I noticed that the subtype of AlterDomainStmt is directly using constants in the code. It is not conducive to the maintenance and reading of the code. Based on the definition of AlterTableType, use "AD_" as the prefix. Define several macros to replace the original characters. The subtype of AlterTableCmd is defined using an enumeration. The subtypes of AlterDomainStmt are relatively few in number, and the original definition uses characters. These definitions still use characters and maintain the values unchanged. If some plugins or tools are also processing AlterDomainStmt, there will be no errors.

--
Quan Zongliang
diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index 54ad38247aa..3f09f85a480 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 0b5652071d1..391e70b594c 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_VaidateConstraint;
                                        n->typeName = $3;
                                        n->name = $6;
                                        $$ = (Node *) n;
diff --git a/src/backend/tcop/utility.c b/src/backend/tcop/utility.c
index 25fe3d58016..5c0a4eb53cb 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_VaidateConstraint:
                                                        address =
                                                                
AlterDomainValidateConstraint(stmt->typeName,
                                                                                
                                          stmt->name);
diff --git a/src/include/nodes/parsenodes.h b/src/include/nodes/parsenodes.h
index 4610fc61293..7349918e35e 100644
--- a/src/include/nodes/parsenodes.h
+++ b/src/include/nodes/parsenodes.h
@@ -2534,14 +2534,7 @@ typedef struct AlterCollationStmt
 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
-                                                                *------------
-                                                                */
+       char            subtype;                /* see AD_xxx constants below */
        List       *typeName;           /* domain to work on */
        char       *name;                       /* column or constraint name to 
act on */
        Node       *def;                        /* definition of default or 
constraint */
@@ -2549,6 +2542,13 @@ typedef struct AlterDomainStmt
        bool            missing_ok;             /* skip error if missing? */
 } AlterDomainStmt;
 
+/* AlterDomainStmt action */
+#define AD_AlterDefault                                'T'                     
/* SET|DROP DEFAULT */
+#define AD_DropNotNull                         'N'                     /* DROP 
NOT NULL */
+#define AD_SetNotNull                          'O'                     /* SET 
NOT NULL */
+#define AD_AddConstraint                       'C'                     /* ADD 
CONSTRAINT */
+#define AD_DropConstraint                      'X'                     /* DROP 
CONSTRAINT */
+#define AD_VaidateConstraint           'V'                     /* VALIDATE 
CONSTRAINT */
 
 /* ----------------------
  *             Grant|Revoke Statement

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