The global TAP schema collects information on tables and columns from known TAP servers. This facilitates locating queriable data by physics (via UCD) or keywords (via description). Note that this shouldn't really be necessary as all information present here should be exposed through Registry records. However, in reality data providers currently are much more liable to give column metadata in their tap_schema than in their Registry records. Hence, for the time being, we maintain this service by harvesting tap_schemas about monthly.