Controls the placement of correlated subquery predicate operators within a query plan.
-1 to 1
0
Can be set for any scope, any user, takes immediate effect.
For correlated subquery operators within a query, the IQ optimizer may have a choice of several different valid locations within that query’s plan. SUBQUERY_PLACEMENT_PREFERENCE allows you to override the optimizer’s cost-based decision when choosing the placement location. It does not override internal rules that determine whether a location is valid, and in some queries, there might be only one valid choice. If you set this option to a nonzero value, it affects every correlated subquery predicate in a query; it cannot be used to selectively modify the placement of one subquery out of several in a query.
This option is normally used for internal testing, and only experienced DBAs should use it. Table 2-18 describes the valid values for this option and their actions.
Value |
Action |
|---|---|
0 |
Let the optimizer choose. |
1 |
Prefer the highest possible location in the query plan, thereby delaying the execution of the subquery to as late as possible within the query. |
-1 |
Prefer the lowest possible location in the query plan, thereby placing the execution of the subquery as early as possible within the query. |
The default setting of this option is almost always appropriate. Occasionally, Sybase Technical Support might ask you to change this value.
