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Tuesday, 20 November 2012

Type of tables in Oracle

Heap organized tables:

These are normal, standard database tables. Data is managed in a heap-like fashion. As data is added, the first free space found in the segment that can fit the data will be used. As data is removed from the table, it allows space to become available for reuse by subsequent INSERTs and UPDATEs. This is the origin of the name “heap” as it refers to this type of table. A heap is a bunch of space, and it is used in a somewhat random fashion.

Index organized tables:

These tables are stored in an index structure. This imposes physical order on the rows themselves. Whereas in a heap the data is stuffed wherever it might fit, in index-organized tables (IOTs) the data is stored in sorted
order, according to the primary key.

Index clustered tables:

Clusters are groups of one or more tables, physically stored on the same database blocks, with all rows that share a common cluster key value being stored physically near each other. Two goals are achieved in this structure. First, many tables may be stored physically joined together. Normally, you would expect data from only one table to be found on a database block, but with clustered tables, data from many tables may be stored on the same block. Second, all data that contains the same cluster key value, such as DEPTNO = 10, will be physically stored together. The data is clustered around the cluster key value. A cluster key is built using a B*Tree index.

Hash clustered tables:

These tables are similar to index clustered tables, but instead of using a B*Tree index to locate the data by cluster key, the hash cluster hashes the key to the cluster to arrive at the database block the data should be on. In a hash cluster, the data is the index (metaphorically speaking). These tables are appropriate for data that is read frequently via an equality comparison on the key.

Sorted hash clustered tables:

This table type is new in Oracle 10g and combines some aspects of a hash-clustered table with those of an IOT. The concept is as follows: you have some key value that rows will be hashed by (say, CUSTOMER_ID), and then a series of records related to that key that arrive in sorted order (timestamp-based records) and are processed in that sorted order. For example, a customer places orders in your order entry system, and these orders are retrieved and processed in a first in, first out (FIFO) manner. In such a system, a sorted hash cluster may be the right data structure for you.

Nested tables: These are part of the object-relational extensions to Oracle. They are simply system-generated and maintained child tables in a parent/child relationship. They work much in the same way as EMP and DEPT in the SCOTT schema with the EMP table being the nested table. EMP is considered to be a child of the DEPT table, since the EMP table has a foreign key, DEPTNO, that points to DEPT. The main difference is that they are not stand-alone tables like EMP.

Temporary tables: These tables store scratch data for the life of a transaction or the life of a session. These tables allocate temporary extents, as needed, from the current user’s temporary tablespace. Each session will see only the extents that session allocates; it will never see any of the data created in any other session.

Object tables: These tables are created based on an object type. They have special attributes not associated with non-object tables, such as a system-generated REF (object identifier) for each row. Object tables are really special cases of heap, index organized, and temporary tables, and they may include nested tables as part of their structure as well.
External tables: The data in these tables are not stored in the database itself; rather, they reside outside of the database in ordinary operating system files. External tables in Oracle9i and above give you the ability to query a file residing outside the database as if it were a normal table inside the database. They are most useful as a means of getting data into the database (they are a very powerful data-loading tool). Furthermore, in Oracle 10g, which introduces an external table unload capability, they provide an easy way to move data between Oracle databases without using database links.