T187226: Do not use count(*) for reading list size limits requires a new column to be added to wikishared.reading_list on x1. It is a single shared table used by all wikis, the code handling it is not exposed to traffic yet so it's practically empty (400 rows) and probably gets approximately zero requests per minute.
The required change is
ALTER TABLE reading_list ADD COLUMN rl_size INTEGER UNSIGNED NOT NULL DEFAULT 0;
This change does not affect existing code and can be deployed at any time.