ASA Database Administration Guide
  Database Options
    Alphabetical list of options

USER_ESTIMATES option [database]

Function 

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

Allowed values 

ENABLED, DISABLED, OVERRIDE-MAGIC

Scope 

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

Default 

OVERRIDE-MAGIC

Description 

Adaptive Server 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:

• ENABLED    All user-supplied selectivity estimates are respected. You can also use ON to turn on this option.

• OVERRIDE-MAGIC    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).

• DISABLED    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.