Oracle Notes Part-5

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CURSOR: A cursor is a temporary work area created in the system memory when a SQL statement is executed. A cursor contains information on a select statement and the rows of data accessed by it.This temporary work area is used to store the data retrieved from the database, and manipulate this data. A cursor can hold more than one row, but can process only one row at a time. The set of rows the cursor holds is called the active set.

Two Types of Cursor :

1)Implicit Cursor

Implicit cursors are automatically created by Oracle whenever an SQL statement is executed, when there is no explicit cursor for the statement. Programmers cannot control the implicit cursors and the information in it.Whenever a DML statement (INSERT, UPDATE and DELETE) is issued, an implicit cursor is associated with this statement. For INSERT operations, the cursor holds the data that needs to be inserted. For UPDATE and DELETE operations, the cursor identifies the rows that would be affected.

2)Explicit Cursor

They must be created when you are executing a SELECT statement that returns more than one row. Even though the cursor stores multiple records, only one record can be processed at a time, which is called as current row. When you fetch a row the current row position moves to next row.

For Example: When you execute INSERT, UPDATE, or DELETE statements the cursor attributes tell us whether any rows are affected and how many have been affected. When a SELECT… INTO statement is executed in a PL/SQL Block, implicit cursor attributes can be used to find out whether any row has been returned by the SELECT statement. PL/SQL returns an error when no data is selected.

In PL/SQL, you can refer to the most recent implicit cursor as the SQL cursor, which always has the attributes like %FOUND, %ISOPEN, %NOTFOUND, and %ROWCOUNT. The SQL cursor has additional attributes, %BULK_ROWCOUNT and %BULK_EXCEPTIONS, designed for use with the FORALL statement.

TRIGGER: Triggers are stored programs, which are automatically executed or fired when some events occur.Trigger automatically associated with DML statement, when DML statement execute trigger implicitly execute.You can create trigger using the CREATE TRIGGER statement. If trigger activated, implicitly fire DML statement and if trigger deactivated can’t fire.

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Triggers could be defined on the table, view, schema, or database with which the event is associated.

Advantages of trigger:

1) Triggers can be used as an alternative method for implementing referential integrity constraints.

2) By using triggers, business rules and transactions are easy to store in database and can be used consistently even if there are future updates to the database.

3) It controls on which updates are allowed in a database.

4) When a change happens in a database a trigger can adjust the change to the entire database.

5) Triggers are used for calling stored procedures.

 

Use the CREATE TRIGGER statement to create and enable a database trigger, which is:

  • A stored PL/SQL block associated with a table, a schema, or the database or
  • An anonymous PL/SQL block or a call to a procedure implemented in PL/SQL or Java

Oracle Database automatically executes a trigger when specified conditions occur.When you create a trigger, the database enables it automatically. You can subsequently disable and enable a trigger with the DISABLE and ENABLE clause of the ALTER TRIGGER or ALTER TABLE statement.

Before a trigger can be created, the user SYS must run a SQL script commonly called DBMSSTDX.SQL. The exact name and location of this script depend on your operating system.

  • To create a trigger in your own schema on a table in your own schema or on your own schema (SCHEMA), you must have the CREATE TRIGGERsystem privilege.
  • To create a trigger in any schema on a table in any schema, or on another user’s schema (schema.SCHEMA), you must have the CREATE ANYTRIGGER system privilege.
  • In addition to the preceding privileges, to create a trigger on DATABASE, you must have the ADMINISTER DATABASE TRIGGER system privilege.

If the trigger issues SQL statements or calls procedures or functions, then the owner of the trigger must have the privileges necessary to perform these operations. These privileges must be granted directly to the owner rather than acquired through roles.

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Data Blocks

At the finest level of granularity, Oracle stores data in data blocks (also called logical blocks, Oracle blocks, or pages). One data block corresponds to a specific number of bytes of physical database space on disk. You set the data block size for every Oracle database when you create the database. This data block size should be a multiple of the operating system’s block size within the maximum limit. Oracle data blocks are the smallest units of storage that Oracle can use or allocate.In contrast, all data at the physical, operating system level is stored in bytes. Each operating system has what is called a block size. Oracle requests data in multiples of Oracle blocks, not operating system blocks. Therefore, you should set the Oracle block size to a multiple of the operating system block size to avoid unnecessary I/O.

Extents

The next level of logical database space is called an extent. An extent is a specific number of contiguous data blocks that is allocated for storing a specific type of information.

Segments

The level of logical database storage above an extent is called a segment. A segment is a set of extents that have been allocated for a specific type of data structure, and that all are stored in the same tablespace. For example, each table’s data is stored in its own data segment, while each index’s data is stored in its own index segment.Oracle allocates space for segments in extents. Therefore, when the existing extents of a segment are full, Oracle allocates another extent for that segment. Because extents are allocated as needed, the extents of a segment may or may not be contiguous on disk. The segments also can span files, but the individual extents cannot.

There are four types of segments used in Oracle databases:

– data segments
– index segments
– rollback segments
– temporary segments

Data Segments:
There is a single data segment to hold all the data of every non clustered table in an oracle database. This data segment is created when you create an object with the CREATE TABLE/SNAPSHOT/SNAPSHOT LOG command. Also, a data segment is created for a cluster when a CREATE CLUSTER command is issued.
The storage parameters control the way that its data segment’s extents are allocated. These affect the efficiency of data retrieval and storage for the data segment associated with the object.

Index Segments:
Every index in an Oracle database has a single index segment to hold all of its data. Oracle creates the index segment for the index when you issue the CREATE INDEX command. Setting the storage parameters directly affects the efficiency of data retrieval and storage.

Rollback Segments
Rollbacks are required when the transactions that affect the database need to be undone. Rollbacks are also needed during the time of system failures. The way the roll-backed data is saved in rollback segment, the data can also be redone which is held in redo segment.

A rollback segment is a portion of the database that records the actions of transactions if the transaction should be rolled back. Each database contains one or more rollback segments. Rollback segments are used to provide read consistency, to rollback transactions, and to recover the database.

Types of rollbacks:
– statement level rollback
– rollback to a savepoint
– rollback of a transaction due to user request
– rollback of a transaction due to abnormal process termination
– rollback of all outstanding transactions when an instance terminates abnormally
– rollback of incomplete transactions during recovery.

Temporary Segments:
The SELECT statements need a temporary storage. When queries are fired, oracle needs area to do sorting and other operation due to which temporary storages are useful.

The commands that may use temporary storage when used with SELECT are:
GROUP BY, UNION, DISTINCT, etc.