What is Shared Pool in Oracle?

Shared memory area Shared Pool

The Shared Pool is made up of two performance-related memory structures:

    Row cache, discussed below.
    Cache "library" examined in the second advanced tutorial on oracle architecture.

Row cache.

When a user submits an SQL query, the server process extracts, during the analysis of the query, information stored in the dictionary tables (user account data, names of data files, names of table segments and indexes , extents locations, table descriptions and user privileges…).

This information is cached in the dictionary for reuse. During subsequent parse scans, the server process searches the dictionary cache for information to resolve object names and validate access.


Caching data dictionary information reduces response time to LMD statements (SELECT, INSERT, UPDATE, DELETE). A sufficient size of this cache contributes significantly to improving performance.

If the dictionary cache is of limited size, recursive calls slower than the queries made directly in the cache, will be operated by the server process on the database dictionary (disk access).

To learn more about the components and basic concepts of the Oracle architecture, take look at our Expert DBA Team Club blog and more advance topics are available at this source.

What is SGA in Oracle?

The SGA (System Global Area) in Oracle Instance

System Global Area (SGA) memory is memory shared by all server and background processes.


The SGA is made up of three mandatory components and three optional elements.

Mandatory components:

     Shared memory area
     Database buffer cache
     Redo log buffer

Optional items:

     LARGE POOL memory area
     Java memory area (Java Pool)
     Streams memory area (streams pool)

To learn more about the components and basic concepts of the Oracle architecture, take look at our Expert DBA Team Club blog and more advance topics are available at this source.

What is Oracle instance

What is Oracle instance?

An Oracle server is made up of an Oracle instance and an Oracle database.

An Oracle database can only be accessed through one instance. See our tutorial on managing an oracle instance to learn how to manipulate the associated database.

An Oracle server instance is made up of a SGA (Global Area System) shared memory structure and several background Oracle processes each with a specific role.


An instance can only open one database at a time.

A database can be opened by multiple instances using the RAC (Oracle Real Application Clusters) option for high availability and load balancing. (see diagram below)


In such a configuration, users waiting to be connected can be directed to the least loaded instance and in addition, if one instance fails, the other takes over for better database availability.

N.B: In Windows environment, each instance is associated with a service named OracleService “InstanceName”. This service must be started in order to start the instance.

When it starts up, the instance uses an initialization settings file.

An instance has a SID. This determines the parameter file used at STARTUP (spfile ”SID” .ora or spfile.ora or init ”SID” .ora in this order of priority, if Oracle does not find one it goes to the next).

The components of the instance (SGA + background processes) are described in the following sections.

To learn more about the components and basic concepts of the Oracle architecture, take look at our Expert DBA Team Club blog and more advance topics are available at this source.

PGA memory in Oracle

What is the PGA memory

An additional memory structure called PGA (Global Area Program) is created for each logged in user.

PGA stores user session specific control information such as private areas for cursor processing, bind variables, session information, sort area ...

Each server process has its own private PGA memory which is exclusively reserved for it.

When the user process disconnects (end of session), the associated server process ends and the PGA memory is freed.

N.B: In a shared server configuration:

    a server process handles requests from multiple user processes.
    a global amount of PGA memory can be allocated to the Oracle instance through the PGA_AGGREGATE_TARGET initialization parameter.

Depending on the technical architecture of the application, the user process exists, either on the user's PC (in the case of a client / server application) or on the application server of the intermediate level (middle-tier) in the case of an n-tier application.

To learn more about the components and basic concepts of the Oracle architecture, take look at our Expert DBA Team Club blog and more advance topics are available at this source.

Characteristics of Oracle Server

Oracle server

An Oracle server, a database management system, consists of an Oracle instance and an Oracle database.

The instance and the database together constitute an Oracle server.

The architecture of Oracle Server can be described in three phases:

    User login to the database
    Memory structures that are part of the Oracle instance
    Background processes that are part of the Oracle instance
    Physical structures of files forming the Oracle database.

User login to the database

Two processes allow a user to interact with the instance and ultimately with the database: the user process and the server process.


Each time a user runs an application, such as an application for human resources management, financial management, or just an SQL command, the client machine first launches a user process to establish a connection from the user to the Oracle instance.
The Oracle Listener Listening Process

The Oracle listener process is the primary server-side Oracle component that establishes the connection between client computers and an Oracle database. The listener can be seen as a big ear that listens for connection requests to Oracle services.

Theoretically, a server machine can host several Oracle databases and one listener and only one to allow a client to connect to the Oracle instance of his choice. The instance name is submitted by the client during the connection process (step 1).

Two cases are possible:

    A Listener process can serve multiple databases configured on the same server machine,
    Several listener can be configured on the same machine (for failover or load balancing purposes to support heavy loads of connection requests).

In a dedicated server configuration, the Listener starts a new server process for each client and gives it control of the client session. Each client connection is served by its own server process.

The diagram above corresponds to a dedicated server configuration and for a client / server application.

The connection process goes through the following steps:

    The customer contacts the Oracle listener by choosing the instance to which he wishes to connect (request for a service name).
    The listener starts a dedicated process called a server process
    The listener sends an acknowledgment to the client with the address of the dedicated server process
    The client establishes a connection with the dedicated server process
    The server process connects to the Oracle instance on behalf of the user process (creation of a user session)

It is the server process that connects to the Oracle instance to serve the user process throughout the client session.

The user process does not directly interact with the Oracle server. Rather, it is the server process that interacts with the Oracle server, responds to user requests and returns results to the user.

To learn more about the components and basic concepts of the Oracle architecture, take look at our Expert DBA Team Club blog and more advance topics are available at this source.

SQL language for Oracle DBA

SQL language

SQL is a standard non-procedural language for relational database management systems (RDBMS)

Examples of SQL queries:

    SELECT ename FROM emp;
    SELECT last_name, department_id FROM employees;
    DELETE FROM department WHERE department_id IN (10, 30, 70);
    CREATE TABLE articles (article_id VARCHAR2 (15), designation VARCHAR2 (40));
    INSERT INTO articles VALUES (‘imp1’, laser printer ’);
    DROP TABLE articles;
    GRANT SELECT ON department TO scott;
    REVOKE DELETE ON department FROM scott;

The terms SELECT, FROM, DELETE, WHERE, IN, CREATE, TABLE, VARCHAR2, INSERT, INTO, VALUES, DROP, GRANT, TO, REVOKE, ON are words reserved for SQL

Oracle SQL statements are classified into categories:

  DML commands for Data Manipulation Language (SELECT, INSERT, UPDATE, DELETE)
    DDL commands for Data Definition Language (CREATE, ALTER, DROP, RENAME, TRUNCATE, GRANT, REVOKE…).
        DDL commands are automatically validated: they generate an implicit commit.
    Control Commands of:
        Transactions (COMMIT, ROLLBACK, SAVEPOINT, SET TRANSACTION)
        Sessions (ALTER SESSION)
        and System (ALTER SYSTEM)

SQL Recursive commands:

When an DDL command is issued, Oracle automatically issues an LMD command to modify the corresponding information in the data dictionary. This last command is called a recursive SQL command.

An DDL "CREATE TABLE" command, for example, issued by a user to create a table in their own schema, automatically generates another recursive LMD command (INSERT) in the SYS schema: data dictionary tables. In fact, the information on the structure of the newly created table must be inserted and available in the data dictionary.


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