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open_cursors Tips
Oracle Database Tips by Donald BurlesonMarch 23, 2015
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The open_cursors
parameter is a governor, a block to prevent runaway tasks from consuming
too much library cache RAM.
Any session may execute many SQL statements and the
open_cursors parameter governs the total number of open cursors for any
given session.
For example, if you set open_cursors=100, Oracle
will be allowed to allocate up to 100 cursor slots in the library cache.
Because the slots are only allocated as they are requested, there is no
added overhead to setting this value higher than actually needed.
The starting value is set by Oracle at instance creation
time.
Just like the sessions and
processes parameters, your application usage determines the value for
open_cursors.
If you set open_cursors value too high, you risk
having a task abort with the ORA-01000 error:
ORA-01000 maximum open cursors exceeded
Whenever you get an ORA-01000 error you need
to determine if the session has a bug or whether the cursor requests are
legitimate. You can change the open_cursors parameter
dynamically while the database is running using an alter system
statement:
alter system set open_cursors = 400 scope=both;
You can monitor your high water mark for open cursors with a query like
this:
col hwm_open_cur format 99,999
col max_open_cur format 99,999
select
max(a.value) as hwm_open_cur,
p.value as max_open_cur
from
v$sesstat a,
v$statname b,
v$parameter p
where
a.statistic# = b.statistic#
and
b.name = 'opened cursors current'
and
p.name= 'open_cursors'
group by p.value;
HWM_OPEN_CUR MAX_OPEN_CUR
---------------- ------------
2,350 4,096
In sum, the open_cursors parameter
default value is usually enough for any application, and it can be increased
as-needed, depending upon your application.
Monitoring open
cursors
To monitor your open cursors, you have several views:
§
v$open_cursor
§
v$sesstat
select
stat.value,
sess.username,
sess.sid,
sess.serial#
from
v$sesstat stat,
v$statname b,
v$session sess
where
stat.statistic# = b.statistic#
and
sess.sid=stat.sid
and
b.name = 'opened cursors current';
select
sum(stat.value)
total_cur,
avg(stat.value) avg_cur,
max(stat.value) max_cur,
sess.username,
sess.machine
from
v$sesstat stat,
v$statname b,
v$session sess
where
stat.statistic# = b.statistic#
and
sess.sid=stat.sid
and
b.name = 'opened cursors current'
group by
sess.username,
sess.machine
order by 1 desc;
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