Oracle DBA interview questions

Oracle DBA interview questions

Explain the difference between a hot backup and a cold backup.

A cold backup is done when there is no user activity going on with the system. Also called as offline backup, is taken when the database is not running and no users are logged in. all files of the database are copied and no changes during the copy are made.

A hot backup is taken when the database needs to run all the time. It is an online backup. All files of the database are copied and there may be changes to the database during the copy.

How many memory layers are in the oracle shared pool? Explain them

Oracles shared pool consists of two layers namely, Library cache and Data dictionary cache.

Library cache: This layer has information about SQL statements that were parsed, information about cursors and any plan data.

Data Dictionary cache: this Layer has information about the accounts of the users, their privileges and segments information.

What is the cache hit ratio, what impact does it have on performance of an Oracle database and what is involved in tuning it?

When the cache client like a CPU, web browser etc finds a successful entry in the cache memory, it is called as a cache hit. The percentage of these successful cache hits is called as cache hit ratio. Higher the cache hit ratio better will be the performance because data read from the cache is faster than the memory.

Explain the concept of the DUAL table.

On installing Oracle database, DUAL table Is present by default. It is a special table with just one row. It has a single column called DUMMY. The data type of this column in VARCHAR2(1). It has a value “X”. It is most commonly used to select pseudo columns in Oracle like sysdate.

Select sysdate from dual

What are the ways tablespaces can be managed and how do they differ?

Objects can be assigned to a table space. The related objects can be then grouped together. Table space can also be managed using extents. Extents consist of a specific number of contiguous data blocks. For the required extent, the free extent closest in size is determined.

Explain what are Oracle Catalog and Oracle Archive log?

Oracle catalog contains tables and views to get information about the database. It helps user to understand the available tables, attributes, constraints etc.

Oracle Archive log mode of a database in Oracle, ensures, the online redo logs are not overwritten before they are archived. This ensures that recovery is possible.

What are PCT Free and PCT Used? What is PCT increase parameter in segment?

PCTFREE is a parameter used to find how much space should be left in a database block for future updates. This means that if the PCTFREE = 20, new rows will be added in the block until it is 80% full.

PCTUSED is a parameter helps Oracle to find when it should consider a database block to be empty enough to be added to the freelist. This means that if the PCTFREE = 50, new rows will be not be added in the block until sufficient rows are deleted from the block so that it falls below 40% empty.

PCTINCREASE parameter is used to find how much will the each subsequent segment will grow. This value is in %.

What is dump destination? What are bdump, cdump and udump?

Trace files for Oracle processes are stored in dump destination.

Bdump- Oracle writes to the trace log and creates trace files for background processes in background dump destination. If this directory becomes full and more files cannot be written, debugging becomes difficult.

Cdump- Oracle writes core files and background processes in Core dump destination directory. If this directory becomes full and more files cannot be written, debugging becomes difficult.

Udump – Oracle creates trace files for the user processes in the User Dump directory if this directory becomes full and more files cannot be written, debugging becomes difficult.

How do you increase the performance of %LIKE operator?

LIKE% works the fastest because it uses the index to search on the column provided an index is specified on the column. Using % after LIKE, results in faster results.

Why use materialized view instead of a table

Materialized views are basically used to increase query performance since it contains results of a query. They should be used for reporting instead of a table for a faster execution.

Why and how the deadlock situation arises

A deadlock situation arises when two or more users wait for the same resource locked by one anther or two or more processes wait to update rows which are locked by other processes. Oracle if detects a deadlock, rolls back the session chosen by the deadlock victim.

What are standby databases? Difference between Physical and logical standby databases

A standby database is a replica of the original database. In order to keep both the database synchronized, archived redo logs can be used. It is mainly used in disaster protection. It can also be opened in read only mode which allows it to be used independently for reporting.

A logical standby database allows new database objects like tables, indexes to be added to the database. On the other hand, the physical standby database is a physical or structural copy of primary database. They can be opened in read only for disaster recovery.

What is Cache Fusion Technology?

In Cache fusion, multiple buffers join to act as one. It eliminates disk i/o operaions by making use of a scalable shared cache. It treats multiple buffer caches as one thereby resolving data consistency issues. Cash fusion technology can provide more resources and increases concurrency of users.

What is the difference between Cloning and Standby databases?

The clone database is a copy of the database which can be opened in read write mode. It is treated as a separate copy of the database that is functionally completely separate. The standby database is a copy of the production database used for disaster protection. In order to update the standby database; archived redo logs from the production database can be used. If the primary database is destroyed or its data becomes corrupted, one can perform a failover to the standby database, in which case the standby database becomes the new primary database.

What is dump destination? What are bdump, cdump and udump? 

Trace files for Oracle processes are stored in dump destination.

Bdump- Oracle writes to the trace log and creates trace files for background processes in background dump destination. If this directory becomes full and more files cannot be written, debugging becomes difficult.

Cdump- Oracle writes core files and background processes in Core dump destination directory. If this directory becomes full and more files cannot be written, debugging becomes difficult.

Udump – Oracle creates trace files for the user processes in the User Dump directory if this directory becomes full and more files cannot be written, debugging becomes difficult.

What are PCT Free and PCT Used? What is PCT increase parameter in segment? 

PCTFREE is a parameter used to find how much space should be left in a database block for future updates. This means that if the PCTFREE = 20, new rows will be added in the block until it is 80% full.

PCTUSED is a parameter helps Oracle to find when it should consider a database block to be empty enough to be added to the freelist. This means that if the PCTFREE = 50, new rows will be not be added in the block until sufficient rows are deleted from the block so that it falls below 40% empty.

PCTINCREASE parameter is used to find how much will the each subsequent segment will grow. This value is in %

Explain what are Oracle Catalog and Oracle Archive log? 

Oracle catalog contains tables and views to get information about the database. It helps user to understand the available tables, attributes, constraints etc.

Oracle Archive log mode of a database in Oracle, ensures, the online redo logs are not overwritten before they are archived. This ensures that recovery is possible