Determining the Size of the Database › Allocating Space for Indexes › Calculating the Size of the Index

Calculating the Size of the Index

To account for the different types of index structures, you use a different set of formulas to calculate the size of each of the following types of indexes:

• Indexes sorted on a symbolic key
• Indexes sorted on the database key
• Unsorted indexes

Formulas for calculating the size of indexes are outlined in the following tables.

For information about sizing an index automatically, see Area statements in "Physical Database DDL Statements" of Volume 1 of CA IDMS Database Administration Guide.

Considerations

Before you calculate the size of your indexes, you should be aware of the following index sizing considerations:

• The method of loading the index determines how the index size should be calculated. The formulas presented in the tables below should be used only to calculate space requirements for indexes that are loaded in sequential order.
• Index sizing calculations should allow ample space for future growth. You have several options for reserving space for expansion of an index:
  • Make a generous estimate of the number of occurrences to be indexed; use this inflated number as the basis for performing your index sizing calculations.
  • Make a generous estimate of the number of pages required for the area in which the index will be stored; the formulas presented below can be used to calculate the minimum number of pages required for an area in which an index will be stored.
  • Specify a page reserve at load time; after the index has been loaded, remove the page reserve and increase the number of entries in each SR8.
  • Indicate how far away from the parent or SR7 the bottom-level SR8s are to be stored. For an indexed relationship or a system-owned index, you can use the DISPLACEMENT clause of the non-SQL schema ADD SET statement or the SQL schema CREATE INDEX statement to cluster bottom-level SR8s away from their parent in a database area. By specifying the number of pages to displace the bottom-level SR8s, you can reserve space in the area for storage of intermediate SR8s.