I went on my first DBA course in 1997 and the lecturer there explained that readers do not block writers in an Oracle database. I had an issue recently which appeared to contradict this so I have reproduced it below in an Oracle 11.2.0.1 database.
I looked at the Blocking Sessions screen and saw that Andrew was blocking Fred:
I clicked on the link to see the SQL which Fred was running:
I clicked on the link to see the SQL which Andrew was running:
So it seemed that:
SELECT COUNT(*)
FROM DBA_TABLES
FROM DBA_TABLES
was blocking:
update andrew.tab1
set col1 = 2
where col1 = 1
set col1 = 2
where col1 = 1
As you might have guessed, I set this test up on purpose and this is what really happened. Andrew created a table:
SQL> conn andrew/reid
Connected.
SQL> create table tab1 (col1 number)
2 /
Table created.
SQL>
He then allowed Fred to update it:
SQL> grant update on tab1 to fred
2 /
Grant succeeded.
SQL>
He added a row to the table, saved it and updated it, setting up a lock in the process:
SQL> insert into tab1
2 values (1)
3 /
1 row created.
SQL> commit
2 /
Commit complete.
SQL> update tab1
2 set col1 = 2
3 where col1 = 1
4 /
1 row updated.
SQL>
Finally, he counted the rows in DBA_TABLES again and again:
SQL> declare
2 row_count number;
3 begin
4 while (1=1) loop
5 select count(*) into row_count
6 from dba_tables;
7 end loop;
8 end;
9 /
Fred then came along and tried to update the same row in TAB1 but had to wait:
SQL> conn fred/bloggs
Connected.
SQL> update andrew.tab1
2 set col1 = 2
3 where col1 = 1
4 /
So, what is the point of this example?
