Database Structures FAQ's

Database Structures FAQ's


1. What are the components of physical database structure of Oracle database?
Oracle database is comprised of three types of files. One or more datafiles, two are more redo log files, and one or more control files.

2. What are the components of logical database structure of Oracle database?
There are tablespaces and database's schema objects.

3. What is a tablespace?
A database is divided into Logical Storage Unit called tablespaces. A tablespace is used to grouped related logical structures together.

4. What is SYSTEM tablespace and when is it created?
Every Oracle database contains a tablespace named SYSTEM, which is automatically created when the database is created. The SYSTEM tablespace always contains the data dictionary tables for the entire database.

5. Explain the relationship among database, tablespace and data file.
Each databases logically divided into one or more tablespaces one or more data files are explicitly created for each tablespace.

6. What is schema?
A schema is collection of database objects of a user.

7. What are Schema Objects?
Schema objects are the logical structures that directly refer to the database's data. Schema objects include tables, views, sequences, synonyms, indexes, clusters, database triggers, procedures, functions packages and database links.

8. Can objects of the same schema reside in different table spaces?

Yes.

9. Can a tablespace hold objects from different schemes?
Yes.

10. What is Oracle table?
A table is the basic unit of data storage in an Oracle database. The tables of a database hold all of the user accessible data. Table data is stored in rows and columns.

11. What is an Oracle view?
A view is a virtual table. Every view has a query attached to it. (The query is a SELECT statement that identifies the columns and rows of the table(s) the view uses.)

12. Do a view contain data?
Views do not contain or store data.

13. Can a view based on another view?
Yes.

14. What are the advantages of views?
- Provide an additional level of table security, by restricting access to a predetermined set of rows and columns of a table.
- Hide data complexity.
- Simplify commands for the user.
- Present the data in a different perspective from that of the base table.
- Store complex queries.

15. What is an Oracle sequence?
A sequence generates a serial list of unique numbers for numerical columns of a database's tables.

16. What is a synonym?
A synonym is an alias for a table, view, sequence or program unit.

17. What are the types of synonyms?
There are two types of synonyms private and public.

18. What is a private synonym?Only its owner can access a private synonym.

19. What is a public synonym?
Any database user can access a public synonym.

20. What are synonyms used for?
- Mask the real name and owner of an object.
- Provide public access to an object
- Provide location transparency for tables, views or program units of a remote database.
- Simplify the SQL statements for database users.

21. What is an Oracle index?
An index is an optional structure associated with a table to have direct access to rows, which can be created to increase the performance of data retrieval. Index can be created on one or more columns of a table.

22. How are the index updates?Indexes are automatically maintained and used by Oracle. Changes to table data are automatically incorporated into all relevant indexes.

23. What are clusters?Clusters are groups of one or more tables physically stores together to share common columns and are often used together.

24. What is cluster key?
The related columns of the tables in a cluster are called the cluster key.

25. What is index cluster?
A cluster with an index on the cluster key.

26. What is hash cluster?A row is stored in a hash cluster based on the result of applying a hash function to the row's cluster key value. All rows with the same hash key value are stores together on disk.

27. When can hash cluster used?
Hash clusters are better choice when a table is often queried with equality queries. For such queries the specified cluster key value is hashed. The resulting hash key value points directly to the area on disk that stores the specified rows.

28. What is database link?A database link is a named object that describes a "path" from one database to another.

29. What are the types of database links? 
Private database link, public database link & network database link.

30. What is private database link?
Private database link is created on behalf of a specific user. A private database link can be used only when the owner of the link specifies a global object name in a SQL statement or in the definition of the owner's views or procedures.

31. What is public database link?
Public database link is created for the special user group PUBLIC. A public database link can be used when any user in the associated database specifies a global object name in a SQL statement or object definition.

32. What is network database link?Network database link is created and managed by a network domain service. A network database link can be used when any user of any database in the network specifies a global object name in a SQL statement or object definition.

33. What is data block?
Oracle database's data is stored in data blocks. One data block corresponds to a specific number of bytes of physical database space on disk.

34. How to define data block size?
A data block size is specified for each Oracle database when the database is created. A database users and allocated free database space in Oracle data blocks. Block size is specified in init.ora file and cannot be changed latter.

35. What is row chaining?In circumstances, all of the data for a row in a table may not be able to fit in the same data block. When this occurs, the data for the row is stored in a chain of data block (one or more) reserved for that segment.

36. What is an extent?An extent is a specific number of contiguous data blocks, obtained in a single allocation and used to store a specific type of information.

37. What is a segment?
A segment is a set of extents allocated for a certain logical structure.

38. What are the different types of segments?
Data segment, index segment, rollback segment and temporary segment.

39. What is a data segment?
Each non-clustered table has a data segment. All of the table's data is stored in the extents of its data segment. Each cluster has a data segment. The data of every table in the cluster is stored in the cluster's data segment.

40. What is an index segment?
Each index has an index segment that stores all of its data.

41. What is rollback segment?
A database contains one or more rollback segments to temporarily store "undo" information.

42. What are the uses of rollback segment?
To generate read-consistent database information during database recovery and to rollback uncommitted transactions by the users.

43. What is a temporary segment?
Temporary segments are created by Oracle when a SQL statement needs a temporary work area to complete execution. When the statement finishes execution, the temporary segment extents are released to the system for future use.

44. What is a datafile?
Every Oracle database has one or more physical data files. A database's data files contain all the database data. The data of logical database structures such as tables and indexes is physically stored in the data files allocated for a database.

45. What are the characteristics of data files?
A data file can be associated with only one database. Once created a data file can't change size. One or more data files form a logical unit of database storage called a tablespace.

46. What is a redo log?
The set of redo log files for a database is collectively known as the database redo log.

47. What is the function of redo log?The primary function of the redo log is to record all changes made to data.

48. What is the use of redo log information?The information in a redo log file is used only to recover the database from a system or media failure prevents database data from being written to a database's data files.

49. What does a control file contains?
- Database name
- Names and locations of a database's files and redolog files.
- Time stamp of database creation.

50. What is the use of control file?
When an instance of an Oracle database is started, its control file is used to identify the database and redo log files that must be opened for database operation to proceed. It is also used in database recovery.
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