Creating Hash Clusters

A hash cluster is created using a CREATE CLUSTER statement, but you specify a HASHKEYS clause. The following example contains a statement to create a cluster named trial_cluster that stores the trial table, clustered by the trialno column (the cluster key); and another statement creating a table in the cluster.

CREATE CLUSTER trial_cluster (trialno NUMBER(5,0))
    TABLESPACE users
    STORAGE (INITIAL 250K     NEXT 50K
      MINEXTENTS 1     MAXEXTENTS 3
      PCTINCREASE 0)
    HASH IS trialno HASHKEYS 150;

CREATE TABLE trial (
    trialno NUMBER(5,0) PRIMARY KEY,
    ...)
    CLUSTER trial_cluster (trialno);

As with index clusters, the key of a hash cluster can be a single column or a composite key (multiple column key). In this example, it is a single column.

The HASHKEYS value, in this case 150, specifies and limits the number of unique hash values that can be generated by the hash function used by the cluster. The database rounds the number specified to the nearest prime number.

If no HASH IS clause is specified, the database uses an internal hash function. If the cluster key is already a unique identifier that is uniformly distributed over its range, you can bypass the internal hash function and specify the cluster key as the hash value, as is the case in the preceding example. You can also use the HASH IS clause to specify a user-defined hash function.

You cannot create a cluster index on a hash cluster, and you need not create an index on a hash cluster key.

For additional information about creating tables in a cluster, guidelines for setting parameters of the CREATE CLUSTER statement common to index and hash clusters, and the privileges required to create any cluster, see Chapter 20, "Managing Clusters". The following sections explain and provide guidelines for setting the parameters of the CREATE CLUSTER statement specific to hash clusters:

Creating a Sorted Hash Cluster
Creating Single-Table Hash Clusters
Controlling Space Use Within a Hash Cluster
Estimating Size Required by Hash Clusters

Creating a Sorted Hash Cluster

In a sorted hash cluster, the rows corresponding to each value of the hash function are sorted on a specified set of columns in ascending order, which can improve response time during subsequent operations on the clustered data.

For example, a telecommunications company needs to store detailed call records for a fixed number of originating telephone numbers through a telecommunications switch. From each originating telephone number there can be an unlimited number of telephone calls.

Calls are stored as they are made and processed later in first-in, first-out order (FIFO) when bills are generated for each originating telephone number. Each call has a detailed call record that is identified by a timestamp. The data that is gathered is similar to the following:

Originating Telephone Numbers	Call Records Identified by Timestamp
650-555-1212	t0, t1, t2, t3, t4, ...
650-555-1213	t0, t1, t2, t3, t4, ...
650-555-1214	t0, t1, t2, t3, t4, ...
...	...

In the following SQL statements, the telephone_number column is the hash key. The hash cluster is sorted on the call_timestamp and call_duration columns. The number of hash keys is based on 10-digit telephone numbers.

CREATE CLUSTER call_detail_cluster ( 
   telephone_number NUMBER, 
   call_timestamp NUMBER SORT, 
   call_duration NUMBER SORT ) 
  HASHKEYS 10000 HASH IS telephone_number 
  SIZE 256; 

CREATE TABLE call_detail ( 
   telephone_number     NUMBER, 
   call_timestamp       NUMBER   SORT, 
   call_duration        NUMBER   SORT, 
   other_info           VARCHAR2(30) ) 
  CLUSTER call_detail_cluster ( 
   telephone_number, call_timestamp, call_duration );

Given the sort order of the data, the following query would return the call records for a specified hash key by oldest record first.

SELECT * WHERE telephone_number = 6505551212;

Creating Single-Table Hash Clusters

You can also create a single-table hash cluster, which provides fast access to rows in a table. However, this table must be the only table in the hash cluster. Essentially, there must be a one-to-one mapping between hash keys and data rows. The following statement creates a single-table hash cluster named peanut with the cluster key variety:

CREATE CLUSTER peanut (variety NUMBER)
   SIZE 512 SINGLE TABLE HASHKEYS 500;

The database rounds the HASHKEYS value up to the nearest prime number, so this cluster has a maximum of 503 hash key values, each of size 512 bytes. The SINGLE TABLE clause is valid only for hash clusters. HASHKEYS must also be specified.

Controlling Space Use Within a Hash Cluster

When creating a hash cluster, it is important to choose the cluster key correctly and set the HASH IS, SIZE, and HASHKEYS parameters so that performance and space use are optimal. The following guidelines describe how to set these parameters.

Choosing the Key

Choosing the correct cluster key is dependent on the most common types of queries issued against the clustered tables. For example, consider the emp table in a hash cluster. If queries often select rows by employee number, the empno column should be the cluster key. If queries often select rows by department number, the deptno column should be the cluster key. For hash clusters that contain a single table, the cluster key is typically the entire primary key of the contained table.

The key of a hash cluster, like that of an index cluster, can be a single column or a composite key (multiple column key). A hash cluster with a composite key must use the internal hash function of the database.

Setting HASH IS

Specify the HASH IS parameter only if the cluster key is a single column of the NUMBER datatype, and contains uniformly distributed integers. If these conditions apply, you can distribute rows in the cluster so that each unique cluster key value hashes, with no collisions (two cluster key values having the same hash value), to a unique hash value. If these conditions do not apply, omit this clause so that you use the internal hash function.

Setting SIZE

SIZE should be set to the average amount of space required to hold all rows for any given hash key. Therefore, to properly determine SIZE, you must be aware of the characteristics of your data:

If the hash cluster is to contain only a single table and the hash key values of the rows in that table are unique (one row for each value), SIZE can be set to the average row size in the cluster.
If the hash cluster is to contain multiple tables, SIZE can be set to the average amount of space required to hold all rows associated with a representative hash value.

Further, once you have determined a (preliminary) value for SIZE, consider the following. If the SIZE value is small (more than four hash keys can be assigned for each data block) you can use this value for SIZE in the CREATE CLUSTER statement. However, if the value of SIZE is large (four or fewer hash keys can be assigned for each data block), then you should also consider the expected frequency of collisions and whether performance of data retrieval or efficiency of space usage is more important to you.

If the hash cluster does not use the internal hash function (if you specified HASH IS) and you expect few or no collisions, you can use your preliminary value of SIZE. No collisions occur and space is used as efficiently as possible.
If you expect frequent collisions on inserts, the likelihood of overflow blocks being allocated to store rows is high. To reduce the possibility of overflow blocks and maximize performance when collisions are frequent, you should adjust SIZE as shown in the following chart.

Available Space for each Block / Calculated SIZE Setting for SIZE

1 SIZE

2 SIZE + 15%

3 SIZE + 12%

4 SIZE + 8%

>4 SIZE

Overestimating the value of SIZE increases the amount of unused space in the cluster. If space efficiency is more important than the performance of data retrieval, disregard the adjustments shown in the preceding table and use the original value for SIZE.

Available Space for each Block / Calculated SIZE	Setting for SIZE
1	`SIZE`
2	`SIZE` + 15%
3	`SIZE` + 12%
4	`SIZE` + 8%
>4	`SIZE`

Setting HASHKEYS

For maximum distribution of rows in a hash cluster, the database rounds the HASHKEYS value up to the nearest prime number.

Controlling Space in Hash Clusters

The following examples show how to correctly choose the cluster key and set the HASH IS, SIZE, and HASHKEYS parameters. For all examples, assume that the data block size is 2K and that on average, 1950 bytes of each block is available data space (block size minus overhead).

Controlling Space in Hash Clusters: Example 1

You decide to load the emp table into a hash cluster. Most queries retrieve employee records by their employee number. You estimate that the maximum number of rows in the emp table at any given time is 10000 and that the average row size is 55 bytes.

In this case, empno should be the cluster key. Because this column contains integers that are unique, the internal hash function can be bypassed. SIZE can be set to the average row size, 55 bytes. Note that 34 hash keys are assigned for each data block. HASHKEYS can be set to the number of rows in the table, 10000. The database rounds this value up to the next highest prime number: 10007.

CREATE CLUSTER emp_cluster (empno 
NUMBER)
. . .
SIZE 55
HASH IS empno HASHKEYS 10000;

Controlling Space in Hash Clusters: Example 2

Conditions similar to the previous example exist. In this case, however, rows are usually retrieved by department number. At most, there are 1000 departments with an average of 10 employees for each department. Department numbers increment by 10 (0, 10, 20, 30, . . . ).

In this case, deptno should be the cluster key. Since this column contains integers that are uniformly distributed, the internal hash function can be bypassed. A preliminary value of SIZE (the average amount of space required to hold all rows for each department) is 55 bytes * 10, or 550 bytes. Using this value for SIZE, only three hash keys can be assigned for each data block. If you expect some collisions and want maximum performance of data retrieval, slightly alter your estimated SIZE to prevent collisions from requiring overflow blocks. By adjusting SIZE by 12%, to 620 bytes (refer to "Setting SIZE"), there is more space for rows from expected collisions.

HASHKEYS can be set to the number of unique department numbers, 1000. The database rounds this value up to the next highest prime number: 1009.

CREATE CLUSTER emp_cluster (deptno NUMBER)
. . .
SIZE 620
HASH IS deptno HASHKEYS 1000;

Estimating Size Required by Hash Clusters

As with index clusters, it is important to estimate the storage required for the data in a hash cluster.

Oracle Database guarantees that the initial allocation of space is sufficient to store the hash table according to the settings SIZE and HASHKEYS. If settings for the storage parameters INITIAL, NEXT, and MINEXTENTS do not account for the hash table size, incremental (additional) extents are allocated until at least SIZE*HASHKEYS is reached. For example, assume that the data block size is 2K, the available data space for each block is approximately 1900 bytes (data block size minus overhead), and that the STORAGE and HASH parameters are specified in the CREATE CLUSTER statement as follows:

STORAGE (INITIAL 100K
    NEXT 150K
    MINEXTENTS 1
    PCTINCREASE 0)
SIZE 1500
HASHKEYS 100

In this example, only one hash key can be assigned for each data block. Therefore, the initial space required for the hash cluster is at least 100*2K or 200K. The settings for the storage parameters do not account for this requirement. Therefore, an initial extent of 100K and a second extent of 150K are allocated to the hash cluster.

Alternatively, assume the HASH parameters are specified as follows:

SIZE 500 HASHKEYS 100

In this case, three hash keys are assigned to each data block. Therefore, the initial space required for the hash cluster is at least 34*2K or 68K. The initial settings for the storage parameters are sufficient for this requirement (an initial extent of 100K is allocated to the hash cluster).