When you update or delete a row from a heap table, the effects on the data cache are similar to the process for inserts. If a page is already in the cache, the row is changed and the entire buffer (a single page or more, depending on the I/O size) is placed on the MRU end of the chain.
If the page is not in cache, it is read from disk into cache and examined to determine whether the rows on the page match query clauses. Its placement on the MRU/LRU chain depends on whether data on the page needs to be changed:
If data on the page needs to be changed, the buffer is placed on the MRU end. It remains in cache, where it can be updated repeatedly, or read by other users before being flushed to disk.
If data on the page does not need to be changed, the buffer is placed immediately before the wash marker in the cache.
