release-24.1: sql: do not rewrite UDF body statement slice while assigning placeholders

pkg/sql/logictest/testdata/logic_test/udf_prepare

@@ -6,3 +6,47 @@ PREPARE p AS SELECT $1::INT
 
 statement error pgcode 0A000 cannot evaluate function in this context
 EXECUTE p(f())
+
+statement ok
+DEALLOCATE p;
+
+# Ensure that stable folding does not affect plans stored in the plan cache.
+subtest regression_147186
+
+statement ok
+CREATE FUNCTION f147186() RETURNS INT LANGUAGE SQL AS $$ SELECT CAST(current_setting('foo.bar') AS INT) $$;
+
+statement ok
+CREATE TABLE t147186 (a INT, b INT DEFAULT f147186());
+
+statement ok
+PREPARE p AS INSERT INTO t147186 (a) VALUES ($1);
+
+statement ok
+SET foo.bar = '100';
+
+statement ok
+EXECUTE p(1);
+
+query II rowsort
+SELECT a, b FROM t147186;
+----
+1  100
+
+statement ok
+SET foo.bar = '200';
+
+statement ok
+EXECUTE p(2);
+
+# The second row should reflect the custom var change.
+query II rowsort
+SELECT a, b FROM t147186;
+----
+1  100
+2  200
+
+statement ok
+DEALLOCATE p;
+
+subtest end

pkg/sql/opt/norm/factory.go

@@ -382,9 +382,20 @@ func (f *Factory) AssignPlaceholders(from *memo.Memo) (err error) {
 				}
 				recursiveRoutines[t.Def] = struct{}{}
 			}
+			// Copy the arguments, if any.
+			var newArgs memo.ScalarListExpr
+			if t.Args != nil {
+				copiedArgs := f.CopyAndReplaceDefault(&t.Args, replaceFn).(*memo.ScalarListExpr)
+				newArgs = *copiedArgs
+			}
+			// Make sure to copy the slice that stores the body statements, rather
+			// than mutating the original.
+			newDef := *t.Def
+			newDef.Body = make([]memo.RelExpr, len(t.Def.Body))
 			for i := range t.Def.Body {
-				t.Def.Body[i] = f.CopyAndReplaceDefault(t.Def.Body[i], replaceFn).(memo.RelExpr)
+				newDef.Body[i] = f.CopyAndReplaceDefault(t.Def.Body[i], replaceFn).(memo.RelExpr)
 			}
+			return f.ConstructUDFCall(newArgs, &memo.UDFCallPrivate{Def: &newDef})
 		case *memo.RecursiveCTEExpr:
 			// A recursive CTE may have the stats change on its Initial expression
 			// after placeholder assignment, if that happens we need to