Calculate the query plan

The optimizer computes the cost for all possible plans and then chooses the lowest-cost plan. Cost includes the number of disk accesses, the number of network accesses, and the amount of work in memory to access rows and sort data.

Selectivity is also a factor in the total cost. Selectivity is the percentage of rows that pass the filter. The optimizer expresses the selectivity as a number from 0 to 1, which represents the percentage of rows in the table that pass the filter.

The larger the selectivity value, the less likely that a row will disqualify the filter. Therefore, the database server generally evaluates a UDR with a smaller selectivity value before it evaluates a UDR with a larger selectivity value. Similarly, the database server generally evaluates a lower-cost UDR before a higher cost one. The ultimate order of UDR filter evaluation depends on a combination of the cost and selectivity of the UDR.

For more information about how the optimizer calculates the query plan, refer to the Informix® Performance Guide.