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.

As with all algorithms, there are limitations. But these limitations did not cause a problem for many years. However, once customers began using Oracle for large data warehouse applications, two significant problems began to occur: