diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index acf11e83c04e..000c6721d489 100644
--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -15696,7 +15696,7 @@ ATPostAlterTypeParse(Oid oldId, Oid oldRelId, Oid refRelId, char *cmd,
 		{
 			AlterDomainStmt *stmt = (AlterDomainStmt *) stm;
 
-			if (stmt->subtype == 'C')	/* ADD CONSTRAINT */
+			if (stmt->subtype == AD_AddConstraint)
 			{
 				Constraint *con = castNode(Constraint, stmt->def);
 				AlterTableCmd *cmd = makeNode(AlterTableCmd);
diff --git a/src/backend/parser/gram.y b/src/backend/parser/gram.y
index 0b5652071d11..103021c0947f 100644
--- a/src/backend/parser/gram.y
+++ b/src/backend/parser/gram.y
@@ -11629,7 +11629,7 @@ AlterDomainStmt:
 				{
 					AlterDomainStmt *n = makeNode(AlterDomainStmt);
 
-					n->subtype = 'T';
+					n->subtype = AD_AlterDefault;
 					n->typeName = $3;
 					n->def = $4;
 					$$ = (Node *) n;
@@ -11639,7 +11639,7 @@ AlterDomainStmt:
 				{
 					AlterDomainStmt *n = makeNode(AlterDomainStmt);
 
-					n->subtype = 'N';
+					n->subtype = AD_DropNotNull;
 					n->typeName = $3;
 					$$ = (Node *) n;
 				}
@@ -11648,7 +11648,7 @@ AlterDomainStmt:
 				{
 					AlterDomainStmt *n = makeNode(AlterDomainStmt);
 
-					n->subtype = 'O';
+					n->subtype = AD_SetNotNull;
 					n->typeName = $3;
 					$$ = (Node *) n;
 				}
@@ -11657,7 +11657,7 @@ AlterDomainStmt:
 				{
 					AlterDomainStmt *n = makeNode(AlterDomainStmt);
 
-					n->subtype = 'C';
+					n->subtype = AD_AddConstraint;
 					n->typeName = $3;
 					n->def = $5;
 					$$ = (Node *) n;
@@ -11667,7 +11667,7 @@ AlterDomainStmt:
 				{
 					AlterDomainStmt *n = makeNode(AlterDomainStmt);
 
-					n->subtype = 'X';
+					n->subtype = AD_DropConstraint;
 					n->typeName = $3;
 					n->name = $6;
 					n->behavior = $7;
@@ -11679,7 +11679,7 @@ AlterDomainStmt:
 				{
 					AlterDomainStmt *n = makeNode(AlterDomainStmt);
 
-					n->subtype = 'X';
+					n->subtype = AD_DropConstraint;
 					n->typeName = $3;
 					n->name = $8;
 					n->behavior = $9;
@@ -11691,7 +11691,7 @@ AlterDomainStmt:
 				{
 					AlterDomainStmt *n = makeNode(AlterDomainStmt);
 
-					n->subtype = 'V';
+					n->subtype = AD_ValidateConstraint;
 					n->typeName = $3;
 					n->name = $6;
 					$$ = (Node *) n;
diff --git a/src/backend/tcop/utility.c b/src/backend/tcop/utility.c
index 25fe3d580166..aff8510755f3 100644
--- a/src/backend/tcop/utility.c
+++ b/src/backend/tcop/utility.c
@@ -1343,7 +1343,7 @@ ProcessUtilitySlow(ParseState *pstate,
 					 */
 					switch (stmt->subtype)
 					{
-						case 'T':	/* ALTER DOMAIN DEFAULT */
+						case AD_AlterDefault:
 
 							/*
 							 * Recursively alter column default for table and,
@@ -1353,30 +1353,30 @@ ProcessUtilitySlow(ParseState *pstate,
 								AlterDomainDefault(stmt->typeName,
 												   stmt->def);
 							break;
-						case 'N':	/* ALTER DOMAIN DROP NOT NULL */
+						case AD_DropNotNull:
 							address =
 								AlterDomainNotNull(stmt->typeName,
 												   false);
 							break;
-						case 'O':	/* ALTER DOMAIN SET NOT NULL */
+						case AD_SetNotNull:
 							address =
 								AlterDomainNotNull(stmt->typeName,
 												   true);
 							break;
-						case 'C':	/* ADD CONSTRAINT */
+						case AD_AddConstraint:
 							address =
 								AlterDomainAddConstraint(stmt->typeName,
 														 stmt->def,
 														 &secondaryObject);
 							break;
-						case 'X':	/* DROP CONSTRAINT */
+						case AD_DropConstraint:
 							address =
 								AlterDomainDropConstraint(stmt->typeName,
 														  stmt->name,
 														  stmt->behavior,
 														  stmt->missing_ok);
 							break;
-						case 'V':	/* VALIDATE CONSTRAINT */
+						case AD_ValidateConstraint:
 							address =
 								AlterDomainValidateConstraint(stmt->typeName,
 															  stmt->name);
diff --git a/src/include/nodes/parsenodes.h b/src/include/nodes/parsenodes.h
index dd00ab420b8a..81314ea1cf07 100644
--- a/src/include/nodes/parsenodes.h
+++ b/src/include/nodes/parsenodes.h
@@ -2536,17 +2536,20 @@ typedef struct AlterCollationStmt
  * this command.
  * ----------------------
  */
+typedef enum AlterDomainType
+{
+	AD_AlterDefault = 'T',				/* SET|DROP DEFAULT */
+	AD_DropNotNull = 'N',				/* DROP NOT NULL */
+	AD_SetNotNull = 'O',				/* SET NOT NULL */
+	AD_AddConstraint = 'C',				/* ADD CONSTRAINT */
+	AD_DropConstraint = 'X',			/* DROP CONSTRAINT */
+	AD_ValidateConstraint = 'V',		/* VALIDATE CONSTRAINT */
+} AlterDomainType;
+
 typedef struct AlterDomainStmt
 {
 	NodeTag		type;
-	char		subtype;		/*------------
-								 *	T = alter column default
-								 *	N = alter column drop not null
-								 *	O = alter column set not null
-								 *	C = add constraint
-								 *	X = drop constraint
-								 *------------
-								 */
+	AlterDomainType	subtype;	/* Type of domain alteration to apply */
 	List	   *typeName;		/* domain to work on */
 	char	   *name;			/* column or constraint name to act on */
 	Node	   *def;			/* definition of default or constraint */