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Lookup tables are like constraints which hold a value. The content in the lookup tables dosen't change often, hence refered as lookup (reference).
Example : Countries table
Countries are not added on regular bases hence one can consider the table as a lookup table. As and when needed in some other table the countries information can be fetched base on PK - Country-Code column. Eg - Emp_Per_Info.
Group of tables physically stored together because they share common columns and are often used together is called Cluster.
different types of joins are
-Cartesian Product or Cross Join
-Inner Join
-Equi Join or Natural Join
-Non Equi Join
-Self Join
-Outer Join
-Left Outer Join
-Right Outer Join
-Full Outer Join
Hash clusters are useful in cases where :
(i) There is a uniform, even and predictable no. of key values.
(ii) Queries using equality predicates.
(iii) The table is NOT growing constantly, and the keys are rarely updated.
A sequence is a database object created by a user that can be used to generate unique integers. A typical usage of sequences is to generate primary key values which are unique for each row.
It is generated and incremented (or decremented) by an internal Oracle routine. It can be used by multiple users and for multiple tables too. A sequence can be used instead of writing an application code for sequence-generating routine.
An Extent is a collection of contagious blocks the second level of granuality of the storage in database is called extent.it is defined as the collectionof contagious datablocks.
oracle uses private sql area to execute sql queries and store the information.pl/sql uses cursors to name these private sql area and access its stored information
there r two types of cursors
implicit
explicit
implicit cursors are used for all dml and single row queries.these are system defined
explicit cursors are used for queries which return multiple row .these are user defined.
This is an alternate to index, as index fast retrieve rows from single column,same way hash cluster fast retrieve rows from group of table and reduce i/o .
An Oracle database consists of one or more logical storage units called tablespaces, which collectivley store all of the database's data.
Each tablespace in an ORACLE database consists of one or more files called datafiles, which are physical structures that conform with the operating system in which Oracle is running.
For Example::: A simple Oracle Database have one tablespace and one datafile.
An index cluster uses an index to maintain data within the cluster. The structure of a cluster index is similar to a normal index.
Although a normal index does not store NULL key values, and index cluster can store it.
And, a cluster index is likely to be smaller than a normal index because, there is only one entry for each key value in the cluster index, even if that is repeating.
DDL commands are auto commited commands, after DDL job execution
no need to commit again
An index is a method of allowing faster retrieval of records.
A view is an virtual table formed using a single table or one or more tables.
A synonym is an alternative name for objects such as tables, views, sequences, stored procedures, and other database objects
It consists of
one or more data files.
one or more control files.
two or more redo log files.
The Database contains
multiple users/schemas
one or more rollback segments
one or more tablespaces
Data dictionary tables
User objects (table,indexes,views etc.,)
The server that access the database consists of
SGA (Database buffer, Dictionary Cache Buffers, Redo log buffers, Shared SQL pool)
SMON (System MONito)
PMON (Process MONitor)
LGWR (LoG Write)
DBWR (Data Base Write)
ARCH (ARCHiver)
CKPT (Check Point)
RECO
Dispatcher
User Process with associated PGS
its called snap shot too old error.
it comes if a user is updates a table with loop option, in another session an user to see the info(select) then its shows ora-1555
error
Exceptions can be handled in various ways in the stored procedures. Method (1) could be using EXCEPTION block in the procedure within which use WHEN <exceptionname> THEN. Method (2) could be declare a variable in DECLARE section as EXCEPTION type, raise it wherever required using RAISE <exception variable name> and handle it in EXCEPTION part using WHEN clause. Method (3) could be using the keywords SQLCODE & SQLERRM to display error no and error code..
A view does not contain any data of its own, but is like a window through which data from other tables can be viewed and changed.
temp segment,
data segment,
table segment,
index segment
information about the database,redologfile and datafile,archivelog file location and current scn no
and redo log file
1. Database name and identifier.
2. Time stamp of database creation.
3. Tablespace names.
4. Backup information.
5. Checkpoint information.
6. Current online redo log file sequence numbe
Two processes wating to update the rows of a table which are locked by the other process then deadlock arises. In a database environment this will often happen because of not issuing proper row lock commands. Poor design of front-end application may cause this situation and the performance of server will reduce drastically.
These locks will be released automatically when a commit/rollback operation performed or any one of this processes being killed externally.
Deadlock is a kind of situation,which occured in a network of database environment while multiple processes is waiting for their completion, whereas some other process is still running till the time we use the rollback or commit commands or stopped the blocked process by database systems program itself.
The use of RollBack Segment is Database is - to maintain read consistency between multiple transactions.
Rollback segments are the areas in your database which are used to temporarily save the previous values when some updates (by 'updates', inserts or deletes as well) or any other transactions going on.
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