Under most conditions, single-table select operations on a heap use:
The largest I/O available to the table and
Fetch-and-discard (MRU) replacement strategy
For heaps, select operations performing large I/O can be very effective. Adaptive Server can read sequentially through all the extents in a table.
Unless the heap is being scanned as the inner table of a nested-loop join, the data pages are needed only once for the query, so MRU replacement strategy reads and discards the pages from cache.
Large I/O on allpages-locked heaps is effective only when the page chains are not fragmented.
See “Maintaining heaps” for information on maintaining heaps.