Technical Blog Post
How does DB2 come up with the access plan when instance gets restarted and if one or more table in the query does not have statistics on it?
If RTS (real time statistics) is enabled then DB2 will collect the statistics internally.
If no RTS is used then it will use whatever statistics were present from before when a statistics was collected last.
If the table never had statistics then it requests the storage layer to tell the physical size of the table. Then it will use the physical size, coupled with the UDI (Update Delete Insert) counter, to fabricate statistics.
A fabrication is not same as running a real runstats.
So, with statistics fabrication a possibility of sub-optimal access plan is more common.