Oracle7 Server Concepts

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Clusters are an optional method of storing table data. A cluster is a group of tables that share the same data blocks because they share common columns and are often used together. For example, the EMP and DEPT table share the DEPTNO column. When you cluster the EMP and DEPT tables (see Figure 5 - 8), Oracle physically stores all rows for each department from both the EMP and DEPT tables in the same data blocks.

Figure 5 - 8. Clustered Table Data

Because clusters store related rows of different tables together in the same data blocks, properly used clusters offer two primary benefits:

Performance Considerations

Clusters can reduce the performance of INSERT statements as compared with storing a table separately with its own index. This disadvantage relates to the use of space and the number of blocks that must be visited to scan a table; because multiple tables have data in each block, more blocks must be used to store a clustered table than if that table were stored non-clustered.

To identify data that would be better stored in clustered form than non-clustered, look for tables that are related via referential integrity constraints and tables that are frequently accessed together using a join. If you cluster tables on the columns used to join table data, you reduce the number of data blocks that must be accessed to process the query; all the rows needed for a join on a cluster key are in the same block. Therefore, performance for joins is improved. Similarly, it might be useful to cluster an individual table. For example, the EMP table could be clustered on the DEPTNO column to cluster the rows for employees in the same department. This would be advantageous if applications commonly process rows department by department.

Like indexes, clusters do not affect application design. The existence of a cluster is transparent to users and to applications. You access data stored in a clustered table via SQL just like data stored in a non-clustered table.

For more information about the performance implications of using clusters, see Oracle7 Server Tuning.

Format of Clustered Data Blocks

In general, clustered data blocks have an identical format to non-clustered data blocks with the addition of data in the table directory. However, Oracle stores all rows that share the same cluster key value in the same data block.

When you create a cluster, specify the average amount of space required to store all the rows for a cluster key value using the SIZE parameter of the CREATE CLUSTER command. SIZE determines the maximum number of cluster keys that can be stored per data block.

For example, if each data block has 1700 bytes of available space and the specified cluster key size is 500 bytes, each data block can potentially hold rows for three cluster keys. If SIZE is greater than the amount of available space per data block, each data block holds rows for only one cluster key value.

Although the maximum number of cluster key values per data block is fixed by SIZE, Oracle does not actually reserve space for each cluster key value nor does it guarantee the number of cluster keys that are assigned to a block. For example, if SIZE determines that three cluster key values are allowed per data block, this does not prevent rows for one cluster key value from taking up all of the available space in the block. If more rows exist for a given key than can fit in a single block, the block is chained, as necessary.

A cluster key value is stored only once in a data block.

The Cluster Key

The cluster key is the column, or group of columns, that the clustered tables have in common. You specify the columns of the cluster key when creating the cluster. You subsequently specify the same columns when creating every table added to the cluster.

For each column specified as part of the cluster key (when creating the cluster), every table created in the cluster must have a column that matches the size and type of the column in the cluster key. No more than 16 columns can form the cluster key, and a cluster key value cannot exceed roughly one-half (minus some overhead) the available data space in a data block. The cluster key cannot include a LONG or LONG RAW column.

You can update the data values in clustered columns of a table. However, because the placement of data depends on the cluster key, changing the cluster key for a row might cause Oracle to physically relocate the row. Therefore, columns that are updated often are not good candidates for the cluster key.

The Cluster Index

You must create an index on the cluster key columns after you have created a cluster. A cluster index is an index defined specifically for a cluster. Such an index contains an entry for each cluster key value. To locate a row in a cluster, the cluster index is used to find the cluster key value, which points to the data block associated with that cluster key value. Therefore, Oracle accesses a given row with a minimum of two I/Os (possibly more, depending on the number of levels that must be traversed in the index).

You must create a cluster index before you can execute any DML statements (including INSERT and SELECT statements) against the clustered tables. Therefore, you cannot load data into a clustered table until you create the cluster index.

Like a table index, Oracle stores a cluster index in an index segment. Therefore, you can place a cluster in one tablespace and the cluster index in a different tablespace.

A cluster index is unlike a table index in the following ways:

If you drop a cluster index, data in the cluster remains but becomes unavailable until you create a new cluster index. You might want to drop a cluster index to move the cluster index to another tablespace or to change its storage characteristics; however, you must re-create the cluster's index to allow access to data in the cluster.

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