Make CDB_Get_Foreign_Updated_At robust to missing CDB_TableMetadata

This may happen with non-carto DB's, when checking the updated_at
times and not finding the corresponding remote.cdb_tablemetadata
imported from the foreign non-carto DB.

Instead of failing, return a NOW() timestampt, so that caching logic
just assumes there may have been changes.

This makes it work today, and leaves open the possibility of adding
the required carto metadata for homogeneous caching in the future.

scripts-available/CDB_ForeignTable.sql

@@ -125,7 +125,15 @@ BEGIN
 
   -- We assume that the remote cdb_tablemetadata is called cdb_tablemetadata and is on the same schema as the queried table.
   SELECT nspname FROM pg_class c, pg_namespace n WHERE c.oid=foreign_table AND c.relnamespace = n.oid INTO fdw_schema_name;
+  BEGIN
     EXECUTE FORMAT('SELECT updated_at FROM %I.cdb_tablemetadata WHERE tabname=%L ORDER BY updated_at DESC LIMIT 1', fdw_schema_name, remote_table_name) INTO time;
+  EXCEPTION
+    WHEN undefined_table THEN
+      -- If you add a GET STACKED DIAGNOSTICS text_var = RETURNED_SQLSTATE
+      -- you get a code 42P01 which corresponds to undefined_table
+      RAISE NOTICE 'CDB_Get_Foreign_Updated_At: could not find %.cdb_tablemetadata while checking % updated_at, returning NOW() timestamp', fdw_schema_name, foreign_table;
+      time := NOW();
+  END;
   RETURN time;
 END
 $$