Oracle 10g Cost Based Optimizer

Hello Friends, here i like to give some brief on Cost based optimizer in Oracle, i am not covering Rule based optimizer as it is not much in use as well not recommended. plz share ur feedback or if u like to contribute more on this topic,

Oracle 10g Cost Based Optimizer

Whenever you execute a SQL statement, a component of the database known as the optimizer must decide how best to access the data operated on by that statement. To figure out the optimal execution path for a statement, the optimizers consider the following:

  • The syntax you’ve specified for the statement
  • Any conditions that the data must satisfy (the WHERE clauses)
  • The database tables your statement will need to access
  • All possible indexes that can be used in retrieving data from the table
  • The Oracle RDBMS version
  • The current optimizer mode
  • SQL statement hints
  • All available object statistics (generated via the ANALYZE command)
  • The physical table location (distributed SQL)
  • INIT.ORA settings (parallel query, async I/O, etc.)

Understanding the Cost-Based Optimizer

The information required by the cost-based optimizer is available once a table has been analyzed via the ANALYZE command, or via the DBMS_STATS facility. Plz note The ANALYZE command and the DBMS_STATS functions collect statistics about tables, clusters, and indexes, and store those statistics in the data dictionary.

The RDBMS kernel defaults to using the cost-based optimizer under a number of situations, including the following:

  • OPTIMIZER_MODE = CHOOSE has been specified in the INIT.ORA file, and statistics exist for at least one table involved in the statement
  • An ALTER SESSION SET OPTIMIZER_MODE = CHOOSE command has been executed, and statistics exist for at least one table involved in the statement
  • An ALTER SESSION SET OPTIMIZER_MODE = FIRST_ROWS (or ALL_ROWS) command has been executed, and statistics exist for at least one table involved in the statement
  • A statement uses the FIRST_ROWS or ALL_ROWS hint (e.g., SELECT /*+ FIRST_ROWS */. . .)

ANALYZE command

The way that you analyze your tables can have a dramatic effect on your SQL performance. If your DBA forgets to analyze tables or indexes after a table re-build, the impact on performance can be devastating. If your DBA analyzes each weekend, a new threshold may be reached and Oracle may change its execution plan. The new plan will more often than not be an improvement, but will occasionally be worse.

If you do want to analyze frequently, use DBMS_STATS.EXPORT_SCHEMA_STATS to back up the existing statistics prior to re-analyzing. This gives you the ability to revert back to the previous statistics if things screw up. When you analyze, you can have Oracle look at all rows in a table (ANALYZE COMPUTE) or at a sampling of rows (ANALYZE ESTIMATE). Typically, When a table is analyzed using ANALYZE, all associated indexes are analyzed as well. If an index is subsequently dropped and recreated, it must be re-analyzed. Be aware that the procedures DBMS_STATS.GATHER_SCHEMA_STATS and GATHER_TABLE_STATS analyze only tables by default, not their indexes. When using those procedures, you must specify the CASCADE=>TRUE option for indexes to be analyzed as well.

Following are some sample ANALYZE statements:



If you analyze a table by mistake, you can delete the statistics. For example:


Analyzing can take an excessive amount of time if you use the COMPUTE option on large objects. We find that on almost every occasion, ANALYZE ESTIMATE 5 PERCENT on a large table forces the optimizer make the same decision as ANALYZE COMPUTE.

Inner workings of the cost-based optimizer

The cost-based optimizer is flexible and can adapt to its environment. This adaptation is possible only once the necessary underlying object statistics have been refreshed (re-analyzed). What is constant is the method by which the cost-based optimizer calculates each possible execution plan and evaluates its cost (efficiency).

The cost-based optimizer’s functionality can be (loosely) broken into the following steps:

1.       Parse the SQL (check syntax, object privileges, etc.).

2.       Generate a list of all potential execution plans.

3.       Calculate (estimate) the cost of each execution plan using all available object statistics.

4.       Select the execution plan with the lowest cost.

The cost-based optimizer will be used only if at least one table within a SQL statement has statistics (table statistics for unanalyzed tables are estimated). If no statistics are available for any table involved in the SQL, the RDBMS will resort to the rule-based optimizer, unless the cost-based optimizer is forced via statement-level HINTS or by an optimizer goal of ALL_ROWS or FIRST_ROWS.

To understand how the cost-based optimizer works and, ultimately, how to exploit it, we need to understand how it thinks.

Primary key and/or UNIQUE index equality —A UNIQUE index’s selectivity is recognized as 100%. No other indexed access method is more precise. For this reason, a unique index is always used when available.

Non-UNIQUE index equality —For non-UNIQUE indexes, index selectivity is calculated. The cost-based optimizer makes the assumption that the table (and subsequent indexes) have uniform data spread unless you use the FOR ALL INDEXED COLUMNS option of the ANALYZE. That option will make the cost-based optimizer aware of how the data in the indexed columns is skewed.

Range evaluation —For index range execution plans, selectivity is evaluated. This evaluation is based on a column’s most recent high-value and low-value statistics. Again, the cost-based optimizer makes the assumption that the table (and subsequent indexes) have uniform data spread unless you use the FOR ALL INDEXED COLUMNS option when analyzing the table.

Range evaluation over bind variables—For index range execution plans, selectivity is guessed. Prior to Oracle9i, because bind variable values are not available at parse time (values are passed to the cursor after the execution plan has been decided), the optimizer cannot make decisions based on bind variable values. The optimizer assumes a rule of thumb of 25% selectivity for unbounded bind variable ranges (e.g., WHERE dept_no = :b1) and 50% selectivity for bounded ranges (WHERE dept_no > :b1 AND dept_no < :b2). Beginning with Oracle9i, the cost-based optimizer obtains bind variable values prior to determining an execution plan.

System resource usage —By default, the cost-based optimizer assumes that you are the only person accessing the database. Oracle9i gives you the ability to store information about system resource usage, and can make much better informed decisions based on workload (read up on the DBMS_STATS.GATHER_SYSTEM_STATS package).

Current statistics are important —The cost-based optimizer can make poor execution plan choices when a table has been analyzed but its indexes have not been, or when indexes have been analyzed but not the tables.

Oracle – Technical FAQ – Part 4

 Q18.) What are the OOPS concepts in Oracle?

A18.) Oracle does implement the OOPS concepts. The best example is the Property Classes. We can categorise the properties by setting the visual attributes and then attach the property classes for the objects. OOPS supports the concepts of objects and classes and we can consider the property classes as classes and the items as objects.


Q19) What is the difference between candidate key, unique key and primary key?

A19) Candidate keys are the columns in the table that could be primary keys and the primary key is the key that has been selected to identify the rows. Unique key is also useful for identifying the distinct rows in the table.


Q20) What is concurrency?

A20) Concurrency is allowing simultaneous access of same data by different users. Locks useful for accessing the database are

a) Exclusive — The exclusive lock is useful for locking the row when an insert , update or delete is being done. This lock should not be applied when we do only select from the row.

b) Share lock — We can do the table as Share_Lock as many share_locks can be put on the same resource.


Q21) What are the Previleges and Grants?

A21) Privileges are the right to execute a particular type of SQL statements. The grant has to be given by the owner to object.


Q22) What are Table Space, Data files, parameter file, control files?

A22) Table space: The table space is useful for storing the data in the database. When a database is created two table spaces are created.

a) System Table space : This data file stores all the tables related to the system and dba tables.

b) User Table Space: This data file stores all the user related tables.

We should have separate table spaces for storing the tables and indexes so that the access is fast.

Data files : Every oracle Data base has one or more physical data files. They store the data for the database. Every datafile is associated with only one database. Once the data file is created the size can not change. To increase the size of the database to store more data we have to add data file.

Parameter file:Parameter file is needed to start an instance. A parameter file contains the list of instance configuration parameters e.g. db_block_buffers = 500

db_name = ORA7

db_domain = u.s.acme lang

Control files : Control files record the physical structure of the data files and redo log files . They contain the DB name, name and locations of dbs , data files, redo log files and time stamp.


Q23) What is physical storage of the Data?

A23) The finest level of granularity of the data base are the blocks.

Extent : Extent is the number of specific number of contigious data blocks.

Segments : Set of Extents allocated for Extents. There are three types of segments

a) Data Segment : Non clustered tables has data segment data of every table is stored in cluster data segment.

b) Index Segment : Each index has index segment that stores data.

c) Rollback Segment : Temporarily store ‘undo’ information.


Q24) What is Normalisation ? Explain 1NF, 2NF, 3NF.

Normalization is a process of eliminating data redudancy.

1NF- No repeating groups.

2NF- Eliminates the functional dependency on the partial key field.

3NF- Eliminates the functional dependency on the non-key field.

Oracle – Technical FAQ – Part 2

Q6) What are the database triggers and stored procedures?
A6) Database triggers are procedures that are automatically executed as a result of insert in, update to or delete from table.
Database triggers have the values old and new to denote the old value in the table before it is deleted and the new indicated the new value that will be used. DT are useful for implementing complex business rules which can not be enforced using the integrity rules. We can have the trigger as before trigger or after trigger and at statement or row level.
e.g. operations insert, update, delete — 3
before, after 3 * 2 — a total of 6 combinations

At statement level (once for the trigger) or row level (for every execution) 6 * 2 — a total of 12.

Thus a total of 12 combinations are there and the restriction of usage of 12 triggers has been lifted from Oracle 7.3 onwards

Stored Procedures : Stored procedures are procedures that are stored in complied form in the database. The advantage of using the stored procedures is that many users can use the same procedure in compiled and ready to use format.


Q7) How many integrity rules are there and what are they?

A7) There are three integrity rules and they are as follows

a)Entity integrity Rule : The entity integrity rule enforces that the primary key can not be Null

b) Foreign key integrity rule : The FKIR denotes that the relationship between the foreign key and the primary key has to be enforced. When there is data in child tables the master tables can not be deleted.

c) Business integrity rules : The third integrity rule is about the complex business processes which can not be implemented by the above 2 rules.


Q.8.) What are the various Master and Detail relationships?

A.8.) The various Master and Detail relationship are

a) Non Isolated : The Master can not be deleted when a child is existing

b) Isolated : The master can be deleted when the child is existing

c) Cascading : The child gets deleted when the master is deleted.


Q9) What are the various Block Coordination properties ?.

A9) The various block coordination properties are

a) Immediate – Default setting. The detail records are shown when the Master Record are shown.

b) Differed with AutoQuery — Oracle forms defer fetching the detail records until the operator navigates to the detail block.

C) Deffered with no auto query — The operator must navigate to the detail block and explicitly executes a query.


Q10) What are the different Optimisation techniques ?

A10) The various optimisation techniques are

a) Execute plan : We can see the plan of the query and change it accordingly based on the indexes

b) Optimizer_hint : Set_item_property (‘DeptBlock’,OPTIMIZER_HINT,’FIRST_ROWS’);

Select /* + First Rows */ Deptno, Dname, Loc, Rowid from dept

where (Deptno > 25)

c) Optimize_sql : By setting the optimize_sql = No, Oracle Forms assign a single cursor for all SQL statements. This slow downs the processing because for everytime the SQL must be parsed whenever they are executed. f45run module = my_firstform useid = scott/tiger optimize_Tp = No


  more questions are here :

Part 3 is here :

Part 1 is here :




Oracle – Technical FAQ – Part 1

Q1) What are the background processes in Oracle and what are they?
Ans: This is one of the most frequently asked question. There are basically 9 processes but in a general system we need t mention the first five background processes. They do the house keeping activities for the Oracle and are common in any system
The various background processes in oracle are
a) Data base writer (DBWR) : Data Base Writer writes modified blocks from databas buffer cache to Datafiles. This is required since the data is not written whenever a transaction is commited.

b) LogWriter(LGWR) : LogWriter writes the redo log entries to disk. Redo log data is generated in redo log buffer of SGA. As transaction commits and logs buffer fills, LGWR writes log entries into a online redo log file.

c) System Monitor (SMON) : The System Monitor performs instance recovery at instance startup. This is useful for recovery from system failure.

d) Process Monitor (PMON) : The Process Monitor performs process recovery when user process fails. Pmon clears and frees resources that process was using.

e) CheckPoint (CKPT): At specified times, all modified database buffers in SGA are written to data files by DBWR at checkpoints and updating all data files and control files of database to indicate the most recent checkpoint.

f) Archiever (ARCH) : The archiever copies online redo log files to archival storal when they are busy.

g) recoveror (RECO) : The recoveror is used to reslove the distributed transaction in network.

h) Dispatcher (Dnnn) : The Dispatcher is useful in Multi Threaded Architecture.

i) Lckn : We can have upto 10 lock processed for inter instance locking in parallel sql.

Q2) How many types of sql statements are there in Oracle?

A2) There are basically 6 types of sql statements. They are

a) Data defination Language (DDL) : The DDL statements define and maintain objects and drop objects.

b) Data Manipulation Language (DML) : The DML statements manipulate database data

c) Transaction Control Statements : Manage change by DML

d) Session Control : Used to control the properties of current session enabling and disabling roles and changing e.g. Alter statements , Set Role

e) System Control Statements : Change properties of Oracle Instance e.g. Alter System

f) Embedded Sql : Incorporate DDL, DML and TCS in processing Languagel e.g. Using the SQL statements in language such as ‘C’, Open, Fetch, Execute and close

Q3) What is a transaction in Oracle ?

A3) A transaction is a logical unit of work that compromises one or more SQL statements executed by a simple user. According to ANSI, a transaction begins with first executable statement and ends when it is explicitly commited or rolled back.

Q4) What are the key words used in Oracle?

A4) The key words that are used in Oracle are

a) Commiting : A transaction is said to be commited when the transaction makes permanent changes resulting from the SQL statements

b) Rollback : A transaction that retracts any of the changes resulting from SQL statements in Transaction.

c) Savepoint : For long transactions that contain many SQL statements , intermediate markers or savepoints are declared. Savepointes can be used to divide a transaction into smaller points.

d) Rolling forward : Process of applying redo log during recovery is called rolling forward.

e) Cursor : A cursor is a handle (name or a pointer) for the memory associated with a specific statement. A cursor is basically an area allocated by Oracle for executing the SQL statement. Oracle uses an implicit cursor statement for single row query and uses Explicit cursor for a multirow query.

f) System Global Area (SGA) : The SGA is a shared memory region allocated by the Oracle that contains Data and control information for one Oracle Instance. It consists of Database buffre cache and redo log buffer.

g) Program Global Area (PGA) : The PGA is a memory buffer that contains data and control information for server process.

h) Database Buffer Cache : Database buffer of SGA stores he most recently used blcoks of database data. The set of database buffers in an instance is called Database Buffer Cache.

i) Redo log buffer : Redo log buffer of SGA stores all the redo log entries.

j) Redo log files :Redo log files are set of files that protect altered database in memory that has been not written to Data files. They are basically used for backup when a database crashes.

k) Process : A process is a ‘thread of control’ or mechanism in Operating System that executes series of steps

Q5) What are procedures, functions and packages?

A5) Procedures and functions consist of set of PL/SQL statements that are grouped together as a unit to solve a specific problem or perform a set of related tasks.

Procedure do not return values while functions return one and only one value.

Packages provide a method of encapsulating and storing related procedures, functions and other package contents.

 more questions are here :

Part 3 is here :




Oracle Database related Questions

Here there are very basic still sometime become critical to know concept if you working in Oracle Database related activities. 

Q: What is an Oracle instance?Every running Oracle database is associated with an Oracle instance. When adatabase is started on a database server (regardless of the type of computer),Oracle allocates a memory area called the System Global Area (SGA) and starts one or more Oracle processes. This combination of the SGA and the Oracle processes is called an Oracle instance. The memory and processes of an instance manage the associated database’s data efficiently and serve the one or multiple users of the database.
The Instance and the Database

After starting an instance, Oracle associates the instance with the specified database. This is called mounting the database. The database is then ready to be opened, which makes it accessible to authorized users. Multiple instances can execute concurrently on the same computer, each accessing its own physical database. In clustered and massively parallel systems (MPP),the Oracle Parallel Server allows multiple instances to mount a single database. Only the database administrator can start up an instance and open the database.If a database is open, the database administrator can shut down the database so that it is closed. When a database is closed, users cannot access the information that it contains. Security for database startup and shutdown is controlled via connections to Oracle with administrator privileges. Normal users do not have control over the current status of an Oracle database.




Q: What is a view?

A view is a tailored presentation of the data contained in one or more tables(or other views). Unlike a table, a view is not allocated any storage space, nor does a view actually contain data; rather, a view is defined by a query that extracts or derives data from the tables the view references. These tables are called base tables. Views present a different representation of the data that resides within thebase tables. Views are very powerful because they allow you to tailor the presentation of data to different types of users. Views are often used to:

• provide an additional level of table security by restricting access to a predetermined set of rows and/or columns of a table

• hide data complexity

• simplify commands for the user

• present the data in a different perspective from that of the base table

• isolate applications from changes in definitions of base tables

• express a query that cannot be expressed without using a view

Q: What is referential integrity?


Rules governing the relationships between primary keys and foreign keys of tables within a relational database that determine data consistency. Referential integrity requires that the value of every foreign key in every table be matched by the value of a primary key in another table.

Q: What is a cursor?A cursor is a private sql work area used to perform manipulations on data using pl\sql, mainly used for multiple row manipulations and locking columns. Data which is populated into the cursor is known as active dataset.


Cursors are of two types 1.implicit   2.explicit

Implicit———attributes or properties for implicit cursor

1.sql%is open:attribute returns a boolean value stating wether the cursor is open or closed.

2.sql % found: returns boolean value stating whether the record is found in the cursor.

3.sql%notfound : returns a boolean value stating whether the record is not found in the cursor

4.sql %rowcount :returns a pneumeric value stating no.of rows executed in the cursor.

Explicit cursors—retrives multiple rows, users can perform locks on th data in the cursor attributes-

1.% is open

2.% found

3.% not found

4.% rowcount




Q: Why Use Sql* Loader in Oracle Database? 
The Sql Loader utility loads data into an existing ORACLE table from an external files.



Sample Example on Viewing Worksheet through Discoverer Plus via URL Link

Sample Example on Viewing Worksheet through Discoverer Plus via URL Link

Sample example on viewing worksheet through discoverer plus via URL link.




Trace a Concurrent Request And Generate TKPROF File

Hello Friends, As in Oracle Application, there always need to tune programs , procedure and it is always difficult to make a perfect program, here i am trying to put a “How to” on tkprof and surely like to know if you find it useful.

How to Trace a Concurrent Request And Generate TKPROF File

 Enable Tracing For The Concurrent Manager  Program 

  • Responsibility: System Administrator
  • Navigate: Concurrent > Program > Define
  • Query Concurrent Program
  • Select the Enable Trace Checkbox 

Turn On Tracing

  • Responsibility: System Administrator
  • Navigate: Profiles > System
  • Query Profile Option Concurrent: Allow Debugging
  • Set profile to Yes

 Run Concurrent Program With Tracing Turned On

  • Logon to the Responsibility that runs the Concurrent Program 
  •  In the Submit Request Screen click on Debug Options (B)
  • Select the Checkbox for SQL Trace

 2. Find Trace File Name

  Run the following SQL to find out the Raw trace name and location for the concurrent program.  The SQL prompts the user for the request id

SELECT ‘Request id: ‘||request_id ,  ‘Trace id: ‘||oracle_Process_id,  ‘Trace Flag: ‘||req.enable_trace,  ‘Trace Name:  ‘||dest.value||’/’||lower(dbnm.value)||’_ora_’||oracle_process_id||’.trc’,  ‘Prog. Name: ‘||prog.user_concurrent_program_name,  ‘File Name: ‘||execname.execution_file_name|| execname.subroutine_name ,  ‘Status : ‘||decode(phase_code,’R’,’Running’)  ||’-‘||decode(status_code,’R’,’Normal’),  ‘SID Serial: ‘||ses.sid||’,’|| ses.serial#,  ‘Module : ‘||ses.module  from fnd_concurrent_requests req, v$session ses, v$process proc,  v$parameter dest, v$parameter dbnm, fnd_concurrent_programs_vl prog,  fnd_executables execname  where req.request_id = &request  and req.oracle_process_id=proc.spid(+)  and proc.addr = ses.paddr(+)  and’user_dump_dest’  and’db_name’  and req.concurrent_program_id = prog.concurrent_program_id  and req.program_application_id = prog.application_id  and prog.application_id = execname.application_id  and prog.executable_id=execname.executable_id; 





3. TKPROF Trace File

Once you have obtained the Raw trace file you need to format the file using TKPROF.

$tkprof raw_trace_file.trc output_file explain=apps/apps sort=(exeela,fchela) sys=no

Where: raw_trace_file.trc: Name of trace file

output_file: tkprof out file

explain: This option provides the explain plan for the sql                      statements

sort: his provides the sort criteria in which all sql statements will be sorted.  This will bring the bad sql at the top of the outputfile.

sys=no:Disables sql statements issued by user SYS 

Another example: To get (TKPROF) sorted by longest running queries first and limits the results to the “Top 10” long running queries



$ tkprof <filename.trc> <output_filename> sys=no explain=apps/<password> sort='(prsela,exeela,fchela)’ print=10 





Thanks – Shivmohan Purohit