Execution Plan Overview and Bad Execution Plan fix in Oracle Database

                              Performance Monitoring

In order to investigate locking session and killing session, DBA need to follow below steps


Login on Database and check blocking session with below commands

1) To check session lock on database

SELECT DECODE(request,0,'Holder: ','Waiter: ') ||
sid sess, id1, id2, lmode, request, type
FROM V$LOCK
WHERE (id1, id2, type) IN (SELECT id1, id2, type FROM V$LOCK WHERE request > 0)
ORDER BY id1, request;


2) To identify session as well as SQL details

select sid,serial#,osuser,username,program,status,SQl_id from v$session where sid=&sid;

or

select s.username, s.sid, s.serial#,t.sql_text "Last SQL"from v$session s, v$sqlarea t where s.sql_address =t.address and
s.sql_hash_value =t.hash_value and s.sid = '&sid';


3) Map OS processes with database process as below

select p.spid,s.sql_id,s.sql_hash_value,s.sql_id,s.sid,s.serial#,s.username,s.event,to_char(s.logon_time,'dd-mon:hh24:mi') from v$session s, v$process p
where p.addr=s.paddr and s.sid  = '&sid';


4) Session can be killed from OS prompt as well as database level

From OS level

$ kill -9 <spid>


From Database level

alter system kill session 'sid,serial#' immediate;
eg

alter system kill session '41,5' immediate;


5) Cross check session at database level.

select sid,serial#,osuser,username,program,status,SQl_id from v$session where sid=&sid;
 

Here are some basic tips to work with Execution Plan stability, using Baselines.

==========================================================

We use the below queries to see the available execution plans and see which plan was running fine.

1.      @?/rdbms/admin/awrsqrpi.sql  ---> This will generate the html page for the required query based on the SQLID and its awr history.

or

2.      dbms_xplan.display_awr()

Ex: select * from TABLE(dbms_xplan.display_awr('47qjdv3ncanhr'));

or

3.       USING GRID Control 12c

 To gather the history of a SQL execution and the plans used during those runs, obtain the SQL Id to be evaluated, connect to the GRID Control 12c:

select the Targets/Databases -->Select the database -->Performance/SQL/Search SQL-->

Check AWR Snapshots -->Enter the SQL ID in the SQL ID filed / Search-->

Verify the executions and the different Hash Plans used.

 The ones with the Smallest Elapsed Times are the best execution Plans for the SQL.

·         If the HASH Plan is still in the Cursor Cache it can be created as a baseline and instructed to run every time that SQL ID is loaded to the Shared Pool.

·         If the HASH Plan is no longer in the Cursor Cache, then it is still possible to load the HASH Plan to a Sql Tuning Set and create a baseline from the STS and assign it the SQL ID as well. Take note of  the Snap ID (from the GRID SQL Search above) for the desired HASH Plan

HASH /SQL plan needed found in the Cursor Cache

Now you know which hash plan hash to be fixed. Now follow the below example. If the needed plan is found in the cursor cache then it is very simple to create a baseline and fixing the plan for the SQL query.

Ex: Determined the Hash Plan: 2601263939 is the best to run against the SQL ID: 47qjdv3ncanhr

1.Create the Baseline:

var v_num number;

exec :v_num:=dbms_spm.load_plans_from_cursor_cache(sql_id =>'47qjdv3ncanhr',plan_hash_value => 2601263939);

OR

Example from Internet for script:

SQL> !cat create_baseline.sql

var ret number

exec :ret := dbms_spm.load_plans_from_cursor_cache(sql_id=>'&sql_id', plan_hash_value=>&plan_hash_value);

 SQL> @create_baseline

Enter value for sql_id: 47qjdv3ncanhr

Enter value for plan_hash_value: 2601263939

2. Verify the baseline got created or not

=================================

SQL> select sql_handle, plan_name, enabled, accepted, fixed from dba_sql_plan_baselines;

SQL_HANDLE                      PLAN_NAME                                              ENA    ACC     FIX

------------------------------             ------------------------------                                     ---         ---         ---

SQL_4bd90f15ef3c1f10           SQL_PLAN_4rq8g2rrms7sh3cc6a555       YES     YES     NO

To see all the details, this will create a file with all the baseline info.:

spool baseline_plan.txt

select * from table(dbms_xplan.display_sql_plan_baseline(sql_handle=>'SQL_4bd90f15ef3c1f10',  format=>'basic'));

verify the spool file to confime the SQL ID and the HASH associated with it.

3.TO MODIFY A SQL PLAN BASELINE

var v_num number;

exec :v_num:=dbms_spm.ALTER_SQL_PLAN_BASELINE (sql_handle =>'SQL_4bd90f15ef3c1f10',plan_name => 'SQL_PLAN_4rq8g2rrms7sh3cc6a555', attribute_name=> 'FIXED',  attribute_value  => 'YES');

Attributes

    ·  enabled (YES/NO) : If YES, the plan is available for the optimizer if it is also marked as accepted.

    ·  fixed (YES/NO) : If YES, the SQL plan baseline will not evolve over time. Fixed plans are used in preference to non-fixed plans.

    ·  autopurge (YES/NO) : If YES, the SQL plan baseline is purged automatically if it is not used for a period of time.

·   plan_name : Used to amend the SQL plan name, up to a maximum of 30 character.

·    description : Used to amend the SQL plan description, up to a maximum of 30 character.

   

Sometimes the required HASH / SQL plan  will not be present in the Cursor Cache, then you have to  load it from a AWR snapshots.

 Steps are as below:

======================================

To load plans to the cursor cache from awr snapshots:

1. -- Drop SQL Tuning Set (STS)

BEGIN

  DBMS_SQLTUNE.DROP_SQLSET(

    sqlset_name => 'SAMPLE_TUNING_SET');

END;

 2. -- Create SQL Tuning Set (STS)

 BEGIN

  DBMS_SQLTUNE.CREATE_SQLSET(

    sqlset_name => 'SAMPLE_TUNING_SET',

    description => 'SQL Tuning Set for loading plan into SQL Plan Baseline');

END;

3.-- Populate STS from AWR using a time duration when the desired plan was used.

Retrieve the begin Snap ID from the same session described in the GRID Contol above or by :   

SELECT SNAP_ID, BEGIN_INTERVAL_TIME, END_INTERVAL_TIME FROM dba_hist_snapshot ORDER BY END_INTERVAL_TIME DESC;

Note: Specify the sql_id in the basic_filter (other predicates are available, see desc dba_hist_snapshot) if necessary.

DECLARE

  cur sys_refcursor;

BEGIN

  OPEN cur FOR

    SELECT VALUE(P)

    FROM TABLE(

       dbms_sqltune.select_workload_repository(begin_snap=>1477, end_snap=>1478,basic_filter=>'sql_id = ''9n82zq1gkpg2t''',attribute_list=>'ALL')

              ) p;

     DBMS_SQLTUNE.LOAD_SQLSET( sqlset_name=> 'SAMPLE_TUNING_SET', populate_cursor=>cur);

  CLOSE cur;

END;

/

4. -- List out SQL Tuning Set contents to check we got what we wanted

SELECT

  first_load_time,

  executions as execs,

  parsing_schema_name,

  elapsed_time  / 1000000 as elapsed_time_secs,

  cpu_time / 1000000 as cpu_time_secs,

  buffer_gets,

  disk_reads,

  direct_writes,

  rows_processed,

  fetches,

  optimizer_cost,

  sql_plan,

  plan_hash_value,

  sql_id,

  sql_text

   FROM TABLE(DBMS_SQLTUNE.SELECT_SQLSET(sqlset_name => 'SAMPLE_TUNING_SET'));

5.-- Finally create the baseline from the STS:

DECLARE

my_plans pls_integer;

BEGIN

  my_plans := DBMS_SPM.LOAD_PLANS_FROM_SQLSET(

    sqlset_name => 'SAMPLE_TUNING_SET',

    basic_filter=>'plan_hash_value = ''1117073691'''

    );

END;

/

6.-- Verify the baseline got created and modify it if necessary

select sql_handle, plan_name, enabled, accepted, fixed from dba_sql_plan_baselines;

exec :v_num:=dbms_spm.ALTER_SQL_PLAN_BASELINE (sql_handle =>'SQL_ab2ab5c194ee0fc8',plan_name => 'SQL_PLAN_aqapps6afw3y81722054c', attribute_name=> 'FIXED',  attribute_value  => 'YES');

7.-- Verify all details for the new Baseline:

spool baseline_plan.txt

select * from table(

    dbms_xplan.display_sql_plan_baseline(

        sql_handle=>'SQL_ab2ab5c194ee0fc8',

        format=>'basic'));

using below query to see which all plan_hash_value are being used by my sql_id...

select ss.snap_id,

ss.instance_number node,

begin_interval_time,

sql_id,

plan_hash_value,

nvl(executions_delta,0) execs,

round((elapsed_time_delta/decode(nvl(executions_delta,0),0,1,executions_delta))/1000000,2) avg_etime,

round((buffer_gets_delta/decode(nvl(buffer_gets_delta,0),0,1,executions_delta)),2) avg_lio

from DBA_HIST_SQLSTAT S,

DBA_HIST_SNAPSHOT SS

where sql_id = 'xxxxxxxx'

and ss.snap_id = S.snap_id

and ss.instance_number = S.instance_number

and executions_delta > 0

order by 1 desc, 2, 3;

HOW TO GET 11G TO USE THE 10G EXECUTION PLAN

The next challenge was to get SQL with exactly the same SQL text as the application used (exactly the same SQL_ID) and set a SQL plan baseline. I work from Australia and the client’s team is based in North America. Therefore, I couldn’t just call the developer and ask to re-execute the SQL. I had to come with a method that would allow me to find the exact SQL text without executing it. The SQL’s execution took a lot of resources Oracle captured SQL in AWR repository. (The client had a Diagnostic license.) I used the following statement to get the SQL and associated “good” execution plan in a shared pool.

alter session set optimizer_features_enable='10.2.0.4';
 declare
  v_sql varchar2(8000);
  c NUMBER;
  begin
  select sql_text into v_sql from DBA_HIST_SQLTEXT where sql_id='djkbyr8vkc64h';
  c := dbms_sql.open_cursor;
  dbms_sql.parse(c, v_sql, dbms_sql.NATIVE);
  dbms_sql.close_cursor(c);
 end;

I confirmed that the “good” execution plan had been used by the following SQL:

  select sql_id,LAST_LOAD_TIME,PLAN_HASH_VALUE, exact_matching_signature, sysdate from v$sqlarea where sql_id='djkbyr8vkc64h';

   

  SQL_ID LAST_LOAD_TIME PLAN_HASH_VALUE EXACT_MATCHING_SIGNATURE SYSDATE

  ------------- ------------------- --------------- ------------------------ -------------------

  djkbyr8vkc64h 2012.11.12 01:25:51 810205201 14465951278806438046 2012.11.12 01:26:04

The final bit was to create a SQL plan baseline based on the pair of SQL_ID and PLAN_HASH_VALUE. From there, it was too easy. :)

  declare

  n number;

  begin

  n:=DBMS_SPM.LOAD_PLANS_FROM_CURSOR_CACHE (

  sql_id => 'djkbyr8vkc64h',

  plan_hash_value => '810205201'

  FIXED => 'YES',

  ENABLED => 'YES');

  dbms_output.put_line(n);

 end;

 /  

 PL/SQL procedure successfully completed.

To validate that the base line was created for the SQL:

  col version for a10

  col SQL_HANDLE for a25

  col CREATED for a30

  col sb.last_executed for a30

  SELECT

  sb.sql_handle,sb.plan_name,

  sb.origin,sb.version,

 cast(sb.created as date) created,

 cast(sb.last_executed as date) last_executed,

 sb.enabled,

 sb.accepted FROM dba_sql_plan_baselines sb

 WHERE  sb.signature = 14465951278806438046;  

 SQL_HANDLE PLAN_NAME ORIGIN VERSION CREATED LAST_EXECUTED ENA ACC

 ------------------------- ------------------------------ -------------- ---------- ------------------------------ ------------------- --- ---

 SQL_c8c1620b4f5d909e SQL_PLAN_cjhb21d7pv44y27600b06 MANUAL-LOAD 11.2.0.3.0 2012.11.12 01:31:58 YES YES

Just in case we would need to rollback the change, we just need to run the following PL/SQL block:

  set output on

   DECLARE

   i NATURAL;

    BEGIN

    i := DBMS_SPM.ALTER_SQL_PLAN_BASELINE (

    sql_handle=>'SQL_c8c1620b4f5d909e',

    PLAN_NAME=> 'SQL_PLAN_cjhb21d7pv44y27600b06',

    attribute_name=>'enabled',attribute_value=>'no');

    dbms_output.put_line(i);

   END;

  /

My query is picking a bad execution plan , how to fix it to use the good execution plan available ( using Oracle baselines ) ?

Using Oracle baselines you can fix the sql plan for a SQLID:                                          

SQL plan management is a preventative mechanism that records and evaluates the execution plans of SQL statements over time. This mechanism can build a SQL plan baseline, which is a set of accepted plans for a SQL statement. The accepted plans have been proven to perform well.

The goal of SQL plan baselines is to preserve the performance of corresponding SQL statements, regardless of changes in the database. Examples of changes include:

·         New optimizer version

·         Changes to optimizer statistics and optimizer parameters

·         Changes to schema and metadata definitions

·         Changes to system settings

·         SQL profile creation

SQL plan baselines cannot help in cases where an event has caused irreversible execution plan changes, such as dropping an index.

The SQL tuning features of Oracle Database generate SQL profiles that help the optimizer to produce well-tuned plans. However, this mechanism is reactive and cannot guarantee stable performance when drastic database changes occur. SQL tuning can only resolve performance issues after they have occurred and are identified. 

For example, a SQL statement may become high-load because of a plan change, but SQL tuning cannot solve this problem until after the plan change occurs.

To know more details, follow the below link

To identify blocking session and killing it after approval

Blocking session occurs when one session acquired an exclusive lock on an object and doesn't release it,another session (one or more) want to modify the same data.First session will block the second until it completes its job.

Mostly DML statement caused this issue

From the view of the user It look like the application is completely hanged  while waiting for the first session to release its lock.

In most of case, session lock release automatically when we get mutliple email regarding session lock
DBA need to investigate and identify block session on database and inform to IM Team/Service management Team with complete SQL details .

These details will help  IM Team/Service management team to get approval shop direct or application team .Once IM Team/Service management team get approval from shop direct or application team
The will inform to DBA to kill session