From 471fda80c41fae835ecbe63ae8505526a37487a9 Mon Sep 17 00:00:00 2001
From: Peter Eisentraut <peter@eisentraut.org>
Date: Wed, 12 Jul 2023 16:12:35 +0200
Subject: [PATCH v2 04/10] Add TupleDescGetDefault()

This unifies some repetitive code.

Note: I didn't push the "not found" error message into the new
function, even though all existing callers would be able to make use
of it.  Using the existing error handling as-is would probably require
exposing the Relation type via tupdesc.h, which doesn't seem
desirable.
---
 src/backend/access/common/tupdesc.c  | 25 +++++++++++++++++++++++++
 src/backend/commands/tablecmds.c     | 17 ++---------------
 src/backend/parser/parse_utilcmd.c   | 13 ++-----------
 src/backend/rewrite/rewriteHandler.c | 16 +---------------
 src/include/access/tupdesc.h         |  2 ++
 5 files changed, 32 insertions(+), 41 deletions(-)

diff --git a/src/backend/access/common/tupdesc.c b/src/backend/access/common/tupdesc.c
index 253d6c86f8..ce2c7bce85 100644
--- a/src/backend/access/common/tupdesc.c
+++ b/src/backend/access/common/tupdesc.c
@@ -927,3 +927,28 @@ BuildDescFromLists(const List *names, const List *types, const List *typmods, co
 
 	return desc;
 }
+
+/*
+ * Get default expression (or NULL if none) for the given attribute number.
+ */
+Node *
+TupleDescGetDefault(TupleDesc tupdesc, AttrNumber attnum)
+{
+	Node	   *result = NULL;
+
+	if (tupdesc->constr)
+	{
+		AttrDefault *attrdef = tupdesc->constr->defval;
+
+		for (int i = 0; i < tupdesc->constr->num_defval; i++)
+		{
+			if (attrdef[i].adnum == attnum)
+			{
+				result = stringToNode(attrdef[i].adbin);
+				break;
+			}
+		}
+	}
+
+	return result;
+}
diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index fb6d1b90dc..0a7101dadc 100644
--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -2796,22 +2796,9 @@ MergeAttributes(List *columns, const List *supers, char relpersistence,
 			 */
 			if (attribute->atthasdef)
 			{
-				Node	   *this_default = NULL;
+				Node	   *this_default;
 
-				/* Find default in constraint structure */
-				if (constr != NULL)
-				{
-					AttrDefault *attrdef = constr->defval;
-
-					for (int i = 0; i < constr->num_defval; i++)
-					{
-						if (attrdef[i].adnum == parent_attno)
-						{
-							this_default = stringToNode(attrdef[i].adbin);
-							break;
-						}
-					}
-				}
+				this_default = TupleDescGetDefault(tupleDesc, parent_attno);
 				if (this_default == NULL)
 					elog(ERROR, "default expression not found for attribute %d of relation \"%s\"",
 						 parent_attno, RelationGetRelationName(relation));
diff --git a/src/backend/parser/parse_utilcmd.c b/src/backend/parser/parse_utilcmd.c
index 55c315f0e2..cf0d432ab1 100644
--- a/src/backend/parser/parse_utilcmd.c
+++ b/src/backend/parser/parse_utilcmd.c
@@ -1358,20 +1358,11 @@ expandTableLikeClause(RangeVar *heapRel, TableLikeClause *table_like_clause)
 				 (table_like_clause->options & CREATE_TABLE_LIKE_GENERATED) :
 				 (table_like_clause->options & CREATE_TABLE_LIKE_DEFAULTS)))
 			{
-				Node	   *this_default = NULL;
-				AttrDefault *attrdef = constr->defval;
+				Node	   *this_default;
 				AlterTableCmd *atsubcmd;
 				bool		found_whole_row;
 
-				/* Find default in constraint structure */
-				for (int i = 0; i < constr->num_defval; i++)
-				{
-					if (attrdef[i].adnum == parent_attno)
-					{
-						this_default = stringToNode(attrdef[i].adbin);
-						break;
-					}
-				}
+				this_default = TupleDescGetDefault(tupleDesc, parent_attno);
 				if (this_default == NULL)
 					elog(ERROR, "default expression not found for attribute %d of relation \"%s\"",
 						 parent_attno, RelationGetRelationName(relation));
diff --git a/src/backend/rewrite/rewriteHandler.c b/src/backend/rewrite/rewriteHandler.c
index b486ab559a..41a362310a 100644
--- a/src/backend/rewrite/rewriteHandler.c
+++ b/src/backend/rewrite/rewriteHandler.c
@@ -1246,21 +1246,7 @@ build_column_default(Relation rel, int attrno)
 	 */
 	if (att_tup->atthasdef)
 	{
-		if (rd_att->constr && rd_att->constr->num_defval > 0)
-		{
-			AttrDefault *defval = rd_att->constr->defval;
-			int			ndef = rd_att->constr->num_defval;
-
-			while (--ndef >= 0)
-			{
-				if (attrno == defval[ndef].adnum)
-				{
-					/* Found it, convert string representation to node tree. */
-					expr = stringToNode(defval[ndef].adbin);
-					break;
-				}
-			}
-		}
+		expr = TupleDescGetDefault(rd_att, attrno);
 		if (expr == NULL)
 			elog(ERROR, "default expression not found for attribute %d of relation \"%s\"",
 				 attrno, RelationGetRelationName(rel));
diff --git a/src/include/access/tupdesc.h b/src/include/access/tupdesc.h
index f6cc28a661..ffd2874ee3 100644
--- a/src/include/access/tupdesc.h
+++ b/src/include/access/tupdesc.h
@@ -151,4 +151,6 @@ extern TupleDesc BuildDescForRelation(const List *columns);
 
 extern TupleDesc BuildDescFromLists(const List *names, const List *types, const List *typmods, const List *collations);
 
+extern Node *TupleDescGetDefault(TupleDesc tupdesc, AttrNumber attnum);
+
 #endif							/* TUPDESC_H */
-- 
2.41.0