Basic DBA Faq's

Basic DBA Faq's

What is Log Switch?
The point at which ORACLE ends writing to one online redo log file and begins writing to another is called a log switch.

What is On-line Redo Log?
The On-line Redo Log is a set of tow or more on-line redo files that record all committed changes made to the database. Whenever a transaction is committed, the corresponding redo entries temporarily stores in redo log buffers of the SGA are written to an on-line redo log file by the background process LGWR. The on-line redo log files are used in cyclical fashion.

Which parameter specified in the DEFAULT STORAGE clause of CREATE TABLESPACE cannot be altered after creating the tablespace?
All the default storage parameters defined for the tablespace can be changed using the ALTER TABLESPACE command. When objects are created their INITIAL and MINEXTENS values cannot be changed.

What are the steps involved in Database Startup?
Start an instance, Mount the Database and Open the Database.

What are the steps involved in Instance Recovery?
Rolling forward to recover data that has not been recorded in data files, yet has been recorded in the on-line redo log, including the contents of rollback segments. Rolling back transactions that have been explicitly rolled back or have not been committed as indicated by the rollback segments regenerated in step a. Releasing any resources (locks) held by transactions in process at the time of the failure. Resolving any pending distributed transactions undergoing a two-phase commit at the time of the instance failure.

Can Full Backup be performed when the database is open?
No.

What are the different modes of mounting a Database with the Parallel Server?
Exclusive Mode If the first instance that mounts a database does so in exclusive mode, only that Instance can mount the database. Parallel Mode If the first instance that mounts a database is started in parallel mode, other instances that are started in parallel mode can also mount the database.

What are the advantages of operating a database in ARCHIVELOG mode over operating it in NO ARCHIVELOG mode?
Complete database recovery from disk failure is possible only in ARCHIVELOG mode. Online database backup is possible only in ARCHIVELOG mode.

What are the steps involved in Database Shutdown?
Close the Database, Dismount the Database and Shutdown the Instance.

What is Archived Redo Log?
Archived Redo Log consists of Redo Log files that have archived before being reused.

What is Restricted Mode of Instance Startup? – An instance can be started in (or later altered to be in) restricted mode so that when the database is open connections are limited only to those whose user accounts have been granted the RESTRICTED SESSION system privilege.

What is Partial Backup?
A Partial Backup is any operating system backup short of a full backup, taken while the database is open or shut down.

What is Mirrored on-line Redo Log?
A mirrored on-line redo log consists of copies of on-line redo log files physically located on separate disks, changes made to one member of the group are made to all members.

What is Full Backup?
A full backup is an operating system backup of all data files, on- line redo log files and control file that constitute ORACLE database and the parameter.

Can a View based on another View?
Yes.

Can a Tablespace hold objects from different Schemes?
Yes.

Can objects of the same Schema reside in different tablespaces?
Yes.

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.

Do View contain Data?
Views do not contain or store data.

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.

What are the type of Synonyms?
There are two types of Synonyms Private and Public

What is a Redo Log?
The set of Redo Log files YSDATE,UID,USER or USERENV SQL functions, or the pseudo columns LEVEL or ROWNUM.

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

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

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

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

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

What is an 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.

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.

What is a 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.)

What is 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.

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

What is a Sequence?
A sequence generates a serial list of unique numbers for numerical columns of a database and its tables.

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

What is schema?
A schema is collection of database objects of a User.

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.

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

What is a Private Synonyms?
A Private Synonyms can be accessed only by the owner.

What is Database Link?
A database link is a schema object in one database that enables you to access objects on another database

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

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

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.

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 datablocks. Block size is specified in INIT.ORA file and can’t be changed later.

What does a Control file Contain?
A Control file records the physical structure of the database. It contains the following information. Database Name Names and locations of a database datafiles and redolog files. Time stamp of database creation. When is hot backup taken. Rman config details.

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 cannot contain Nulls.

What is Index Cluster?
A Cluster with an index on the Cluster Key 48.When does a Transaction end? – When it is committed or Rollbacked.

What are the factors that affect OPTIMIZER in choosing an Optimization approach?
The OPTIMIZER_MODE initialization parameter Statistics in the Data Dictionary the OPTIMIZER_GOAL parameter of the ALTER SESSION command hints in the statement.

What is the effect of setting the value ALL_ROWS for OPTIMIZER_GOAL, parameter of the ALTER SESSION Command?
The Optimizer chooses Cost_based approach and optimizes with the goal of best throughput if statistics for atleast one of the tables accessed by the SQL statement exist in the data dictionary. Otherwise the OPTIMIZER chooses RULE_based approach.

What is the function of Optimizer?
The goal of the optimizer is to choose the most efficient way to execute a SQL statement.

What is Execution Plan?
The combinations of the steps the optimizer chooses to execute a statement is called an execution plan.

What are the different approaches used by Optimizer in choosing an execution plan?
Rule-based and Cost-based.