Use SUBQUERY_FLATTENING_PREFERENCE and SUBQUERY_FLATTENING_PERCENT to control subquery flattening.
Subquery flattening is an optimization technique in which the optimizer rewrites a query containing a subquery into a query that uses a join. SAP Sybase IQ flattens many but not all subqueries. Use SUBQUERY_FLATTENING_PREFERENCE and SUBQUERY_FLATTENING_PERCENT to control when the optimizer chooses to use this optimization.