“How Adaptive Server performs I/O for heap operations” introduces the basic concepts of the Adaptive Server data cache, and shows how caches are used when reading heap tables.
Index pages get special handling in the data cache, as follows:
Root and intermediate index pages always use LRU strategy.
Index pages can use one cache while the data pages use a different cache, if the index is bound to a different cache.
Covering index scans can use fetch-and-discard strategy.
Index pages can cycle through the cache many times, if number of index trips is configured.
When a query that uses an index is executed, the root, intermediate, leaf, and data pages are read in that order. If these pages are not in cache, they are read into the MRU end of the cache and are moved toward the LRU end as additional pages are read in.
Each time a page is found in cache, it is moved to the MRU end of the page chain, so the root page and higher levels of the index tend to stay in the cache.