You can obtain the dbspace_id from the SYSDBSPACE system view. The estimates returned by the sa_get_dtt_groupreads system procedure are part of the cost
model, and are used to select group reads of appropriate sizes during operations such as sorting.
This procedure, intended for internal diagnostic purposes, retrieves data from the ISYSOPTSTAT system table. If no entries
are recorded in this table, typical values are returned. To tailor estimates for your hardware, execute the following statement: