Tablespaces are the bridge between certain physical and logical components of the Oracle database. Tablespaces are where you store Oracle database objects such as tables, indexes and rollback segments.  You can think of a tablespace like a shared disk drive in Windows. You can store files on your shared drive, move files around and remove files. The same is true with tablespaces. 

A tablespace is made up of one or more database datafiles. The total amount of storage space available in a tablespace is the sum of the physical disk size of all the datafiles associated with that tablespace (less some system overhead). The datafiles are created automatically when the tablespace is defined. In most cases, all datafile space is pre-allocated; that is, the space is set aside when the datafile is created. Thus, when you create a tablespace, you define the initial size of the associated datafile. We will discuss the specifics of creating tablespaces later in this book.

Tablespaces are given names as they are created. For example, the first tablespaces that are created are named SYSTEM and SYSAUX (though SYSAUX is only created in 10g).

Tablespaces are generally named based on the objects within the tablespace.  For example you might name a tablespace PAYROLL_DATA if it?s going to store payroll related information. We will be discussing tablespaces in great detail in later chapters in this book.

Oracle Block Concept

As we discussed earlier, a block is the smallest unit of storage in Oracle. The size of a database block is fixed when the database is created, and can not be changed except by rebuilding the database from scratch. The database block size is fixed at 2K, 4K, 8K, 16K, or 32K in size. Once the base block size is defined, you can create new tablespace with alternate block sizes.

Remember that the Oracle instance also includes a RAM buffer cache which is made-up of RAM blocks which map to the data block in the physical datafiles.

Oracle Extents Concepts

An extent is an uninterrupted (or contiguous) allocation of blocks within a segment. Extents are assigned to a segment automatically by Oracle, so you will rarely deal directly with an extent, rather you will deal directly with its associated segment.

An extent must be on contiguous blocks within a single datafile, so an extent cannot span multiple Oracle datafiles. Oracle will allocate the size of the extents based on the type of tablespace. We will discuss more about extent allocation in later chapters when we discuss table and index creation. 

Oracle Segments Concepts

Segments are the storage objects within the Oracle database. A segment might be a table, an index, a cluster or any one of more than 20 object types.

The DBA creates the segments and assigns them to a specific tablespace. In most cases a single segment cannot reside in more than one tablespace. However, you can split up, or partition, a segment into different tablespaces.

When you create a segment, you define its initial size. We will look at the creation of segments in Oracle in much more detail in chapters to come.

Logical Oracle Structures - The Big Picture

The following picture shows the relationship between segments, extents and blocks in the database. In this example we see that we have created a single segment.   This segment actually consists of two extents. Each of the two extents consists of a number of contiguous blocks.

Well, here we are at the end of Chapter One, and we are now ready to see a complete map of an Oracle database.

Don?t worry if you don?t get everything yet. As you proceed through the next chapters, and actually work with the database, this will all make much more sense!

Here we see the RAM region (the system global area) interacting with Oracle?s programs (CKPT, ARC0) and we see how Oracle programs read and write data to our disk files.

The main points of this chapter include:

* Oracle is the world? most powerful, flexible and robust database.

* Oracle is very complex.

* Oracle 10g administration can be simplified with automation features.

* The configuration of Oracle is governed by initialization parameters, called init.ora.

* An Oracle instance consists of a RAM memory region called the SGA, plus a set of Oracle programs called background processes.

This is an excerpt from the bestselling "Easy Oracle Jumpstart" by Robert Freeman and Steve Karam (Oracle ACE and Oracle Certified Master).  It?s only $19.95 when you buy it directly from the publisher here.

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