Fix(postgres): _date_add_sql bug w/ interval inputs (#7440)

Author: georgesittas

sqlglot/generators/postgres.py

@@ -61,7 +61,9 @@ def func(self: PostgresGenerator, expression: DATE_ADD_OR_SUB) -> str:
         unit = expression.args.get("unit")
 
         e = self._simplify_unless_literal(expression.expression)
-        if isinstance(e, exp.Literal):
+        if isinstance(e, exp.Interval):
+            return f"{this} {kind} {self.sql(e)}"
+        elif isinstance(e, exp.Literal):
             e.set("is_string", True)
         elif e.is_number:
             e = exp.Literal.string(e.to_py())

tests/dialects/test_postgres.py

@@ -168,6 +168,10 @@ def test_postgres(self):
             "pg_catalog.PG_TABLE_IS_VISIBLE(c.oid) "
             "ORDER BY 2, 3"
         )
+        self.validate_identity(
+            "SELECT date_add(current_date, interval '7' day)",
+            "SELECT CURRENT_DATE + INTERVAL '7 DAY'",
+        )
         self.validate_identity(
             "SELECT e'foo \\' bar'",
             "SELECT e'foo '' bar'",