fix: search_path in RDS (#24739)

(cherry picked from commit 7675e0db10)

superset/db_engine_specs/base.py

@@ -1082,22 +1082,45 @@ class BaseEngineSpec:  # pylint: disable=too-many-public-methods
 
         For some databases (like MySQL, Presto, Snowflake) this requires modifying the
         SQLAlchemy URI before creating the connection. For others (like Postgres), it
-        requires additional parameters in ``connect_args``.
+        requires additional parameters in ``connect_args`` or running pre-session
+        queries with ``set`` parameters.
 
-        When a DB engine spec implements this method it should also have the attribute
-        ``supports_dynamic_schema`` set to true, so that Superset knows in which schema a
-        given query is running in order to enforce permissions (see #23385 and #23401).
+        When a DB engine spec implements this method or ``get_prequeries`` (see below) it
+        should also have the attribute ``supports_dynamic_schema`` set to true, so that
+        Superset knows in which schema a given query is running in order to enforce
+        permissions (see #23385 and #23401).
 
         Currently, changing the catalog is not supported. The method accepts a catalog so
         that when catalog support is added to Superset the interface remains the same.
         This is important because DB engine specs can be installed from 3rd party
-        packages.
+        packages, so we want to keep these methods as stable as possible.
         """
         return uri, {
             **connect_args,
             **cls.enforce_uri_query_params.get(uri.get_driver_name(), {}),
         }
 
+    @classmethod
+    def get_prequeries(
+        cls,
+        catalog: str | None = None,  # pylint: disable=unused-argument
+        schema: str | None = None,  # pylint: disable=unused-argument
+    ) -> list[str]:
+        """
+        Return pre-session queries.
+
+        These are currently used as an alternative to ``adjust_engine_params`` for
+        databases where the selected schema cannot be specified in the SQLAlchemy URI or
+        connection arguments.
+
+        For example, in order to specify a default schema in RDS we need to run a query
+        at the beggining of the session:
+
+            sql> set search_path = my_schema;
+
+        """
+        return []
+
     @classmethod
     def patch(cls) -> None:
         """