This is indeed the situation with Oracle! Oracle blocks are not randomly accessed because SQL is not randomly run from random users doing random tasks (at least, we hope not). So unless Oracle uses a more advanced hashing algorithm, the hashed results can easily look like those in Figure 6-8. This forces Oracle to develop a specific hashing function to transform a Figure 6-8 situation into a Figure 6-7 situation. As you might expect, Oracle's buffer hashing algorithm is proprietary information, but I would be willing to wager that prime numbers are involved somehow.

Figure 6-8. Given nonrandom numbers, a simple modulus hash function is not likely to produce an even distribution, but more like what this figure shows. This graph is based on 1,000 inputs, the first 500 between 0 and 5, and the remaining 500 between 0 and 1,000.

When Oracle is searching for a buffer, the hash value is created based on a combination of the block's file number and block number, which is known as the data block address (DBA). So the hash function essentially hashes on the buffer's file number and block number. It is a very convenient situation and allows for a very quick hash.