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SQL Anywhere 10.0.1 » SQL Anywhere Server - Database Administration » Database Options » Introduction to database options » Alphabetical list of options

update_statistics option [database] Next Page

user_estimates option [database]


Controls whether or not user selectivity estimates in query predicates are respected or ignored by the query optimizer.

Allowed values

Enabled, Disabled, Override-Magic

Default

Override-Magic

Scope

Can be set for an individual connection or for the PUBLIC group. Takes effect immediately.

Remarks

SQL Anywhere allows you to specify user selectivity estimates can improve the optimizer's performance when the 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 selectivity.

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 server may not select an optimal plan if you use inaccurate estimates.

For more information about user selectivity estimates, see Explicit selectivity 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: