Fix!: exp.Merge condition for Trino/Postgres

sqlglot/dialects/dialect.py

@@ -1578,7 +1578,11 @@ def normalize(identifier: t.Optional[exp.Identifier]) -> t.Optional[str]:
             then.transform(
                 lambda node: (
                     exp.column(node.this)
-                    if isinstance(node, exp.Column) and normalize(node.args.get("table")) in targets
+                    if (
+                        isinstance(node, exp.Column)
+                        and normalize(node.args.get("table")) in targets
+                        and not isinstance(node.parent, exp.Func)
+                    )
                     else node
                 ),
                 copy=False,

tests/dialects/test_dialect.py

@@ -2336,6 +2336,17 @@ def test_merge(self):
             },
         )
 
+        # needs to preserve the target alias in then WHEN condition and function but not in the THEN clause
+        self.validate_all(
+            """MERGE INTO foo AS target USING (SELECT a, b FROM tbl) AS src ON src.a = target.a
+            WHEN MATCHED THEN UPDATE SET target.b = COALESCE(src.b, target.b)
+            WHEN NOT MATCHED THEN INSERT (target.a, target.b) VALUES (src.a, src.b)""",
+            write={
+                "trino": """MERGE INTO foo AS target USING (SELECT a, b FROM tbl) AS src ON src.a = target.a WHEN MATCHED THEN UPDATE SET b = COALESCE(src.b, target.b) WHEN NOT MATCHED THEN INSERT (a, b) VALUES (src.a, src.b)""",
+                "postgres": """MERGE INTO foo AS target USING (SELECT a, b FROM tbl) AS src ON src.a = target.a WHEN MATCHED THEN UPDATE SET b = COALESCE(src.b, target.b) WHEN NOT MATCHED THEN INSERT (a, b) VALUES (src.a, src.b)""",
+            },
+        )
+
     def test_substring(self):
         self.validate_all(
             "SUBSTR('123456', 2, 3)",