Query Plan uses temp-file, why?

A quick question regarding to SQL Query Plan:
Why these two requests have different plans, in particularly, why second request needs temp file? To me, temp file is a bad thing which should be avoided, right?

1. select * from Test.Log where cdate = 1
2. select * from Test.Log where cdate > 1

Plan for 1:

Read index map Test.Log.cdateIndex, using the given %SQLUPPER(cdate), and looping on ID.
For each row:
 Read master map Test.Log.IDKEY, using the given idkey value.
 Output the row.

Plan for 2:

Call module B, which populates bitmap temp-file A.
Read bitmap temp-file A, looping on ID.

For each row:

 Read master map Test.Log.IDKEY, using the given idkey value.
 Output the row.
module B
Read index map Test.Log.cdateIndex, looping on %SQLUPPER(cdate) (with a range condition) and ID.

For each row:
 Add ID bit to bitmap temp-file A.

The class:

Class Test.Log Extends %Persistent
{

Property message As %String;

Property cdate As %String;

Index cdateIndex On cdate;

Storage Default
{
...
<Type>%Library.CacheStorage</Type>
}

}