Running SQL / Long Running SQL / Last executed SQL / SQLID from SID

Last/Latest Running SQL

-----------------------
set pages 50000 lines 32767
col "Last SQL" for 100
SELECT t.inst_id,s.username, s.sid, s.serial#,t.sql_id,t.sql_text "Last SQL"
FROM gv$session s, gv$sqlarea t
WHERE s.sql_address =t.address AND
s.sql_hash_value =t.hash_value
/




Current Running SQLs

--------------------
set pages 50000 lines 32767
col HOST_NAME for a20
col EVENT for a40
col MACHINE for a30
col SQL_TEXT for a50
col USERNAME for a15

select sid,serial#,a.sql_id,a.SQL_TEXT,S.USERNAME,i.host_name,machine,S.event,S.seconds_in_wait sec_wait,
to_char(logon_time,'DD-MON-RR HH24:MI') login
from gv$session S,gV$SQLAREA A,gv$instance i
where S.username is not null
-- and S.status='ACTIVE'
AND S.sql_address=A.address
and s.inst_id=a.inst_id and i.inst_id = a.inst_id
and sql_text not like 'select S.USERNAME,S.seconds_in_wait%'
/



Current Running SQLs

--------------------
set pages 50000 lines 32767
col program format a20
col sql_text format a50

select b.sid,b.status,b.last_call_et,b.program,c.sql_id,c.sql_text
from v$session b,v$sqlarea c
where b.sql_id=c.sql_id
/

Last/Latest Running SQL

-----------------------
set pages 50000 lines 32767
select inst_id,sample_time,session_id,session_serial#,sql_id from gv$active_session_history
where sql_id is not null
order by 1 desc
/

SQLs Running from longtime

--------------------------
alter session set nls_date_format = 'dd/mm/yyyy hh24:mi';
set pages 50000 lines 32767
col target format a25
col opname format a40
select sid
,opname
,target
,round(sofar/totalwork*100,2) as percent_done
,start_time
,last_update_time
,time_remaining
from
v$session_longops
/

Active Sessions running for more than 1 hour

---------------------------------------------
set pages 50000 lines 32767
col USERNAME for a10
col MACHINE for a15
col PROGRAM for a40

SELECT USERNAME,machine,inst_id,sid,serial#,PROGRAM,
to_char(logon_time,'dd-mm-yy hh:mi:ss AM')"Logon Time",
ROUND((SYSDATE-LOGON_TIME)*(24*60),1) as MINUTES_LOGGED_ON,
ROUND(LAST_CALL_ET/60,1) as Minutes_FOR_CURRENT_SQL
From gv$session
WHERE STATUS='ACTIVE'
AND USERNAME IS NOT NULL and ROUND((SYSDATE-LOGON_TIME)*(24*60),1) > 60
ORDER BY MINUTES_LOGGED_ON DESC;

Session details associated with SID and Event waiting for

---------------------------------------------------------
set pages 50000 lines 32767
col EVENT for a40

select a.sid, a.serial#, a.status, a.program, b.event,to_char(a.logon_time, 'dd-mon-yy hh24:mi') LOGON_TIME,to_char(Sysdate, 'dd-mon-yy-hh24:mi') CURRENT_TIME, (a.last_call_et/3600) "Hrs connected" from v$session a,v$session_wait b where a.sid in(&SIDs) and a.sid=b.sid order by 8;

Session details associated with Oracle SID

-------------------------------------------
set head off
set verify off
set echo off
set pages 1500
set linesize 100
set lines 120
prompt
prompt Details of SID / SPID / Client PID
prompt ==================================
select /*+ CHOOSE*/
'Session Id.............................................: '||s.sid,
'Serial Num..............................................: '||s.serial#,
'User Name ..............................................: '||s.username,
'Session Status .........................................: '||s.status,
'Client Process Id on Client Machine ....................: '||'*'||s.process||'*' Client,
'Server Process ID ......................................: '||p.spid Server,
'Sql_Address ............................................: '||s.sql_address,
'Sql_hash_value .........................................: '||s.sql_hash_value,
'Schema Name ..... ......................................: '||s.SCHEMANAME,
'Program ...............................................: '||s.program,
'Module .................................................: '|| s.module,
'Action .................................................: '||s.action,
'Terminal ...............................................: '||s.terminal,
'Client Machine .........................................: '||s.machine,
'LAST_CALL_ET ...........................................: '||s.last_call_et,
'S.LAST_CALL_ET/3600 ....................................: '||s.last_call_et/3600
from v$session s, v$process p
where p.addr=s.paddr and
s.sid=nvl('&sid',s.sid)
/
set head on

Checking for Active Transactions SID

------------------------------------
select username,t.used_ublk,t.used_urec from v$transaction t,v$session s where t.addr=s.taddr;

Session details from Session longops

-------------------------------------
select inst_id,SID,SERIAL#,OPNAME,SOFAR,TOTALWORK,START_TIME,LAST_UPDATE_TIME, username from gv$session_longops;


Session details with SPID

-------------------------
select sid, serial#, USERNAME, STATUS, OSUSER, PROCESS,
MACHINE, MODULE, ACTION, to_char(LOGON_TIME,'yyyy-mm-dd hh24:mi:ss')
from v$session where paddr in (select addr from v$process where spid = '&spid')
/
To find Undo Generated For a given session

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


select username,
t.used_ublk ,t.used_urec
from gv$transaction t,gv$session s
where t.addr=s.taddr and
s.sid='&sid';

To list count of connections from other machines

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


select count(1),machine from gv$session where inst_id='&inst_id' group by machine;

To get total count of sessions and processes

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


select count(*) from v$session;

select count(*) from v$process;

select (select count(*) from v$session) sessions, (select count(*) from v$process) processes from dual;

To find sqltext thru sqladdress

-------------------------------
select sql_address from v$session where sid=1999;

select sql_text from v$sqltext where ADDRESS='C00000027FF00AF0' order by PIECE;

To find sqltext for different sql hashvalue

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


select hash_value,sql_text from v$sql where hash_value in (1937378691,1564286875,
248741712,2235840973,2787402785)

To list long running forms user sessions

----------------------------------------
select s.sid,s.process,p.spid,s.status ,s.action,s.module, (s.last_call_et/3600) from
v$session s, v$process p where round(last_call_et/3600) >4 and action like '%FRM%' and
p.addr=s.paddr ;

To list inactive Sessions respective username------------------------------------


SELECT username,count(*) num_inv_sess
FROM v$session
where last_call_et > 3600
and username is not null
AND STATUS='INACTIVE'
group by username
order by num_inv_sess DESC;

SELECT count(*) FROM v$session where last_call_et > 43200 and username is not null AND
STATUS='INACTIVE';
SELECT count(*) FROM v$session where last_call_et > 3600 and username is not null AND
STATUS='INACTIVE';

To find session id with set of SPIDs

------------------------------------
select sid from v$session, v$process where addr=paddr and spid in ('11555','26265','11533');

To find Sql Text given SQLHASH & SQLADDR

----------------------------------------
select piece,sql_text from v$sqltext where HASH_VALUE = &hash and ADDRESS ='&addr' order by piece;
select piece,sql_text from v$sqltext where ADDRESS ='&addr' order by piece;


To find SQL ID / SQL TEXT and SID of running SQLs

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

set lines 300

set pages 3000

set long 50000

COL USERNAME FOR A15

COL SQL_FULLTEXT FOR A100

SELECT A.INST_ID, A.SID, A.SERIAL#, A.USERNAME, A.SQL_ID, A.STATUS , B.SQL_FULLTEXT , B.PLAN_HASH_VALUE FROM GV$SESSION A , GV$SQLAREA B




WHERE A.SQL_ID=B.SQL_ID;

View SQL that run more than one hour from AWR views

select  sql_text 

from    dba_hist_sqltext 
where   sql_id in 
  (select   sql_id
   from     dba_hist_sqlstat 
   where    snap_id between &Start_SnapID and &End_SnapID
   and      elapsed_time_total > 7200000000 -- number for 1 hour
)
/

Identify hot blocks using AWR

First set the start and end snapshot ID as variables. 

To get a general idea of where the majority of WAITS's occur,
run the following SQL statement to view the counts of wait classes
in descending order;
---------------------------------------------------------

select   d.wait_class_id                as Wait_Class_ID
        ,d.wait_class                   as Wait_Class
        ,count(*)                       as Cnt
from     dba_hist_active_sess_history   d
where    d.nap_id between :p_Start_Snap_ID and :p_End_Snap_ID
group by d.wait_class_id
        ,d.wait_class
order by 3 desc;


Next, List a breakdown of Events per Wait class identified
in the previous result set;
---------------------------------------------------------
select   d.wait_class_id                as Wait_Class_id
        ,d.wait_class                   as Wait_Class_Name
        ,e.Name                         as Event_Name
        ,count(*)                       as Cnt
from     dba_hist_active_sess_history   d
        ,v$Event_Name                   e
where    d.snap_id between :p_Start_Snap_ID and :p_End_Snap_ID
and      d.Event_ID                     = e.Event_ID
group by d.wait_class_id
        ,d.wait_class
        ,e.Name
order by 4 desc;


Now attempt to identify which users are responsible for the
waits (broken down per event type).

select   d.wait_class_id                as Wait_Class_ID
        ,d.wait_class                   as Wait_Class_Name
        ,u.Username                     as User_Name
        ,e.Name                         as Event_Name
        ,count(*)                       as Cnt
from     dba_hist_active_sess_history   d
        ,v$Event_Name                   e
        ,all_users                      u
where    d.snap_id between :p_Start_Snap_ID and :p_End_Snap_ID
and      d.Event_ID         = e.Event_ID
and      d.User_id          = u.User_ID
group by u.Username
        ,d.wait_class_id
        ,d.wait_class
        ,e.Name
order by 4, 5 desc;


You may want to exclude WAITS's for SYS and focus only on the
application specific schemas, in which case, add
the additional predicate "u.Username != 'SYS'"

Also, you would probably want to exclude SQL*Net related WAIT's,
therefore add "e.Name not like 'SQL*Net%'" as a predicate.

select   d.wait_class                   as Wait_Class_Name
        ,u.Username                     as User_Name
        ,e.Name                         as Event_Name
        ,count(*)                       as Cnt
from     dba_hist_active_sess_history   d
        ,v$Event_Name                   e
        ,all_users                      u
where    d.snap_id between  :p_Start_Snap_ID and :p_End_Snap_ID
and      d.Event_ID         =     e.Event_ID
and      d.User_id          =     u.User_ID
and      u.Username         !=    'SYS'
and      e.Name not         like  'SQL*Net%'
group by u.Username
        ,d.wait_class
        ,e.Name
order by 4, 5 desc;


To drill down on hot blocks, the WAIT class to target would be;
"User I/O".
Therefore add an additional predicate;
      "d.Wait_Class       like  'User I/O'".

select   d.wait_class                   as Wait_Class_Name
        ,u.Username                     as User_Name
        ,e.Name                         as Event_Name
        ,count(*)                       as Cnt
from     dba_hist_active_sess_history   d
        ,v$Event_Name                   e
        ,all_users                      u
where    d.snap_id between  :p_Start_Snap_ID and :p_End_Snap_ID
and      d.Event_ID         =     e.Event_ID
and      d.User_id          =     u.User_ID
and      u.Username         !=    'SYS'
and      e.Name not         like  'SQL*Net%'
and      d.Wait_Class       like  'User I/O'
group by u.Username
        ,d.wait_class
        ,e.Name
order by 1, 4 desc;


To drill down on which Objects the hot blocks occur in,
join to the all_Objects dictionary view.

Remove the Event Name from the grouping and select list
since we know longer want to focus on individual reasons
for the general "User I/O" (of which there are several).

select   d.wait_class                   as Wait_Class_Name
        ,u.Username                     as User_Name
        ,a.Object_Name                  as Object_Name
        ,count(*)                       as Cnt
from     dba_hist_active_sess_history   d
        ,all_users                      u
        ,all_objects                    a
        ,v$Event_Name                   e
where    d.snap_id between  :p_Start_Snap_ID and :p_End_Snap_ID
and      d.Event_ID         =     e.Event_ID
and      d.User_id          =     u.User_ID
and      u.Username         !=    'SYS'
and      e.Name not         like  'SQL*Net%'
and      d.Wait_Class       like  'User I/O'
and      d.Current_Obj#     =     a.Object_ID
and      a.Object_Type      =     'TABLE'
group by u.Username
        ,d.wait_class
        ,a.Object_Name
order by 4 desc,  2, 3;


And finally, to identify the most read ROWs relative to a Top-N
number passed in as a parameter.

select User_Name
      ,Object_Name
      ,Hot_Row_ID
      ,Cnt
from
  (
  select   u.Username                     as User_Name
          ,a.Object_Name                  as Object_Name
          ,dbms_rowid.rowid_create(1, d.Current_Obj#
                                     ,d.Current_File#
                                     ,d.Current_Block#
                                     ,d.Current_Row#) as Hot_Row_ID
          ,count(*)                       as Cnt
  from     dba_hist_active_sess_history   d
          ,all_users                      u
          ,all_objects                    a
          ,v$Event_Name                   e
  where    d.snap_id between  :p_Start_Snap_ID and :p_End_Snap_ID
  and      d.Event_ID         =     e.Event_ID
  and      d.User_id          =     u.User_ID
  and      u.Username         !=    'SYS'
  and      e.Name not         like  'SQL*Net%'
  and      d.Wait_Class       like  'User I/O'
  and      d.Current_Obj#     =     a.Object_ID
  and      a.Object_Type      =     'TABLE'
  group by u.Username
          ,a.Object_Name
          ,dbms_rowid.rowid_create(1, d.Current_Obj#
                                     ,d.Current_File#
                                     ,d.Current_Block#
                                     ,d.Current_Row#)
  order by 4 desc,  2
  )
where rownum < &top_n;

Identify most scanned tables

Identify the top 20 most scanned tables. 

set linesize 400
set pagesize 3000
col Owner format a15
col Object_Name format a30

select /*+ all_rows */
       *
from
    (select Inst_ID
           ,owner
           ,object_name
           ,value
     from   gv$segment_statistics
     where  statistic_name ='logical  reads'
     and    object_type='TABLE'
     order by 3 desc)
where rownum < 21
/

Identify SQL in blocking and waiting sessions

col Event format a25

col DML_BLOCKING format a45
col DML_In_Waiting format a45
set linesize 400
set pagesize 3000

-- LABEL: STATEMENT A
select  distinct
        a.sid                     as Waiting_SID
       ,d.sql_text                as DML_In_Waiting
       ,o.Owner                   as Object_Owner
       ,o.Object_Name             as Locked_Object
       ,a.Blocking_Session        as Blocking_SID
       ,c.sql_text                as DML_Blocking
from
        v$session                 a
       ,v$active_session_history  b
       ,v$sql                     c
       ,v$sql                     d
       ,all_objects               o
where
        a.event                   = 'enq: TX - row lock contention'
and     a.sql_id                  = d.sql_id
and     a.blocking_session        = b.session_id
and     c.sql_id                  = b.sql_id
and     a.Row_Wait_Obj#           = o.Object_ID
and     b.Current_Obj#            = a.Row_Wait_Obj#
and     b.Current_File#           = a.Row_Wait_File#
and     b.Current_Block#          = a.Row_Wait_Block#

-- LABEL: STATEMENT B
select  distinct
        a.sid                     as Waiting_SID
       ,a.event                   as Event
       ,c.sql_text                as DML_Blocking
       ,b.sid                     as Blocking_SID
       ,b.event                   as Event
       ,b.sql_id                  as Blocking_SQL_ID
       ,b.prev_sql_id             as Blocking_Prev_SQL_ID
       ,d.sql_text                as DML_Blocking
from
        v$session                 a
       ,v$session                 b
       ,v$sql                     c
       ,v$sql                     d
where
        a.event                   = 'enq: TX - row lock contention'
and     a.blocking_session        = b.sid
and     c.sql_id                  = a.sql_id
and     d.sql_id                  = nvl(b.sql_id,b.prev_sql_id);

Script to find session caused the most load (foreground sessions)

The following statement indicates the sessions that have caused the most load as a percentage of all foreground sessions;

select ah.session_id
      ,ah.session_type
      ,nvl(ah.sql_id,'xx')                     as SQL_ID
      ,count(*)                                as Session_Cnt
      ,round(count(*)/sum(count(*)) over(), 2) as Percent_Load
from
       v$active_session_history ah
where
       ah.sample_time >to_date('31-OCT-11 11:40','dd-MON-yy hh24:mi')
and    ah.session_type='FOREGROUND'
group by
       ah.session_id
      ,ah.session_type
      ,ah.sql_id
order by count(*) desc

/

WARNING: Heavy swapping observed on system in last 5 mins

while importing / Exporting Data Oracle Goes to High Memory Utilization this cause SWAP alert in Alert Log file,

hence resolution is simple,

increaase TEMP file size in your Oracle Database and this alert will be gone.


it generally goes to SWAP when TEMP space is not enough to process the data .

How to Export and Import Statistics

procedure with a scenerio
Case Definition
A critical application suddenly seems to hang, wait events show long table scans running on the OLTP environment. It comes out that the DBA in charge of this system did run statistics on the tables of the user that owns the application. The gather statistics got stuck and the process was killed. Since this moment the application started to perform extremely slowly.
The production database has several clones; we decide to export back statistics from one of these clones, to the production database.

Steps in Brief
1) Create a table to hold statistics on the source database
2) Generate a script that export table statistics on the clone database
3) Generate a script that import statistics on the clone database
4) Export statistics on clone database
5) Export table containing the exported statistics from clone database
6) Ftp export file with clone statistics table, and the script to import statistics from clone server to production server
7) Import table containing clone statistics into production database
8) Import statistics on production server using the script to import statisctics generated on the clone server

1. Create tables to hold statistics on the clone database
— On Cloned Database

SQL> execute BMS_STATS.create_stat_table(‘OM’,’OLD_STATS’);
PL/SQL procedure successfully completed.
SQL> grant SELECT,INSERT,UPDATE,DELETE on OM.OLD_STATS to public;
Grant succeeded.

2. Generate a script that export table statistics on the clone database
The purpose of this script is to generate one export statistics command per table, the export is directed into the table created on step 1.
Variables:
&tabname = the table created on the previous step to hold the statistics
&usrname = The name of the owner of &tabname
—- script to generate export table stats start here ———-
set linesize 130 pagesize 0
spool exportstats.sql
select ‘exec dbms_stats.export_table_stats(‘||chr(39)||owner||chr(39)||’,’||chr(39)||table_name||chr(39)||’,null,’||chr(39)||’&tabname’
||chr(39)||’,null,true,’||chr(39)||’INV’||chr(39)||’)’
from dba_tables where owner =’&usrname’
/
spool off

—- script to generate export table stats end here ———-
Note: you may also use instead of the script this command:
exec DBMS_STATS.export_schema_stats(‘&usrname’,’&tabname’)
This syntax will run in 10g. It may fail on 8i – 9i databases with some objects. That’s why I prefer the script on these versions.

3. Generate a script that import statistics on the clone database
The purpose of this script is to generate one import statistics command per table, the source is the table created on step 1.
&tabname = the table created on the previous step to hold the statistics
&usrname = The name of the owner of &tabname

—- script to generate import table stats start here ———-
set linesize 130 pagesize 0
spool importstats.sql
select ‘exec dbms_stats.import_table_stats(‘||chr(39)||owner||chr(39)||’,’||chr(39)||table_name||chr(39)||’,null,’||chr(
39)||’&tabname’||chr(39)||’,null,true,’||chr(39)||’&usrname’||chr(39)||’)’
from dba_tables where owner =’&usrname’
/
spool off
—- script to generate import table stats end here ———-
Execute this script to generate impstats.sql that will import the statistics on the production database.

4. Export statistics on clone database
Using the script expstat.sql; generated on step 2, export statistics into the statistics table created on step 1.

5. Export table containing the exported statistics from clone database
vmractest:/oradisk/av/expstats>exp avargas file=exp_stats_from_clone tables=avr.old_stats feedback=1000
Export: Release 9.2.0.5.0 – Production on Tue Feb 20 11:57:02 2007
Copyright (c) 1982, 2002, Oracle Corporation. All rights reserved.
Password:
Connected to: Oracle9i Enterprise Edition Release 9.2.0.5.0 – Production
With the Partitioning, OLAP and Oracle Data Mining options
JServer Release 9.2.0.5.0 – Production
Export done in IW8ISO8859P8 character set and AL16UTF16 NCHAR character set
About to export specified tables via Conventional Path …
Current user changed to AVR
. . exporting table OLD_STATS
….
4115 rows exported
Export terminated successfully without warnings.

6. Ftp export file with clone statistics table from clone server to production server and script to import statistics from clone server to production server
Execute FTP session from target server, get both the table that contains the exported statistics and the script to import them, generated on step :proddb > ftp vmractest
Connected to vmractest
220 vmractest FTP server (SunOS 5.8) ready.
Name (vmractest:oracle): oracle
331 Password required for oracle.
Password:
230 User oracle logged in.
ftp> cd /oradisk/av/expstats
250 CWD command successful.
ftp> get exp_stats_from_clone.dmp
200 PORT command successful.
150 ASCII data connection for exp_stats_from_clone.dmp (10.5.180.72,64082) (473088 bytes).
226 ASCII Transfer complete.
local: exp_stats_from_clone.dmp remote: exp_stats_from_clone.dmp
478390 bytes received in 0.17 seconds (2680.69 Kbytes/s)
ftp> get impstats.sql
200 PORT command successful.
150 ASCII data connection for impstats.sql (10.5.180.72,64776) (31461 bytes).
226 ASCII Transfer complete.
local: impstats.sql remote: impstats.sql
31704 bytes received in 0.033 seconds (947.63 Kbytes/s)
ftp> bye
221 Goodbye.

7. Import table containing clone statistics into production database
On the production database import the table that contains the exported statistics.
proddb >imp avargas file= exp_stats_from_clone.dmp full =y
Import: Release 9.2.0.5.0 – Production on Tue Feb 20 12:19:11 2007
Copyright (c) 1982, 2002, Oracle Corporation. All rights reserved.
Password:
Connected to: Oracle9i Enterprise Edition Release 9.2.0.5.0 – Production
With the Partitioning, OLAP and Oracle Data Mining options
JServer Release 9.2.0.5.0 – Production
Export file created by EXPORT:V09.02.00 via conventional path
import done in UTF8 character set and UTF8 NCHAR character set
export client uses IW8ISO8859P8 character set (possible charset conversion)
export server uses AL16UTF16 NCHAR character set (possible ncharset conversion)
. importing AVARGAS’s objects into AVARGAS
. importing AVR’s objects into AVR
. . importing table “OLD_STATS” 4115 rows imported
Import terminated successfully without warnings.

8. Import statistics on production server using the script to import statistics generated on the clone server
Using the script impstats.sql; generated on step 3, import statistics into the production database.

All About Statistics In Oracle

In this post I'll try to summarize all sorts of statistics in Oracle, I strongly recommend reading the full article, as it contains information you may find it valuable in understanding Oracle statistics.

Database | Schema | Table | Index Statistics
#####################################

Gather Database Statistics:
=======================
SQL> EXEC DBMS_STATS.GATHER_DATABASE_STATS(
     ESTIMATE_PERCENT=>100,METHOD_OPT=>'FOR ALL COLUMNS SIZE SKEWONLY',
    CASCADE => TRUE,
    degree => 4,
    OPTIONS => 'GATHER STALE',
    GATHER_SYS => TRUE,
    STATTAB => PROD_STATS);

CASCADE => TRUE :Gather statistics on the indexes as well. If not used Oracle will decide whether to collect index statistics or not.
DEGREE => 4 :Degree of parallelism.
options: 
       =>'GATHER' :Gathers statistics on all objects in the schema.
       =>'GATHER AUTO' :Oracle determines which objects need new statistics, and determines how to gather those statistics.
       =>'GATHER STALE':Gathers statistics on stale objects. will return a list of stale objects.
       =>'GATHER EMPTY':Gathers statistics on objects have no statistics.will return a list of no stats objects.
        =>'LIST AUTO' : Returns a list of objects to be processed with GATHER AUTO.
        =>'LIST STALE': Returns a list of stale objects as determined by looking at the *_tab_modifications views.
        =>'LIST EMPTY': Returns a list of objects which currently have no statistics.
GATHER_SYS => TRUE :Gathers statistics on the objects owned by the 'SYS' user.
STATTAB => PROD_STATS :Table will save the current statistics. see SAVE & IMPORT STATISTICS section -last third in this post-.

Note: All above parameters are valid for all kind of statistics (schema,table,..) except Gather_SYS.
Note: Skew data means the data inside a column is not uniform, there is a particular one or more value are being repeated much than other values in the same column, for example the gender column in employee table with two values (male/female), in a construction or security service company, where most of employees are male workforce,the gender column in employee table is likely to be skewed but in an entity like a hospital where the number of males almost equal the number of female workforce, the gender column is likely to be not skewed.

For faster execution:

SQL> EXEC DBMS_STATS.GATHER_DATABASE_STATS(
ESTIMATE_PERCENT=>DBMS_STATS.AUTO_SAMPLE_SIZE,degree => 8);

What's new?
ESTIMATE_PERCENT=>DBMS_STATS.AUTO_SAMPLE_SIZE => Let Oracle estimate skewed values always gives excellent results.(DEFAULT).
Removed "METHOD_OPT=>'FOR ALL COLUMNS SIZE SKEWONLY'" => As histograms is not recommended to be gathered on all columns.
Removed  "cascade => TRUE" To let Oracle determine whether index statistics to be collected or not.
Doubled the "degree => 8" but this depends on the number of CPUs on the machine and accepted CPU overhead during gathering DB statistics.

Starting from Oracle 10g, Oracle introduced an automated task gathers statistics on all objects in the database that having [stale ormissing] statistics, To check the status of that task:
SQL> select status from dba_autotask_client where client_name = 'auto optimizer stats collection';

To Enable Automatic Optimizer Statistics task:
SQL> BEGIN
    DBMS_AUTO_TASK_ADMIN.ENABLE(
    client_name => 'auto optimizer stats collection', 
    operation => NULL, 
    window_name => NULL);
    END;
    /

In case you want to Disable Automatic Optimizer Statistics task:
SQL> BEGIN
    DBMS_AUTO_TASK_ADMIN.DISABLE(
    client_name => 'auto optimizer stats collection', 
    operation => NULL, 
    window_name => NULL);
    END;
    /

To check the tables having stale statistics:

SQL> exec DBMS_STATS.FLUSH_DATABASE_MONITORING_INFO;
SQL> select OWNER,TABLE_NAME,LAST_ANALYZED,STALE_STATS from DBA_TAB_STATISTICS where STALE_STATS='YES';

[update on 03-Sep-2014]
Note: In order to get an accurate information from DBA_TAB_STATISTICS or (*_TAB_MODIFICATIONS, *_TAB_STATISTICS and *_IND_STATISTICS) views, you should manually run DBMS_STATS.FLUSH_DATABASE_MONITORING_INFO procedure to refresh it's parent table mon_mods_all$ from SGA recent data, or you have wait for an Oracle internal that refresh that table  once a day in 10g onwards [except for 10gR2] or every 15 minutes in 10gR2 or every 3 hours in 9i backwards. or when you run manually run one of GATHER_*_STATS procedures.
[Reference: Oracle Support and MOS ID 1476052.1]

Gather SCHEMA Statistics:
======================
SQL> Exec DBMS_STATS.GATHER_SCHEMA_STATS (
     ownname =>'SCOTT',
     estimate_percent=>10,
     degree=>1,
     cascade=>TRUE,
     options=>'GATHER STALE');


Gather TABLE Statistics:
====================
Check table statistics date:
SQL> select table_name, last_analyzed from user_tables where table_name='T1';

SQL> Begin DBMS_STATS.GATHER_TABLE_STATS (
    ownname => 'SCOTT',
    tabname => 'EMP',
    degree => 2,
    cascade => TRUE,
    METHOD_OPT => 'FOR COLUMNS SIZE AUTO',
    estimate_percent => DBMS_STATS.AUTO_SAMPLE_SIZE);
    END;
    /

CASCADE => TRUE : Gather statistics on the indexes as well. If not used Oracle will determine whether to collect it or not.
DEGREE => 2: Degree of parallelism.
ESTIMATE_PERCENT => DBMS_STATS.AUTO_SAMPLE_SIZE : (DEFAULT) Auto set the sample size % for skew(distinct) values (accurate and faster than setting a manual sample size).
METHOD_OPT=>  :  For gathering Histograms:
 FOR COLUMNS SIZE AUTO :  You can specify one column between "" instead of all columns.
 FOR ALL COLUMNS SIZE REPEAT :  Prevent deletion of histograms and collect it only for columns already have histograms.
 FOR ALL COLUMNS  :  Collect histograms on all columns.
 FOR ALL COLUMNS SIZE SKEWONLY :  Collect histograms for columns have skewed value should test skewness first>.
 FOR ALL INDEXED COLUMNS :  Collect histograms for columns have indexes only.

Note: Truncating a table will not update table statistics, it will only reset the High Water Mark, you've to re-gather statistics on that table.

Inside "DBA BUNDLE", there is a script called "gather_stats.sh", it will help you easily & safely gather statistics on specific schema or table plus providing advanced features such as backing up/ restore new statistics in case of fallback.
To learn more about "DBA BUNDLE" please visit this post:
http://dba-tips.blogspot.com/2014/02/oracle-database-administration-scripts.html


Gather Index Statistics:
===================
SQL> exec DBMS_STATS.GATHER_INDEX_STATS(ownname => 'SCOTT',indname => 'EMP_I',
     estimate_percent =>DBMS_STATS.AUTO_SAMPLE_SIZE);

####################
Fixed OBJECTS Statistics
####################

What are Fixed objects:
----------------------------
-Fixed objects are the x$ tables (been loaded in SGA during startup) on which V$ views are built (V$SQL etc.).
-If the statistics are not gathered on fixed objects, the Optimizer will use predefined default values for the statistics. These defaults may lead to inaccurate execution plans.
-Statistics on fixed objects are not being gathered automatically nor within gathering DB stats.

How frequent to gather stats on fixed objects?
-------------------------------------------------------
Only one time for a representative workload unless you've one of these cases:

- After a major database or application upgrade.
- After implementing a new module.
- After changing the database configuration. e.g. changing the size of memory pools (sga,pga,..).
- Poor performance/Hang encountered while querying dynamic views e.g. V$ views.


Note:
- It's recommended to Gather the fixed object stats during peak hours (system is busy) or after the peak hours but the sessions are still connected (even if they idle), to guarantee that the fixed object tables been populated and the statistics well represent the DB activity.
- Also note that performance degradation may be experienced while the statistics are gathering.
- Having no statistics is better than having a non representative statistics.

How to gather stats on fixed objects:
---------------------------------------------

First Check the last analyzed date:
------ -----------------------------------
SQL> select OWNER, TABLE_NAME, LAST_ANALYZED
       from dba_tab_statistics where table_name='X$KGLDP';
Second Export the current fixed stats in a table: (in case you need to revert back)
------- -----------------------------------
SQL> EXEC DBMS_STATS.CREATE_STAT_TABLE
       ('OWNER','STATS_TABLE_NAME','TABLESPACE_NAME');
SQL> EXEC dbms_stats.export_fixed_objects_stats
       (stattab=>'STATS_TABLE_NAME',statown=>'OWNER');
Third Gather the fixed objects stats:
-------  ------------------------------------
SQL> exec dbms_stats.gather_fixed_objects_stats; 

Note:
In case you experienced a bad performance on fixed tables after gathering the new statistics:

SQL> exec dbms_stats.delete_fixed_objects_stats(); SQL> exec DBMS_STATS.import_fixed_objects_stats
       (stattab =>'STATS_TABLE_NAME',STATOWN =>'OWNER');


#################
SYSTEM STATISTICS
#################

What is system statistics:
-------------------------------
System statistics are statistics about CPU speed and IO performance, it enables the CBO to
effectively cost each operation in an execution plan. Introduced in Oracle 9i.

Why gathering system statistics:
----------------------------------------
Oracle highly recommends gathering system statistics during a representative workload,
ideally at peak workload time, in order to provide more accurate CPU/IO cost estimates to the optimizer.
You only have to gather system statistics once.

There are two types of system statistics (NOWORKLOAD statistics & WORKLOAD statistics):

NOWORKLOAD statistics:
-----------------------------------
This will simulates a workload -not the real one but a simulation- and will not collect full statistics, it's less accurate than "WORKLOAD statistics" but if you can't capture the statistics during a typical workload you can use noworkload statistics.
To gather noworkload statistics:
SQL> execute dbms_stats.gather_system_stats(); 

WORKLOAD statistics:
-------------------------------
This will gather statistics during the current workload [which supposed to be representative of actual system I/O and CPU workload on the DB].
To gather WORKLOAD statistics:
SQL> execute dbms_stats.gather_system_stats('start');
Once the workload window ends after 1,2,3.. hours or whatever, stop the system statistics gathering:
SQL> execute dbms_stats.gather_system_stats('stop');
You can use time interval (minutes) instead of issuing start/stop command manually:
SQL> execute dbms_stats.gather_system_stats('interval',60); 

Check the system values collected:
-------------------------------------------
col pname format a20
col pval2 format a40
select * from sys.aux_stats$; 

cpuspeedNW:  Shows the noworkload CPU speed, (average number of CPU cycles per second).
ioseektim:    The sum of seek time, latency time, and OS overhead time.
iotfrspeed:  I/O transfer speed,tells optimizer how fast the DB can read data in a single read request.
cpuspeed:      Stands for CPU speed during a workload statistics collection.
maxthr:          The maximum I/O throughput.
slavethr:      Average parallel slave I/O throughput.
sreadtim:     The Single Block Read Time statistic shows the average time for a random single block read.
mreadtim:     The average time (seconds) for a sequential multiblock read.
mbrc:             The average multiblock read count in blocks.

Notes:
-When gathering NOWORKLOAD statistics it will gather (cpuspeedNW, ioseektim, iotfrspeed) system statistics only.
-Above values can be modified manually using DBMS_STATS.SET_SYSTEM_STATS procedure.
-According to Oracle, collecting workload statistics doesn't impose an additional overhead on your system.

Delete system statistics:
------------------------------
SQL> execute dbms_stats.delete_system_stats();


####################
Data Dictionary Statistics
####################

Facts:
-------
> Dictionary tables are the tables owned by SYS and residing in the system tablespace.
> Normally data dictionary statistics in 9i is not required unless performance issues are detected.
> In 10g Statistics on the dictionary tables will be maintained via the automatic statistics gathering job run during the nightly maintenance window.

If you choose to switch off that job for application schema consider leaving it on for the dictionary tables. You can do this by changing the value of AUTOSTATS_TARGET from AUTO to ORACLE using the procedure:

SQL> Exec DBMS_STATS.SET_PARAM(AUTOSTATS_TARGET,'ORACLE');  

When to gather Dictionary statistics:
---------------------------------------------
-After DB upgrades.
-After creation of a new big schema.
-Before and after big datapump operations.

Check last Dictionary statistics date:
---------------------------------------------
SQL> select table_name, last_analyzed from dba_tables
     where owner='SYS' and table_name like '%$' order by 2; 

Gather Dictionary Statistics:  
-----------------------------------
SQL> EXEC DBMS_STATS.GATHER_DICTIONARY_STATS;
->Will gather stats on 20% of SYS schema tables.
or...
SQL> EXEC DBMS_STATS.GATHER_SCHEMA_STATS ('SYS');
->Will gather stats on 100% of SYS schema tables.
or...
SQL> EXEC DBMS_STATS.GATHER_DATABASE_STATS(gather_sys=>TRUE);
->Will gather stats on the whole DB+SYS schema.



################
Extended Statistics "11g onwards"
################

Extended statistics can be gathered on columns based on functions or column groups.

Gather extended stats on column function:
====================================
If you run a query having in the WHERE statement a function like upper/lower the optimizer will be off and index on that column will not be used:
SQL> select count(*) from EMP where lower(ename) = 'scott'; 

In order to make optimizer work with function based terms you need to gather extended stats:

1-Create extended stats:
>>>>>>>>>>>>>>>>>>>>
SQL> select dbms_stats.create_extended_stats('SCOTT','EMP','(lower(ENAME))') from dual;

2-Gather histograms:
>>>>>>>>>>>>>>>>>
SQL> exec dbms_stats.gather_table_stats('SCOTT','EMP', method_opt=> 'for all columns size skewonly');

OR
----
*You can do it also in one Step:
>>>>>>>>>>>>>>>>>>>>>>>>>

SQL> Begin dbms_stats.gather_table_stats
     (ownname => 'SCOTT',tabname => 'EMP',
     method_opt => 'for all columns size skewonly for
     columns (lower(ENAME))');
     end;
     /

To check the Existence of extended statistics on a table:
----------------------------------------------------------------------
SQL> select extension_name,extension from dba_stat_extensions where owner='SCOTT'and table_name = 'EMP';
SYS_STU2JLSDWQAFJHQST7$QK81_YB (LOWER("ENAME"))

Drop extended stats on column function:
------------------------------------------------------
SQL> exec dbms_stats.drop_extended_stats('SCOTT','EMP','(LOWER("ENAME"))');

Gather extended stats on column group: -related columns-
=================================
Certain columns in a table that are part of a join condition (where statement  are correlated e.g.(country,state). You want to make the optimizer aware of this relationship between two columns and more instead of using separate statistics for each columns. By creating extended statistics on a group of columns, the Optimizer can determine a more accurate the relation between the columns are used together in a where clause of a SQL statement. e.g. columns like country_id and state_name the have a relationship, state like Texas can only be found in USA so the value of state_name are always influenced by country_id.
If there are extra columns are referenced in the "WHERE statement  with the column group the optimizer will make use of column group statistics.

1- create a column group:
>>>>>>>>>>>>>>>>>>>>>
SQL> select dbms_stats.create_extended_stats('SH','CUSTOMERS', '(country_id,cust_state_province)')from dual;
2- Re-gather stats|histograms for table so optimizer can use the newly generated extended statistics:
>>>>>>>>>>>>>>>>>>>>>>>
SQL> exec dbms_stats.gather_table_stats ('SH','customers',method_opt=> 'for all columns size skewonly');

OR
---

*You can do it also in one Step:
>>>>>>>>>>>>>>>>>>>>>>>>>

SQL> Begin dbms_stats.gather_table_stats
     (ownname => 'SH',tabname => 'CUSTOMERS',
     method_opt => 'for all columns size skewonly for
     columns (country_id,cust_state_province)');
     end; 
     /

Drop extended stats on column group:
--------------------------------------------------
SQL> exec dbms_stats.drop_extended_stats('SH','CUSTOMERS', '(country_id,cust_state_province)');


#########
Histograms
#########

What are Histograms?
-----------------------------
> Holds data about values within a column in a table for number of occurrences for a specific value/range.
> Used by CBO to optimize a query to use whatever index Fast Full scan or table full scan.
> Usually being used against columns have data being repeated frequently like country or city column.
> gathering histograms on a column having distinct values (PK) is useless because values are not repeated.
> Two types of Histograms can be gathered:
  -Frequency histograms: is when distinct values (buckets) in the column is less than 255 (e.g. the number of countries is always less than 254).
  -Height balanced histograms: are similar to frequency histograms in their design, but distinct values  > 254
    See an Example: http://aseriesoftubes.com/articles/beauty-and-it/quick-guide-to-oracle-histograms
> Collected by DBMS_STATS (which by default doesn't collect histograms, it deletes them if you didn't use the parameter).
> Mainly being gathered on foreign key columns/columns in WHERE statement.
> Help in SQL multi-table joins.
> Column histograms like statistics are being stored in data dictionary.
> If application exclusively uses bind variables, Oracle recommends deleting any existing histograms and disabling Oracle histograms generation.

Cautions:
   – Do not create them on Columns that are not being queried.
   – Do not create them on every column of every table.
   – Do not create them on the primary key column of a table.

Verify the existence of histograms:
---------------------------------------------
SQL> select column_name,histogram from dba_tab_col_statistics
     where owner='SCOTT' and table_name='EMP'; 

Creating Histograms:
---------------------------
e.g.
SQL> Exec dbms_stats.gather_schema_stats
     (ownname => 'SCOTT',
     estimate_percent => dbms_stats.auto_sample_size,
     method_opt => 'for all columns size auto',
     degree => 7); 


method_opt:
FOR COLUMNS SIZE AUTO                 => Fastest. you can specify one column instead of all columns.
FOR ALL COLUMNS SIZE REPEAT     => Prevent deletion of histograms and collect it only for columns already have histograms.
FOR ALL COLUMNS => collect histograms on all columns .
FOR ALL COLUMNS SIZE SKEWONLY => collect histograms for columns have skewed value .
FOR ALL INDEXES COLUMNS      => collect histograms for columns have indexes.

Note: AUTO & SKEWONLY will let Oracle decide whether to create the Histograms or not.

Check the existence of Histograms:
SQL> select column_name, count(*) from dba_tab_histograms
     where OWNER='SCOTT' table_name='EMP' group by column_name; 

Drop Histograms: 11g
----------------------
e.g.
SQL> Exec dbms_stats.delete_column_stats
     (ownname=>'SH', tabname=>'SALES',
     colname=>'PROD_ID', col_stat_type=> HISTOGRAM);

Stop gather Histograms: 11g
------------------------------
[This will change the default table options]
e.g.
SQL> Exec dbms_stats.set_table_prefs
     ('SH', 'SALES','METHOD_OPT', 'FOR ALL COLUMNS SIZE AUTO,FOR COLUMNS SIZE 1 PROD_ID');
>Will continue to collect histograms as usual on all columns in the SALES table except for PROD_ID column.

Drop Histograms: 10g
----------------------
e.g.
SQL> exec dbms_stats.delete_column_stats(user,'T','USERNAME');


################################
Save/IMPORT & RESTORE STATISTICS:
################################
====================
Export /Import Statistics:
====================
In this way statistics will be exported into table then imported later from that table.

1-Create STATS TABLE:
-  -----------------------------
SQL> Exec dbms_stats.create_stat_table(ownname => 'SYSTEM', stattab => 'prod_stats',tblspace => 'USERS'); 

2-Export statistics to the STATS table:
---------------------------------------------------
For Database stats:
SQL> Exec dbms_stats.export_database_stats(statown => 'SYSTEM', stattab => 'prod_stats');
For System stats:
SQL> Exec dbms_stats.export_SYSTEM_stats(statown => 'SYSTEM', stattab => 'prod_stats');
For Dictionary stats:
SQL> Exec dbms_stats.export_Dictionary_stats(statown => 'SYSTEM', stattab => 'prod_stats');
For Fixed Tables stats:
SQL> Exec dbms_stats.export_FIXED_OBJECTS_stats(statown => 'SYSTEM', stattab => 'prod_stats');
For Schema stas:
SQL> EXEC DBMS_STATS.EXPORT_SCHEMA_STATS('ORIGINAL_SCHEMA','STATS_TABLE',NULL,'STATS_TABLE_OWNER');
For Table: 
SQL> Conn scott/tiger
SQL> Exec dbms_stats.export_TABLE_stats(ownname => 'SCOTT',tabname => 'EMP',stattab => 'prod_stats');
For Index:
SQL> Exec dbms_stats.export_INDEX_stats(ownname => 'SCOTT',indname => 'PK_EMP',stattab => 'prod_stats');
For Column:
SQL> Exec dbms_stats.export_COLUMN_stats (ownname=>'SCOTT',tabname=>'EMP',colname=>'EMPNO',stattab=>'prod_stats');

3-Import statistics from PROD_STATS table to the dictionary:
---------------------------------------------------------------------------------
For Database stats:
SQL> Exec DBMS_STATS.IMPORT_DATABASE_STATS
     (stattab => 'prod_stats',statown => 'SYSTEM');
For System stats:
SQL> Exec DBMS_STATS.IMPORT_SYSTEM_STATS
     (stattab => 'prod_stats',statown => 'SYSTEM');
For Dictionary stats:
SQL> Exec DBMS_STATS.IMPORT_Dictionary_STATS
     (stattab => 'prod_stats',statown => 'SYSTEM');
For Fixed Tables stats:
SQL> Exec DBMS_STATS.IMPORT_FIXED_OBJECTS_STATS
     (stattab => 'prod_stats',statown => 'SYSTEM');
For Schema stats:
SQL> Exec DBMS_STATS.IMPORT_SCHEMA_STATS
     (ownname => 'SCOTT',stattab => 'prod_stats', statown => 'SYSTEM');
For Table stats and it's indexes: 
SQL> Exec dbms_stats.import_TABLE_stats
     ( ownname => 'SCOTT', stattab => 'prod_stats',tabname => 'EMP');
For Index:
SQL> Exec dbms_stats.import_INDEX_stats
     ( ownname => 'SCOTT', stattab => 'prod_stats', indname => 'PK_EMP');
For COLUMN:
SQL> Exec dbms_stats.import_COLUMN_stats
     (ownname=>'SCOTT',tabname=>'EMP',colname=>'EMPNO',stattab=>'prod_stats');

4-Drop STAT Table:
--------------------------
SQL> Exec dbms_stats.DROP_STAT_TABLE (stattab => 'prod_stats',ownname => 'SYSTEM');

===============
Restore statistics: -From Dictionary-
===============
Old statistics are saved automatically in SYSAUX for 31 day.

Restore Dictionary stats as of timestamp:
------------------------------------------------------
SQL> Exec DBMS_STATS.RESTORE_DICTIONARY_STATS(sysdate-1); 

Restore Database stats as of timestamp:
----------------------------------------------------
SQL> Exec DBMS_STATS.RESTORE_DATABASE_STATS(sysdate-1); 

Restore SYSTEM stats as of timestamp:
----------------------------------------------------
SQL> Exec DBMS_STATS.RESTORE_SYSTEM_STATS(sysdate-1); 

Restore FIXED OBJECTS stats as of timestamp:
----------------------------------------------------------------
SQL> Exec DBMS_STATS.RESTORE_FIXED_OBJECTS_STATS(sysdate-1); 

Restore SCHEMA stats as of timestamp:
---------------------------------------
SQL> Exec dbms_stats.restore_SCHEMA_stats
     (ownname=>'SYSADM',AS_OF_TIMESTAMP=>sysdate-1); 
OR:
SQL> Exec dbms_stats.restore_schema_stats
     (ownname=>'SYSADM',AS_OF_TIMESTAMP=>'20-JUL-2008 11:15:00AM');

Restore Table stats as of timestamp:
------------------------------------------------
SQL> Exec DBMS_STATS.RESTORE_TABLE_STATS
     (ownname=>'SYSADM', tabname=>'T01POHEAD',AS_OF_TIMESTAMP=>sysdate-1);

=========
Advanced:
=========

To Check current Stats history retention period (days):
-------------------------------------------------------------------
SQL> select dbms_stats.get_stats_history_retention from dual;
SQL> select dbms_stats.get_stats_history_availability from dual;
To modify current Stats history retention period (days):
-------------------------------------------------------------------
SQL> Exec dbms_stats.alter_stats_history_retention(60); 

Purge statistics older than 10 days:
------------------------------------------
SQL> Exec DBMS_STATS.PURGE_STATS(SYSDATE-10);

Procedure To claim space after purging statstics:
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
Space will not be claimed automatically when you purge stats, you must claim it manually using this procedure:

Check Stats tables size:
>>>>>>
        col Mb form 9,999,999        col SEGMENT_NAME form a40        col SEGMENT_TYPE form a6        set lines 120        select sum(bytes/1024/1024) Mb,
        segment_name,segment_type from dba_segments
         where  tablespace_name = 'SYSAUX'        and segment_name like 'WRI$_OPTSTAT%'        and segment_type='TABLE'        group by segment_name,segment_type order by 1 asc        /

Check Stats indexes size:
>>>>>
        col Mb form 9,999,999        col SEGMENT_NAME form a40        col SEGMENT_TYPE form a6        set lines 120        select sum(bytes/1024/1024) Mb, segment_name,segment_type
        from dba_segments        where  tablespace_name = 'SYSAUX'        and segment_name like '%OPT%'        and segment_type='INDEX'        group by segment_name,segment_type order by 1 asc        /
Move Stats tables in same tablespace:
>>>>>
        select 'alter table '||segment_name||'  move tablespace
        SYSAUX;' from dba_segments
        where tablespace_name = 'SYSAUX'        and segment_name like '%OPT%' and segment_type='TABLE'        /
Rebuild stats indexes:
>>>>>>
        select 'alter index '||segment_name||'  rebuild online;'
        from dba_segments where tablespace_name = 'SYSAUX'        and segment_name like '%OPT%' and segment_type='INDEX'        /

Check for un-usable indexes:
>>>>>
        select  di.index_name,di.index_type,di.status  from
        dba_indexes di , dba_tables dt        where  di.tablespace_name = 'SYSAUX'        and dt.table_name = di.table_name        and di.table_name like '%OPT%'        order by 1 asc        /

Delete Statistics:
==============
For Database stats:
SQL> Exec DBMS_STATS.DELETE_DATABASE_STATS ();
For System stats:
SQL> Exec DBMS_STATS.DELETE_SYSTEM_STATS ();
For Dictionary stats:
SQL> Exec DBMS_STATS.DELETE_DICTIONARY_STATS ();
For Fixed Tables stats:
SQL> Exec DBMS_STATS.DELETE_FIXED_OBJECTS_STATS ();
For Schema stats:
SQL> Exec DBMS_STATS.DELETE_SCHEMA_STATS ('SCOTT');
For Table stats and it's indexes:
SQL> Exec dbms_stats.DELETE_TABLE_stats(ownname=>'SCOTT',tabname=>'EMP');
For Index:
SQL> Exec dbms_stats.DELETE_INDEX_stats(ownname => 'SCOTT',indname => 'PK_EMP');
For Column:
SQL> Exec dbms_stats.DELETE_COLUMN_stats(ownname =>'SCOTT',tabname=>'EMP',colname=>'EMPNO');

Note: This procedure can be rollback by restoring STATS using DBMS_STATS.RESTORE_ procedure.