Cisco Cisco Email Security Appliance C170 사용자 가이드

Because the least expensive test is performed first, switching the order of the 
items in the test will have no effect. If you want to guarantee the order in which 
tests are performed, use nested 

if

 statements. This is also the best way to ensure 

that an expensive test is avoided whenever possible:

expensiveAvoid:

if (<simple tests>) 

  { if (<expensive test>)

    { <action> }

  }

In a somewhat more complicated example, consider:

if (test1 AND test2 AND test3) { ... }

The system groups the expression from left to right, so this becomes:

if ((test1 AND test2) AND test3) { ... }

This means the first thing the system does is compare the cost of 

(test1 AND 

test2)

 against the cost of 

test3

, evaluating the second AND first. If all three tests 

have the same cost, then 

test3

 will be performed first because 

(test1 AND 

test2)

 would be twice as expensive.

Each message filter contains a rule that defines the collection of messages that a 
filter can act upon. You define the filter rules, and then you define a filter action 
for messages that return 

true

.