Why some of the tables showing no stat collected even if auto runstats is turned ON

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It's possible some of the  tables showing  STATS_TIME  "-"  in  syscat.tables catalog table  even when the auto-runstats in the database is tunred  ON.

The main reason might be, if a table is never accessed, then auto-runstats will not action on  that table, even if the table has never had runstats done. This is by-design.  This  could be applicable for system tables as well as  user tables.

Regarding system catalog tables there might be differences of observations from version to version on what table might show the stat not collected. That is based on how the version utilize that catalog table.

Another reason of  not showing stat could be due to runstats policy in place to exclude certain tables. But,  that is a conscious decision.