I have some of the auto generated `_WA_Sys_` statistics in my database, which have not been updated in a while (in comparison to other statistics in the same table).

The rule of thumb appears to be in tables >500 rows, statistics are updated after (20% + 500 rows) changes.

However, I can see using the following query:

    SELECT	t.name,
    		i.name,
    		i.rowcnt,
    		i.rowmodctr,
    		p.last_updated
    FROM	sys.sysindexes i
    		JOIN sys.tables t
    			ON i.id = t.object_id
    		JOIN sys.stats s
    			ON s.object_id = t.object_id AND i.name = s.name
    		CROSS APPLY sys.dm_db_stats_properties(s.object_id,s.stats_id) p
    WHERE	rowmodctr > 0
    ORDER BY i.rowmodctr DESC

...that there is a table with a number of `_WA_Sys_` statistics that are way out of date (and have a `rowmodctr` that is higher than 20% + 500).

If a run a query against the table and add one of the columns associated with the out of date `_WA_Sys_` statistics in the `WHERE` clause, and check the *updated date* of the statistic, I can see it has updated.

If I run the same query with the `WHERE` clause again, the statistic doesn't update.

It seems like the `_WA_Sys` statistics update when a query is run that will use them and they are out of date?