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buffer gets.
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.
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Are there other columns in the table? Do you see row_chaining in the table?
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Originally Posted by skhanal
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
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Originally Posted by jayasankar
select * from chained_rows where table_name='EMPLOYEES'
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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.
Last edited by PAVB; 10-04-2010 at 04:25 PM.
Reason: typo
Pablo (Paul) Berzukov
Author of Understanding Database Administration available at amazon and other bookstores.
Disclaimer: Advice is provided to the best of my knowledge but no implicit or explicit warranties are provided. Since the advisor explicitly encourages testing any and all suggestions on a test non-production environment advisor should not held liable or responsible for any actions taken based on the given advice.
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