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An Oracle database has many schemas contained in it. The schema is a logical structure that contains objects like segments, views, procedures, functions, packages, triggers, user-defined objects, collection types, sequences, synonyms, and database links. A segment is a data structure that can be a table, index, or temporary or undo segment. The schema name is the user that controls the schema. Examples of schemas are the System, Sys, Scott, and SH schemas. Figure 3-1 shows the relationship between these schema objects.
As you can see in Figure 3-1, a schema can have many segments and many segment types. Each segment is a single instance of a table, partition, cluster, index, or temporary or undo segment. So, for example, a table with two indexes is implemented as three segments in the schema. A segment is broken down further into extents, which are a collection of contiguous data blocks. As data is added to Oracle, it will first fill the blocks in the allocated extents and once those extents are full, new extents can be added to the segment as long as space allows. Oracle segment types are listed here:
Tables are where the data is kept in rows and columns. This is the heart of your database and a table is implemented in one schema and one tablespace. The exception to this is a special type of table called a partitioned table where the table can be split into different ranges or sets of values called a partition and each partition can be implemented in a different tablespace.
FIGURE 3-1. The database and user schemas
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