From f685d2b6dd5dbc7a34234b6b3cdbd9f2aef1c29d Mon Sep 17 00:00:00 2001
From: jian he <jian.universality@gmail.com>
Date: Mon, 16 Dec 2024 20:22:16 +0800
Subject: [PATCH v10 1/1] make ALTER TABLE SET EXPRESSION an no-op if not
 changed

if new and old generated expression is the same,
make ALTER TABLE ALTER COLUMN SET EXPRESSION a no-op.
---
 src/backend/commands/tablecmds.c | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index 28623672fa..effa3767e4 100644
--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -8407,6 +8407,11 @@ ATExecSetExpression(AlteredTableInfo *tab, Relation rel, const char *colName,
 	Expr	   *defval;
 	NewColumnValue *newval;
 	RawColumnDefault *rawEnt;
+	ParseState *pstate;
+	Node	   *expr;
+	Node	   *old_default;
+	TupleDesc	tupdesc = RelationGetDescr(rel);
+	ParseNamespaceItem *nsitem;
 
 	tuple = SearchSysCacheAttName(RelationGetRelid(rel), colName);
 	if (!HeapTupleIsValid(tuple))
@@ -8462,6 +8467,24 @@ ATExecSetExpression(AlteredTableInfo *tab, Relation rel, const char *colName,
 
 	ReleaseSysCache(tuple);
 
+	old_default = TupleDescGetDefault(tupdesc, attnum);
+	pstate = make_parsestate(NULL);
+	pstate->p_sourcetext = NULL;
+	nsitem = addRangeTableEntryForRelation(pstate,
+										   rel,
+										   AccessShareLock,
+										   NULL,
+										   false,
+										   true);
+	addNSItemToQuery(pstate, nsitem, true, true, true);
+	expr = cookDefault(pstate, newExpr,
+					   attTup->atttypid, attTup->atttypmod,
+					   NameStr(attTup->attname),
+					   attTup->attgenerated);
+
+	if(equal(expr, old_default))
+		return InvalidObjectAddress;
+
 	if (rewrite)
 	{
 		/*
-- 
2.34.1