When a nonclustered index or a clustered index on a data-only-locked table does not cover the query, Adaptive Server:
Uses the index to locate the first qualifying row at the leaf level of the nonclustered index
Follows the pointer to the data page for that index, and reads the page
Finds the next row on the index page, and locates its data page, and continues this process until all matching keys have been used
For each subsequent key, the data row could be on the same page as the row for the previous key, or the data row may be on a different page in the table. The clustering of key values for each index is measured by a value called the data row cluster ratio. The data row cluster ratio is applied to estimate the number of logical and physical I/Os.
When the data row cluster ratio is 1.0, clustering is very high. High cluster ratios are always seen immediately after creating a clustered index; cluster ratios are 1.00000 or .999997, for example. Rows on the data pages are stored the same order as the rows in the index. The number of logical and physical I/Os needed for the data pages is (basically) the number of rows to be returned, divided by the number of rows per page. For a table with 10 rows per page, a query that needs to return 500 rows needs to read 50 pages if the data row cluster ratio is 1.
When the data row cluster ratio is extremely low, the data rows are scattered on data pages with no relationship to the ordering of the keys. Nonclustered indexes often have low data row cluster ratios, since there is no relationship between the ordering of the index keys and the ordering of the data rows on data pages. When the data row cluster ratio is 0, or close to 0, the number of physical and logical I/Os required could be as much as 1 data page I/O for each row to be returned. A query that needs to return 500 rows needs to read 500 pages, or nearly 500 pages, if the data row cluster ratio is near 0 and the rows are widely scattered on the data pages. In a huge table, this still provides good performance, but in a table with less than 500 pages, the optimizer chooses the cheaper alternative – a table scan.
The size of the data cache is also used in calculating the physical I/O. If the data row cluster ratio is very low, and the cache is small, pages may be flushed from cache before they can be reused. If the cache is large, the optimizer estimates that some pages will be found in cache.