Get rid of ParseState.p_value_substitute; use a columnref hook instead.

I noticed that p_value_substitute, which is a single-purpose kluge I added
in 2002 (commit b0422b21), could be replaced by having domainAddConstraint
install a parser hook that looks for the name "value".  The parser hook
code only dates back to 2009, so it's not surprising that we had to kluge
this in 2002, but we can do it more cleanly now.

src/backend/commands/typecmds.c

@@ -104,6 +104,8 @@ static char *domainAddConstraint(Oid domainOid, Oid domainNamespace,
 					Oid baseTypeOid,
 					int typMod, Constraint *constr,
 					char *domainName, ObjectAddress *constrAddr);
+static Node *replace_domain_constraint_value(ParseState *pstate,
+								ColumnRef *cref);
 
 
 /*
@@ -3022,7 +3024,8 @@ domainAddConstraint(Oid domainOid, Oid domainNamespace, Oid baseTypeOid,
 	domVal->collation = get_typcollation(baseTypeOid);
 	domVal->location = -1;		/* will be set when/if used */
 
-	pstate->p_value_substitute = (Node *) domVal;
+	pstate->p_pre_columnref_hook = replace_domain_constraint_value;
+	pstate->p_ref_hook_state = (void *) domVal;
 
 	expr = transformExpr(pstate, constr->raw_expr, EXPR_KIND_DOMAIN_CHECK);
 
@@ -3099,6 +3102,35 @@ domainAddConstraint(Oid domainOid, Oid domainNamespace, Oid baseTypeOid,
 	return ccbin;
 }
 
+/* Parser pre_columnref_hook for domain CHECK constraint parsing */
+static Node *
+replace_domain_constraint_value(ParseState *pstate, ColumnRef *cref)
+{
+	/*
+	 * Check for a reference to "value", and if that's what it is, replace
+	 * with a CoerceToDomainValue as prepared for us by domainAddConstraint.
+	 * (We handle VALUE as a name, not a keyword, to avoid breaking a lot of
+	 * applications that have used VALUE as a column name in the past.)
+	 */
+	if (list_length(cref->fields) == 1)
+	{
+		Node	   *field1 = (Node *) linitial(cref->fields);
+		char	   *colname;
+
+		Assert(IsA(field1, String));
+		colname = strVal(field1);
+		if (strcmp(colname, "value") == 0)
+		{
+			CoerceToDomainValue *domVal = copyObject(pstate->p_ref_hook_state);
+
+			/* Propagate location knowledge, if any */
+			domVal->location = cref->location;
+			return (Node *) domVal;
+		}
+	}
+	return NULL;
+}
+
 
 /*
  * Execute ALTER TYPE RENAME

src/backend/parser/parse_expr.c

@@ -577,27 +577,6 @@ transformColumnRef(ParseState *pstate, ColumnRef *cref)
 					/*
 					 * Not known as a column of any range-table entry.
 					 *
-					 * Consider the possibility that it's VALUE in a domain
-					 * check expression.  (We handle VALUE as a name, not a
-					 * keyword, to avoid breaking a lot of applications that
-					 * have used VALUE as a column name in the past.)
-					 */
-					if (pstate->p_value_substitute != NULL &&
-						strcmp(colname, "value") == 0)
-					{
-						node = (Node *) copyObject(pstate->p_value_substitute);
-
-						/*
-						 * Try to propagate location knowledge.  This should
-						 * be extended if p_value_substitute can ever take on
-						 * other node types.
-						 */
-						if (IsA(node, CoerceToDomainValue))
-							((CoerceToDomainValue *) node)->location = cref->location;
-						break;
-					}
-
-					/*
 					 * Try to find the name as a relation.  Note that only
 					 * relations already entered into the rangetable will be
 					 * recognized.

src/include/parser/parse_node.h

@@ -149,9 +149,6 @@ typedef Node *(*CoerceParamHook) (ParseState *pstate, Param *param,
  * p_locked_from_parent: true if parent query level applies FOR UPDATE/SHARE
  * to this subquery as a whole.
  *
- * p_value_substitute: replacement for VALUE references, if we're parsing
- * a domain CHECK constraint.
- *
  * p_hasAggs, p_hasWindowFuncs, etc: true if we've found any of the indicated
  * constructs in the query.
  *
@@ -184,7 +181,6 @@ struct ParseState
 	List	   *p_locking_clause;		/* raw FOR UPDATE/FOR SHARE info */
 	bool		p_locked_from_parent;	/* parent has marked this subquery
 										 * with FOR UPDATE/FOR SHARE */
-	Node	   *p_value_substitute;		/* what to replace VALUE with, if any */
 
 	/* Flags telling about things found in the query: */
 	bool		p_hasAggs;