Sunday 11 October 2015

Oracle DBA Interview Questions -3






1. 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.

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

3. What is the different type of Segments?
Data Segment, Index Segment, Rollback Segment and Temporary Segment.

4. 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.

5. What is an Index Segment?
Each Index has an Index segment that stores all of its data.

6. What is Rollback Segment?
A Database contains one or more Rollback Segments to temporarily store "undo" information.

7. What are the uses of Rollback Segment?
Rollback Segments are used:
To generate read-consistent database information during database recovery to rollback uncommitted transactions for users.

8. 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.

9. What is a Data File?
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.

10. 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.

11. What is a Data Dictionary?
The data dictionary of an ORACLE database is a set of tables and views that are used as a read-only reference about the database.
It stores information about both the logical and physical structure of the database, the valid users of an ORACLE database, integrity constraints defined for tables in the database and space allocated for a schema object and how much of it is being used.

12. What is an Integrity Constrains?
An integrity constraint is a declarative way to define a business rule for a column of a table.

13. Can an Integrity Constraint be enforced on a table if some existing table data does not satisfy the constraint?
No.

14. Describe the different type of Integrity Constraints supported by ORACLE?
NOT NULL Constraint - Disallows NULL's in a table's column.
UNIQUE Constraint - Disallows duplicate values in a column or set of columns.
PRIMARY KEY Constraint - Disallows duplicate values and NULLs in a column or set of columns.
FOREIGN KEY Constrain - Require each value in a column or set of columns match a value in a related table's UNIQUE or PRIMARY KEY.
CHECK Constraint - Disallows values that do not satisfy the logical expression of the constraint.

15. What is difference between UNIQUE constraint and PRIMARY KEY constraint?
A column defined as UNIQUE can contain NULLs while a column defined as PRIMARY KEY can't contain Nulls.

16. Describe Referential Integrity?
A rule defined on a column (or set of columns) in one table that allows the insert or update of a row only if the value for the column or set of columns (the dependent value) matches a value in a column of a related table (the referenced value). It also specifies the type of data manipulation allowed on referenced data and the action to be performed on dependent data as a result of any action on referenced data.

17. What are the Referential actions supported by FOREIGN KEY integrity constraint?
UPDATE and DELETE Restrict - A referential integrity rule that disallows the update or deletion of referenced data.
DELETE Cascade - When a referenced row is deleted all associated dependent rows are deleted.

18. What is self-referential integrity constraint?
If a foreign key reference a parent key of the same table is called self-referential integrity constraint.

19. What are the Limitations of a CHECK Constraint?
The condition must be a Boolean expression evaluated using the values in the row being inserted or updated and can't contain sub-queries, sequence, the SYSDATE, UID, USER or USERENV SQL functions, or the pseudo columns LEVEL or ROWNUM.

20. What is the maximum number of CHECK constraints that can be defined on a column?

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