Defining a new set of strategies is beyond the scope of this discussion, but we'll explain how B-tree strategies work because you'll need to know that to add a new operator class. For B-tree s, this number is 5. These strategies correspond to Table 9-2. The access method code can use these strategy numbers, regardless of data type, to figure out how to partition the B-tree , compute selectivity, and so on. Sometimes, strategies aren't enough information for the system to figure out how to use an index.
Some access methods require other support routines in order to work.
For example, the B-tree access method must be able to compare two keys and determine whether one is greater than, equal to, or less than the other. Similarly, the R-tree access method must be able to compute intersections, unions, and sizes of rectangles. This attribute records the number of support routines used by an access method. Similarly, amsupport is just the number of support routines required by the access method. The actual routines are listed elsewhere.