Slides from Openworl 2019. A look at how to safely (and unsafely) kill sessions in the Oracle database, and how to perhaps avoid killing them altogether.

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An assassins guide to Oracle sessions
Connor McDonald
Database Advocate
Copyright © 2019 Oracle and/or its affiliates.
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Connor McDonald
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Stuff
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400+ posts mainly on database & development
250 technical videos, new uploads every week
rants and raves on tech and the world :-)
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etc...
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8 https://asktom.oracle.com
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The following is intended to outline our general product direction. It is intended for information purposes only,
and may not be incorporated into any contract. It is not a commitment to deliver any material, code, or
functionality, and should not be relied upon in making purchasing decisions. The development, release,
timing, and pricing of any features or functionality described for Oracle’s products may change and remains at
the sole discretion of Oracle Corporation.
Statements in this presentation relating to Oracle’s future plans, expectations, beliefs, intentions and prospects
are “forward-looking statements” and are subject to material risks and uncertainties. A detailed discussion of
these factors and other risks that affect our business is contained in Oracle’s Securities and Exchange
Commission (SEC) filings, including our most recent reports on Form 10-K and Form 10-Q under the heading
“Risk Factors.” These filings are available on the SEC’s website or on Oracle’s website
at http://www.oracle.com/investor. All information in this presentation is current as of September 2019 and
Oracle undertakes no duty to update any statement in light of new information or future events.
Safe Harbor
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if it's not in the documentation
- contact Support
- clarify usage before proceeding
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let's be honest
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this is all about ...
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Apology:
gratuitous insertion
of family album pictures
into a presentation
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1990's
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idiot in database
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times have changed...
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idiot in database
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rogue session
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SQL> select sid, last_call_et, status,
3 from v$session s;
SID LAST_CALL_ET STATUS
---------- ------------ ----------
39 7376 ACTIVE
40 412 INACTIVE
44 421 INACTIVE
...
...
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what most of us do
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SQL> alter system kill session ...
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is that our only option?
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is that our best option?
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issues with "kill session"
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issue #1 - it doesn't kill !
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SQL> alter system kill session '89,1781';
*
ERROR at line 1:
ORA-00031: session marked for kill
?
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the immediate solution ... isn't
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SQL> alter system kill session '89,1781' immediate;
responsive
alert.log
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it might be doing its best
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SQL> select SES_ADDR, USED_UREC
2 from V$TRANSACTION;
SES_ADDR USED_UREC
---------------- ----------
000000008A841258 1421
SQL> /
SES_ADDR USED_UREC
---------------- ----------
000000008A841258 377
SQL> /
no rows selected.
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it might be ghosting you
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hello?
are you there?
why wont you talk?
SQL> select s.username, p.pid, p.spid
2 from v$session s, v$process p
3 where s.paddr = p.addr
4 and s.sid = 39;
USERNAME PID SPID
-------------------- ---------- ----------
SCOTT 51 3865
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# kill -9 pid
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C:> orakill MYDB pid
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"I have no OS access !!!"
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SQL> oradebug event immediate crash;
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it still might ghost you
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SQL> select program, pid
2 from v$process
3 where upper(program) like '%PMON%';
PROGRAM PID
---------------------------------------- ----------
oracle@db192.localdomain (PMON) 2
SQL> oradebug wakeup 2
Statement processed.
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issue #2 - you killed the session
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rogue session
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alternatives to kill session
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18c
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aka, #1 reason to upgrade to 18c
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SQL> alter system cancel sql '123,456';
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Upgrade
from 8.1.6
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# kill -URG pid
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sys.dbms_system.set_ev(sid,serial#,10237,1,'');
sys.dbms_system.set_ev(sid,serial#,10237,0,'');
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other supported alternatives
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SQL> begin
2 DBMS_RESOURCE_MANAGER.SWITCH_CONSUMER_GROUP_FOR_SESS
3 (
4 session_id =>sid#,
5 session_serial =>serial#,
6 consumer_group =>'CANCEL_SQL'
7 );
8 end;
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maybe ...
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... you don't want to even cancel
ORA-01013: user requested cancel of current operation
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SQL> select sid, last_call_et, status,
3 from v$session s;
SID LAST_CALL_ET STATUS
---------- ------------ ----------
39 7376 ACTIVE
40 412 INACTIVE
44 421 INACTIVE
46 12 ACTIVE
51 9 ACTIVE
53 15 ACTIVE
58 8 ACTIVE
69 22 ACTIVE
...
80 453 INACTIVE
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SQL> oradebug setorapid nnn
SQL> oradebug suspend
SID LAST_CALL_ET STATUS
---------- ------------ ----------
39 7376 ACTIVE
40 412 INACTIVE
44 421 INACTIVE
46 12 ACTIVE
51 9 ACTIVE
53 15 ACTIVE
58 8 ACTIVE
SQL> oradebug resume
back to bad SQL
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maybe killing won't help
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BAD
SQL
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bad SQL just keeps coming back...
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19c
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aka, #1 reason for upgrading to 19c
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quarantine of bad SQL
elapsed time
resource consumption
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error on subsequent execution
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manually
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SQL> set feedback on sql_id
SQL> set timing on
SQL> select count(*) from scott.emp, scott.emp, scott.emp,
2 ( select 1 from dual connect by level <= 100000 );
COUNT(*)
----------
274400000
1 row selected.
SQL_ID: dcrqdagwxagth
Elapsed: 00:00:15.72
SQL> declare
2 q varchar2(30);
3 begin
4 q := dbms_sqlq.create_quarantine_by_sql_id(sql_id => 'dcrqdagwxagth');
5 end;
6 /
PL/SQL procedure successfully completed.
SQL> select count(*) from scott.emp, scott.emp, scott.emp,
2 ( select 1 from dual connect by level <= 100000 );
*
ERROR at line 1:
ORA-56955: quarantined plan used
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automatic via resource manager
SQL> begin
2 dbms_resource_manager.create_pending_area();
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4 -- switch into this group before running a possibly
5 -- long running SQL statement
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7 dbms_resource_manager.create_consumer_group(
8 CONSUMER_GROUP=>'CG_EXEC_TIME_LIMIT_CANCEL',
9 COMMENT=>'Limited execution time per statement'
10 );
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12 -- and we need a resource plan
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14 dbms_resource_manager.create_plan(
15 PLAN=> 'EXEC_TIME_LIMIT',
16 COMMENT=>'Kill statement after exceeding total execution time'
17 );
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[cont]
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19 -- now create a plan directive for that special user group
20 -- to cancel current SQL if it runs for more than 'n' sec
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22 dbms_resource_manager.create_plan_directive(
23 PLAN=> 'EXEC_TIME_LIMIT',
24 GROUP_OR_SUBPLAN=>'CG_EXEC_TIME_LIMIT_CANCEL',
25 COMMENT=>'Kill statement after exceeding total execution time',
36 SWITCH_GROUP=>'CANCEL_SQL',
37 SWITCH_TIME=>10,
38 SWITCH_ESTIMATE=>false,
39 SWITCH_FOR_CALL=> TRUE
40 );
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42 DBMS_RESOURCE_MANAGER.VALIDATE_PENDING_AREA;
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44 DBMS_RESOURCE_MANAGER.SUBMIT_PENDING_AREA();
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46 end;
47 /
PL/SQL procedure successfully completed.
SQL> select count(*) from emp, emp, emp,
2 ( select 1 from dual connect by level <= 50000 );
COUNT(*)
----------
27440000
Elapsed: 00:00:04.11
SQL> select count(*) from emp, emp, emp,
2 ( select 1 from dual connect by level <= 1000000 );
ERROR at line 1:
ORA-00040: active time limit exceeded - call aborted
Elapsed: 00:00:12.00
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SQL> select count(*) from scott.emp, scott.emp, scott.emp,
2 ( select 1 from dual connect by level <= 100000 );
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ERROR at line 1:
ORA-56955: quarantined plan used
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wrap up
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start with aiming for better SQL
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but if you have to kill ...
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... be gentle :-)
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Ten Amazing SQL Features
Monday, September 16, 02:30 PM - 03:15 PM, Moscone South - Room 203
Proven Ways to Make Applications Slower and Less Secure [THT4796]
Tuesday, September 17, 04:30 PM - 04:50 PM, The Exchange - Theater 3
The Best Oracle Database Feature Ever Invented [THT4798]
Wednesday, September 18, 10:15 AM - 10:35 AM, The Exchange - Theater 3
Flashback - Not Just for DBAs
Wednesday, September 18, 04:00 PM - 04:45 PM, Moscone South - Room 306
PL/SQL: Still the Best Data Access Language
Thursday, September 19, 12:15 PM - 01:00 PM, Moscone South - Room 155A
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Have a great OpenWorld !
youtube youtube.com/c/ConnorMcDonaldOracle
blog connor-mcdonald.com
twitter @connor_mc_d
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