buffer gets.

[jayasankar]

Can somebody tell me why these queries are written two different results and why?

desc employees;

employee_id number(6);
first_name varchar2(20)
last_name varchar2(25)
email varchar2(25)
phone_number number(10)
job_id number(4)
salary number(8,2)


1)select * from employees;

Buffrer-gets----165

2)select job_id,salary,email,first_name from employees

BUFFER_GETS----17

In second query it is accessing only few fields that's why it has accessed only few blocks, but oracle normal does block wise read not row or column wise read , so why difference has come.

[skhanal]

Are there other columns in the table? Do you see row_chaining in the table?

[jayasankar]

Are there other columns in the table? Do you see row_chaining in the table?

select * from chained_rows where table_name='EMPLOYEES'
---------
no rows.

That means no chained rows..i think so

[PAVB]

select * from chained_rows where table_name='EMPLOYEES'
---------
no rows.

That means no chained rows..i think so

It depends on how statistics are gathered. Analyze command updates chained_rows but dbms_stats doesn't.

Eitherway buffer_gets should be identical for both queries - assuming query is doing full table scan for each one of them.

You don't have index on (job_id,salary,email,first_name) - don't you?

Would you mind in doing/posting...

sqlplus /
set autotrace traceonly;
select * from employees;
select job_id,salary,email,first_name from employees;

Thank you.