TAILIEUCHUNG - OCA /OCP Oracle Database 11g A ll-in-One Exam Guide- P10

OCA /OCP Oracle Database 11g A ll-in-One Exam Guide- P10:There is an ever increasing demand for staff with IT industry certification. The benefits to employers are significant—they can be certain that staff have a certain level of competence—and the benefits to the individuals, in terms of demand for their services, are equally great. Many employers are now requiring technical staff to have certifications, and many IT purchasers will not buy from firms that do not have certified staff. | OCA OCP Oracle Database 11g All-in-One Exam Guide 46 one block at a time would be a time-consuming process blocks are grouped into extents. An extent is a contiguous series of blocks that are consecutively numbered within a datafile and segments will grow by an extent at a time. These extents need not be adjacent to each other or even in the same datafile they can come from any datafile that is part of the tablespace within which the segment resides. Figure 1-8 shows the Oracle data storage hierarchy with the separation of logical from physical storage. The figure shows the relationships between the storage structures. Logically a tablespace can contain many segments each consisting of many extents. An extent is a set of Oracle blocks. Physically a datafile consists of many operating system blocks. The two sides of the model are connected by the relationships showing that one tablespace can consist of multiple datafiles and at the lowest level that one Oracle block consists of one or more operating system blocks. The Data Dictionary The data dictionary contains metadata that describes the database both physically and logically and its contents. User definitions security information integrity constraints and with release 10g and later performance monitoring information are all stored in the data dictionary. It is stored as a set of segments in the SYSTEM and SYSAUX tablespaces. In many ways the segments that make up the data dictionary are segments like any other just tables and indexes. The critical difference is that the data dictionary tables are generated at database creation time and you are not allowed to access them directly. There is nothing to stop an inquisitive DBA from investigating the data dictionary directly but if you do any updates to it you may cause irreparable damage to your database and certainly Oracle Corporation will not support you. Creating a data Figure 1-8 The Oracle logical and physical storage hierarchy The logical structures The .

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