Oracle's well-known modified LRU algorithm was released with Oracle 6. It was touted as a major accomplishment, and Oracle developers were indeed very proud of their advanced buffer cache algorithm. After all, it did solve the key standard LRU algorithm problem.

Figure 6-17 shows the key aspects of Oracle's modified LRU algorithm. The only difference from the standard LRU algorithm is the creation of a window of just a few buffers at the LRU end of the LRU chain. The size of this window was only a few buffers (for example, four) and could be altered by the hidden instance parameter _small_table_threshold. This ensured that, regardless of how large the table being full-table scanned was, it would not obliterate a nicely developed cache.

Figure 6-17. Oracle's modified LRU algorithm created a window of a few buffer headers, which all full-table scanned (FTS) buffer headers flowed through when read into the buffer cache. This ensured the more popular buffers residing in the MRU LRU chain area would not be replaced.