issued instead.
      </para>
     </listitem>
    </varlistentry>

src/backend/commands/typecmds.c

@@ -2264,7 +2264,7 @@ AlterDomainNotNull(List *names, bool notNull)
  */
 void
 AlterDomainDropConstraint(List *names, const char *constrName,
-						  DropBehavior behavior)
+						  DropBehavior behavior, bool missing_ok)
 {
 	TypeName   *typename;
 	Oid			domainoid;
@@ -2274,6 +2274,7 @@ AlterDomainDropConstraint(List *names, const char *constrName,
 	SysScanDesc conscan;
 	ScanKeyData key[1];
 	HeapTuple	contup;
+	bool		found = false;
 
 	/* Make a TypeName so we can use standard type lookup machinery */
 	typename = makeTypeNameFromNameList(names);
@@ -2317,6 +2318,7 @@ AlterDomainDropConstraint(List *names, const char *constrName,
 			conobj.objectSubId = 0;
 
 			performDeletion(&conobj, behavior);
+			found = true;
 		}
 	}
 	/* Clean up after the scan */
@@ -2324,6 +2326,19 @@ AlterDomainDropConstraint(List *names, const char *constrName,
 	heap_close(conrel, RowExclusiveLock);
 
 	heap_close(rel, NoLock);
+
+	if (!found)
+	{
+		if (!missing_ok)
+			ereport(ERROR,
+					(errcode(ERRCODE_UNDEFINED_OBJECT),
+					 errmsg("constraint \"%s\" of domain \"%s\" does not exist",
+					   constrName, TypeNameToString(typename))));
+		else
+			ereport(NOTICE,
+					(errmsg("constraint \"%s\" of domain \"%s\" does not exist, skipping",
+							constrName, TypeNameToString(typename))));
+	}
 }
 
 /*

src/backend/nodes/copyfuncs.c

@@ -2568,6 +2568,7 @@ _copyAlterDomainStmt(const AlterDomainStmt *from)
 	COPY_STRING_FIELD(name);
 	COPY_NODE_FIELD(def);
 	COPY_SCALAR_FIELD(behavior);
+	COPY_SCALAR_FIELD(missing_ok);
 
 	return newnode;
 }

src/backend/nodes/equalfuncs.c

@@ -1034,6 +1034,7 @@ _equalAlterDomainStmt(const AlterDomainStmt *a, const AlterDomainStmt *b)
 	COMPARE_STRING_FIELD(name);
 	COMPARE_NODE_FIELD(def);
 	COMPARE_SCALAR_FIELD(behavior);
+	COMPARE_SCALAR_FIELD(missing_ok);
 
 	return true;
 }

src/backend/parser/gram.y

@@ -7616,6 +7616,18 @@ AlterDomainStmt:
 					n->typeName = $3;
 					n->name = $6;
 					n->behavior = $7;
+					n->missing_ok = false;
+					$$ = (Node *)n;
+				}
+			/* ALTER DOMAIN <domain> DROP CONSTRAINT IF EXISTS <name> [RESTRICT|CASCADE] */
+			| ALTER DOMAIN_P any_name DROP CONSTRAINT IF_P EXISTS name opt_drop_behavior
+				{
+					AlterDomainStmt *n = makeNode(AlterDomainStmt);
+					n->subtype = 'X';
+					n->typeName = $3;
+					n->name = $8;
+					n->behavior = $9;
+					n->missing_ok = true;
 					$$ = (Node *)n;
 				}
 			/* ALTER DOMAIN <domain> VALIDATE CONSTRAINT <name> */

src/backend/tcop/utility.c

@@ -768,7 +768,8 @@ standard_ProcessUtility(Node *parsetree,
 					case 'X':	/* DROP CONSTRAINT */
 						AlterDomainDropConstraint(stmt->typeName,
 												  stmt->name,
-												  stmt->behavior);
+												  stmt->behavior,
+												  stmt->missing_ok);
 						break;
 					case 'V':	/* VALIDATE CONSTRAINT */
 						AlterDomainValidateConstraint(stmt->typeName,

src/include/commands/typecmds.h

@@ -33,7 +33,7 @@ extern void AlterDomainNotNull(List *names, bool notNull);
 extern void AlterDomainAddConstraint(List *names, Node *constr);
 extern void AlterDomainValidateConstraint(List *names, char *constrName);
 extern void AlterDomainDropConstraint(List *names, const char *constrName,
-						  DropBehavior behavior);
+									  DropBehavior behavior, bool missing_ok);
 
 extern List *GetDomainConstraints(Oid typeOid);
 

src/include/nodes/parsenodes.h

@@ -1264,6 +1264,7 @@ typedef struct AlterDomainStmt
 	char	   *name;			/* column or constraint name to act on */
 	Node	   *def;			/* definition of default or constraint */
 	DropBehavior behavior;		/* RESTRICT or CASCADE for DROP cases */
+	bool		missing_ok;		/* skip error if missing? */
 } AlterDomainStmt;
 
 

src/test/regress/expected/domain.out

@@ -358,6 +358,10 @@ alter domain con drop constraint t;
 insert into domcontest values (-5); --fails
 ERROR:  value for domain con violates check constraint "con_check"
 insert into domcontest values (42);
+alter domain con drop constraint nonexistent;
+ERROR:  constraint "nonexistent" of domain "con" does not exist
+alter domain con drop constraint if exists nonexistent;
+NOTICE:  constraint "nonexistent" of domain "con" does not exist, skipping
 -- Test ALTER DOMAIN .. CONSTRAINT .. NOT VALID
 create domain things AS INT;
 CREATE TABLE thethings (stuff things);

src/test/regress/sql/domain.sql

@@ -259,6 +259,9 @@ alter domain con drop constraint t;
 insert into domcontest values (-5); --fails
 insert into domcontest values (42);
 
+alter domain con drop constraint nonexistent;
+alter domain con drop constraint if exists nonexistent;
+
 -- Test ALTER DOMAIN .. CONSTRAINT .. NOT VALID
 create domain things AS INT;
 CREATE TABLE thethings (stuff things);