Apress.Expert.Oracle.Database.Architecture.9i.and.10g.Programming.Techniques.and.Solutions.Sep.2005

CHAPTER 10 ■ DATABASE TABLES 379
Figure 10-9. Hash cluster depiction
When you create a hash cluster, you’ll use the same CREATE CLUSTER statement you used to
create the index cluster with different options. You’ll just be adding a HASHKEYS option to it to
specify the size of the hash table. Oracle will take your HASHKEYS value and round it up to the
nearest prime number (the number of hash keys will always be a prime). Oracle will then
compute a value based on the SIZE parameter multiplied by the modified HASHKEYS value. It
will allocate at least that much space in bytes for the cluster. This is a big difference from the
preceding index cluster, which dynamically allocates space as it needs it. A hash cluster preallocates
enough space to hold (HASHKEYS/trunc(blocksize/SIZE)) bytes of data. For example,
if you set your SIZE to 1,500 bytes and you have a 4KB blocksize, Oracle will expect to store two
keys per block. If you plan on having 1,000 HASHKEYs, Oracle will allocate 500 blocks.
It is interesting to note that unlike a conventional hash table in a computer language, it is
OK to have hash collisions—in fact, it is desirable in many cases. If you take the same DEPT/EMP
example from earlier, you could set up a hash cluster based on the DEPTNO column. Obviously,
many rows will hash to the same value, and you expect them to (they have the same DEPTNO).
This is what the cluster is about in some respects: clustering like data together. This is why
Oracle asks you to specify the HASHKEYs (how many department numbers you anticipate over
time) and SIZE (what the size of the data is that will be associated with each department number).
It allocates a hash table to hold HASHKEY number of departments of SIZE bytes each. What
you do want to avoid is unintended hash collisions. It is obvious that if you set the size of the
hash table to 1,000 (really 1,009, since the hash table size is always a prime number and Oracle
rounds up for you), and you put 1,010 departments in the table, there will be at least one collision
(two different departments hashing to the same value). Unintended hash collisions are to
be avoided, as they add overhead and increase the probability of block chaining occurring.
To see what sort of space hash clusters take, we’ll use a small utility stored procedure,
SHOW_SPACE (for details on this procedure, see the “Setup” section at the beginning of the
book), that we’ll use in this chapter and in the next chapter. This routine just uses the
DBMS_SPACE-supplied package to get details about the storage used by segments in the
database.