performance tuning is used to see the status of object 
means make that object portable and effcience, and replace 
with new suitable statements,  method for the performance 
tuning is sql tracer,....

we can analyze performance of any program by using t.code
se30( runtime analysis). there we can see performance of
database server , application server and processor  . If
database is taken more time , try to modify select staments.
Hence performance of program can be improved.

Performance tuning is something related to incresing the 
performance of the program ie not giving the full load to 
the application server.
this can be done in following methods,
1. using corresponding buffering.
2. not using SELECT *
3. maximum usage of WHERE condition
4. using VIEWs
5. mentioning exactly 'occurs n' 
and so on

Performance tuning is a technique used to fine tune your 
ABAP programs when they have complex select statements and 
huge set of data to be accessed from database.

Performacne of a scelect statement or a program can be 
tested using tools like  SE 30- Runtime analysis and SQL 
Trace ST05.

Other than this there are  set of thumb rules that can be 
followed:

1)	Keep the data selection small.
	Avoid unnecessary data across the network
	Always use the where clause
	Avoid selecting useless data we filter later.
	Use the index of the relevant database tables to 
make the where clause more efficient
	Avoid using complex where clauses
	If possible avoid using the NOT in conjunction with 
the where clause.
2)	Transport as little data as possible
•	transport only fields that are really used
•	use the aggregate functions in the select clause 
for calculations
•	Consider using the distinct statement if there is 
possibility of duplicate entries
3)	Use fewer data base access
•	Transfer all of the data at once frm data base to 
internal tables.
•	When possible avoid accessing the same data again 
and again
•	Avoid using nested loops instead of that use an 
internal table and use the select for all entires statement.