When the optimizer costs the use of an index to resolve a query, it first estimates the number of qualifying rows, and then estimates the number of pages that need to be read.
The examples in Chapter 19, “Adaptive Server Optimizer,” show how Adaptive Server estimates the number of rows for a search argument or join using statistics. Once the number of rows has been estimated, the optimizer estimates the number of data pages and index leaf pages that need to be read:
For tables, the optimizer divides the number of rows in the table by the number of pages to determine the average number of rows per data page.
To estimate the average number of rows per page on the leaf level of an index, the optimizer divides the number of rows in the table by the number of leaf pages in the index.
After the number of pages is estimated, data page and index page cluster ratios are used to adjust the page estimates for queries using large I/O, and data row cluster ratios are used to estimate the number of data pages for queries using noncovering indexes.