SQL Anywhere allows you to specify user selectivity estimates can improve the optimizer's performance when the database server
is unable to accurately predict the selectivity of a predicate. However, user selectivity estimates should be used only in
appropriate circumstances. For example, it may be useful to supply a selectivity estimate for a predicate that involves one
or more functions if the Override-Magic selectivity estimate used by the optimizer is significantly different from the actual
If you have used selectivity estimates that are inaccurate as a workaround to performance problems where the software-selected
access plan was poor, it is recommended that you set this option to Disabled. The database server may not select an optimal
plan if you use inaccurate estimates.
When a user selectivity estimate is supplied with a predicate, the estimate is respected or ignored based on the setting of
this option. The following values are accepted:
All user-supplied selectivity estimates are respected. You can also use On to turn on this option.
A user selectivity estimate is respected and used only if the optimizer would otherwise choose to use its last-resort,
heuristic value (also called the magic value).
All user estimates are ignored and magic values are used when no other estimate data is available. You can also use
Off to turn off this option.
You can override any temporary or public settings for this option within individual INSERT, UPDATE, DELETE, SELECT, UNION,
EXCEPT, and INTERSECT statements by including an OPTION clause in the statement.